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Atlantic Tropical Cyclone Monitoring with AMSU-A: Estimation of Maximum Sustained Wind Speeds

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Abstract

The first Advanced Microwave Sounding Unit temperature sounder (AMSU-A) was launched on the NOAA-15 satellite on 13 May 1998. The AMSU-A's higher spatial and radiometric resolutions provide more useful information on the strength of the middle- And upper-tropospheric warm cores associated with tropical cyclones than have previous microwave temperature sounders. The gradient wind relationship suggests that the temperature gradient near the core of tropical cyclones increases nonlinearly with wind speed. The gradient wind equation is recast to include AMSU-A-derived variables. Stepwise regression is used to determine which of these variables is most closely related to maximum sustained winds (Vmax). The satellite variables investigated include the radially averaged gradients at two spatial resolutions of AMSU-A channels 1-10 Tb data (δrTb), the squares of these gradients, a channel-15-based scattering index (SI89), and area-averaged Tb. Calculations of Tb and δrTb from mesoscale model simulations of Andrew (1992) reveal the effects of the AMSU spatial sampling on the cyclone warm core presentation. Stepwise regression of 66 AMSU-A terms against National Hurricane Center Vmax estimates from the 1998 and 1999 Atlantic hurricane season confirms the existence of a nonlinear relationship between wind speed and radially averaged temperature gradients near the cyclone warm core. Of six regression terms, four are dominated by temperature information, and two are interpreted as correcting for hydrometeor contamination. Jackknifed regressions were performed to estimate the algorithm performance on independent data. For the 82 cases that had in situ measurements of Vmax, the average error standard deviation was 4.7 m s-1. For 108 cases without in situ wind data, the average error standard deviation was 7.5 m s-1. Operational considerations, including the detection of weak cyclones and false alarm reduction, are also discussed.

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... [12] described and evaluated limb adjustment procedures in the AMSU-A with a combined physical and statistical technique. [13] monitored and estimated the maximum sustained wind speeds of Atlantic tropical cyclones with information on the strength of the middle and upper tropospheric warm cores provided by AMSU-A data. [14] used a single Microwave Sounding Unit (MSU) channel 2 to detect global warming in the lower troposphere, ranging from 0.08 K to 0.22 K per decade. ...
... The smaller the spread is, the more accurate the calculated bias is. Figure 5 shows the inter-sensor biases between NOAA-18 and all other satellites and the responding standard deviations. The standard deviations of most channels were less than 0.2 K, except for the near-surface channels (1-3 and 15) and the stratospheric channels (13)(14). The uncertainty was relatively larger for biases of channels 1-3 and 13-15 than channels 4-13. ...
... Having carried out an inter-sensor bias correction and a diurnal correction, we calculated the linear trends of all AMSU-A channels over the Amazon rainforest from 1998 to 2020 (Figure 14). We observed a warming trend in the AMSU-A window and tropospheric channels (1-9 and 15) and a cooling trend in stratospheric channels (10)(11)(12)(13)(14). The warming (cooling) trends of channels 7-9 (10) were relatively smaller in magnitude than those of the other channels. ...
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Microwave temperature sounding observations from polar-orbiting meteorological satellites have been widely used for research on climate trends of atmospheric temperature at different heights around the world. Taking the Amazon rainforest as the target area, this study combined the Microwave Temperature Sounder-2 (MWTS-2) data onboard the Chinese FengYun-3D (FY-3D) satellite with the Advanced Microwave Sounding unit-A (AMSU-A) data onboard the National Oce-anic and Atmospheric Administration (NOAA) and the European Meteorological Operational (MetOp) polar-orbiting meteorological satellites (i.e., NOAA-15, −18, −19, MetOp-A,-B). The double difference method was used to estimate and thus eliminate the inter-sensor bias, and a decadal di-urnal correction was used to reduce the impact of different local equator crossing times on climate trends. The "no-rain" conditions were determined for AMSU-A data by channels 1 and 15, and for MWTS-2 data by channels 1 and 7. Finally, the decadal linear trends of atmospheric temperature from 1998 to 2020 were obtained after applying the inter-sensor bias calibration and inter-decadal diurnal correction to AMSU-A and MWTS-2 data from NOAA-15, −18, −19; MetOp-A,-B; and FY-3D. A warming trend was found in the AMSU-A window and tropospheric channels (1-9 and 15) and a cooling trend in stratospheric channels (10-14). The warming (cooling) trends of channels 7-9 (10) were relatively small. The warming (cooling) trends of AMSU-A channels 1-6 (14-15) were significantly reduced after the inter-decadal diurnal correction.
... Since NOAA launched the Advanced Microwave Sounding Unit (AMSU) aboard their polar-orbiting satellite series in May 1998, passive microwave warm-core measurements of TCs have been made. The science behind the measurement is that vertical temperature soundings yield information about the mean sea level pressure and wind fields within a TC through thermodynamic and dynamic constraints (Spencer and Braswell 2001; Brueske and Velden 2003). Demuth et al. (2004) derived a method for estimating TC wind radii via a statistical procedure utilizing AMSU-derived parameters. ...
Article
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Geostationary infrared (IR) satellite data are used to provide estimates of the symmetric and total low-level wind fields in tropical cyclones, constructed from estimations of an azimuthally averaged radius of maximum wind (RMAX), a symmetric tangential wind speed at a radius of 182 km (V182), a storm motion vector, and the maximum intensity (VMAX). The algorithm is derived using geostationary IR data from 405 cases from 87 tropical systems in the Atlantic and east Pacific Ocean basins during the 1995-2003 hurricane seasons that had corresponding aircraft data available. The algorithm is tested on 50 cases from seven tropical storms and hurricanes during the 2004 season. Aircraft-reconnaissance-measured RMAX and V182 are used as dependent variables in a multiple linear regression technique, and VMAX and the storm motion vector are estimated using conventional methods. Estimates of RMAX and V182 exhibit mean absolute errors (MAEs) of 27.3 km and 6.5 kt, respectively, for the dependent samples. A modified combined Rankine vortex model is used to estimate the one-dimensional symmetric tangential wind field from VMAX, RMAX, and V182. Next, the storm motion vector is added to the symmetric wind to produce estimates of the total wind field. The MAE of the IR total wind retrievals is 10.4 kt, and the variance explained is 53%, when compared with the two-dimensional wind fields from the aircraft data for the independent cases.
... D04 capitalized on the efficacy of passive microwave remote sensing for observing TCs by using the Advanced Microwave Sounding Unit (AMSU), the followon to the Microwave Sounding Unit. For more details of the AMSU instrument and its use for TC applications , see Kidder et al. (2000), Knaff et al. (2000 , Spencer and Braswell (2001), Brueske and Velden (2003), Knaff et al. (2004), and Bessho et al. (2006. D04 used AMSU-derived data from 1999 to 2001 to develop algorithms that provide objective estimates of intensity [ The sample size consisted of 473 cases (i.e., passes over a TC) for the intensity estimation models and 129, 92, and 68 cases for the models estimating the 34-, 50-, and 64-kt wind radii, respectively. ...
Article
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Previous work, in which Advanced Microwave Sounding Unit (AMSU) data from the Atlantic Ocean and east Pacific Ocean basins during 1999 2001 were used to provide objective estimates of 1-min maximum sustained surface winds, minimum sea level pressure, and the radii of 34-, 50-, and 64-kt (1 kt ≡ 0.5144 m s-1) winds in the northeast, southeast, southwest, and northwest quadrants of tropical cyclones, is updated to reflect larger datasets, improved statistical analysis techniques, and improved estimation through dependent variable transforms. A multiple regression approach, which utilizes best-subset predictor selection and cross validation, is employed to develop the estimation models, where the dependent data (i.e., maximum sustained winds, minimum pressure, wind radii) are from the extended best track and the independent data consist of AMSU-derived parameters that give information about retrieved pressure, winds, temperature, moisture, and satellite resolution. The developmental regression models result in mean absolute errors (MAE) of 10.8 kt and 7.8 hPa for estimating maximum winds and minimum pressure, respectively. The MAE for the 34-, 50-, and 64-kt azimuthally averaged wind radii are 16.9, 13.3, and 6.8 n mi (1 n mi ≡ 1852 m), respectively.
... The National Hurricane Center (NHC) best track was used for the ground truth values of intensity (maximum 1-min-sustained surface winds and minimum sea level pressure), and the wind structure was measured by the radii of 34-, 50-, and 64-kt winds in four quadrants relative to the TC center (1 kt 0.5144 m s 1 ). Spencer and Braswell (2001) used a somewhat similar statistical approach to estimate the TC maximum wind from several AMSU channels. ...
Article
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Horizontal winds at 850 hPa from tropical cyclones retrieved using the nonlinear balance equation, where the mass field was determined from Advanced Microwave Sounding Unit (AMSU) temperature soundings, are compared with the surface wind fields derived from NASA's Quick Scatterometer (QuikSCAT) and Hurricane Research Division H*Wind analyses. It was found that the AMSU-derived wind speeds at 850 hPa have linear relations with the surface wind speeds from QuikSCAT or H*Wind. There are also characteristic biases of wind direction between AMSU and QuikSCAT or H*Wind. Using this information to adjust the speed and correct for the directional bias, a new algorithm was developed for estimation of the tropical cyclone surface wind field from the AMSU-derived 850-hPa winds. The algorithm was evaluated in two independent cases from Hurricanes Floyd (1999) and Michelle (2001), which were observed simultaneously by AMSU, QuikSCAT, and H*Wind. In this evaluation the AMSU adjustment algorithm for wind speed worked well. Results also showed that the bias correction algorithm for wind direction has room for improvement.
... While the ADT currently provides forecasters with an objective tool based on IR imagery, the use of supplementary spectral information has the potential to advance satellite-based intensity estimation considerably further than can be achieved with the IR band alone. For example, polar-orbiting microwave sensors are being used to denote TC structure and infer intensity (Herndon and Velden 2004;Demuth et al. 2004;Edson and Lander 2002;Spencer and Braswell 2001;Hawkins et al. 2001;Bankert and Tag 2002). Employment of these instruments and methods in conjunction with the existing ADT into an integrated algorithm should provide TC analysts with an even more powerful tool for estimating tropical cyclone intensity. ...
Article
An automated method to estimate tropical cyclone intensity using Special Sensor Microwave Imager (SSM/ I) data is developed and tested. SSM/I images (512 km × 512 km) centered on a given tropical cyclone (TC), with a known best-track intensity, are collected for 142 different TCs (1988-98) from the North Pacific, Atlantic, and Indian Oceans. Over 100 characteristic features are computed from the 85-GHz (H-pol) imagery data and the derived rain-rate imagery data associated with each TC. Of the 1040 sample images. 942 are selected as training samples. These training samples are examined in a feature-selection algorithm to select an optimal subset of the characteristic features that could accurately estimate TC intensity on unknown samples in a K- nearest-neighbor (K-NN) algorithm. Using the 15 selected features as the representative vector and the best-track intensity as the ground truth, the 98 testing samples (taken from four TCs) are presented to the K-NN algorithm. A root-mean-square error (rmse) of 19.8 kt is produced. This "snapshot" approach is enhanced (rmse is 18.1 kt) when a TC intensity history feature is added to 71 of the 98 samples. Reconnaissance data are available for two recent (1999) Atlantic hurricanes, and a comparison is made in the rmse using those data as ground truth versus best track. For these two TCs (17 SSM/I images), an rmse of 15.6 kt is produced when best track is used and an rmse of 19.7 kt is produced when reconnaissance data are used as the ground truth.
... For example, the use of passive microwave instruments mounted on a variety of satellites has assisted in the monitoring of eyewall replacement cycles, one important aspect of the intensification process (Willoughby et al. 1982; Hawkins et al. 2006; Jones et al. 2006). In addition, the Advanced Microwave Sounding Unit (AMSU) series of satellites has assisted researchers and forecasters alike through the ability to monitor the three-dimensional (3D) warm core of a TC, another important aspect of intensification that leads directly to lowered surface pressures and increased winds through thermal wind adjustment (Kidder et al. 2000; Spencer and Braswell 2001; Brueske and Velden 2003; Knaff et al. 2004). Airborne Doppler radar has arguably had the most extensive and fruitful role in TC research, particularly in the observation of storm structure and dynamics. ...
Article
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A synthesis of remote sensing and in situ observations throughout the life cycle of Hurricane Dennis (2005) during the NASA Tropical Cloud Systems and Processes (TCSP) experiment is presented. Measurements from the ER-2 Doppler radar (EDOP), the Advanced Microwave Sounding Unit (AMSU), airborne radiometer, and flight-level instruments are used to provide a multiscale examination of the storm. The main focus is an episode of deep convective bursts (“hot towers”) occurring during a mature stage of the storm and preceding a period of rapid intensification (11-hPa pressure drop in 1 h 35 min). The vigorous hot towers penetrated to 16-km height, had maximum updrafts of 20 m s−1 at 12–14-km height, and possessed a strong transverse circulation through the core of the convection. Significant downdrafts (maximum of 10–12 m s−1) on the flanks of the updrafts were observed, with their cumulative effects hypothesized to result in the observed increases in the warm core. In one ER-2 overpass, subsidence was transported toward the eye by 15–20 m s−1 inflow occurring over a deep layer (0.5–10 km) coincident with a hot tower. Fourier analysis of the AMSU satellite measurements revealed a large shift in the storm’s warm core structure, from asymmetric to axisymmetric, ∼12 h after the convective bursts began. In addition, flight-level wind calculations of the axisymmetric tangential velocity and inertial stability showed a contraction of the maximum winds and an increase in the stiffness of the vortex, respectively, after the EDOP observations. The multiscale observations presented here reveal unique, ultra-high-resolution details of hot towers and their coupling to the parent vortex, the balanced dynamics of which can be generally explained by the axisymmetrization and efficiency theories.
... The advantages of using this method are that microwaves of different wavelengths can penetrate below cloud tops and can sense the temperature, the average amount of cloud liquid water, and rain rates at different levels in the troposphere. The disadvantage is that the spatial resolution is coarse: even in the Advanced Microwave Sounding Unit (AMSU) used by Spencer and Braswell (2001), Brueske and Velden (2003), and Demuth et al. (2004), it varies between 48 and 100 km. Errors in cyclone intensity forecasts using AMSU data are comparable to those by Dvorak's technique: 72.5% of errors were within 15 kt and 13% were between 20 and 57 kt, the larger errors being associated with cyclones having small radius of maximum wind (Demuth et al. 2004). ...
Article
Objective streamline analyses and digitized high-resolution IR satellite cloud data have been used to examine in detail the changes in the environmental circulation and in the cloud structure that took place in and around Tropical Cyclone Kathy (1984) when it started to intensify, and during its intensification and dissipation stages. The change of low-level circulation around Tropical Cyclone Kathy was measured by the change in the angle of inflow (α4) at a radius of 4° latitude from the cyclone center. When Kathy started to intensify, α4 increased suddenly from 20° to 42.5° in the northerly airstream to the north and northeast of the depression, and decreased to 0° to the south of the depression. At that stage the low-level circulation around the depression appeared as a giant swirl that started some 600 km to the north and northeast of the depression and spiraled inward toward its center, while trade air, which is usually cool, dry, and stable, did not enter the cyclonic circulation. The angle α4 remained the same during intensification. During the dissipation stage, α4 returned to 20° and trade air started to participate in the cyclonic circulation. Satellite cloud data were used to determine the origin, evolution, and importance of the feeder bands in the intensification of the cyclone, to follow the moist near-equatorial air that flowed through them and to estimate the maximum height of cumulonimbi that developed in them, to observe the changes in the convective activity in the central dense overcast (CDO) area, as well as in the area around the CDO. Most of the observed changes in Kathy have also been observed in other tropical cyclones during intensification and dissipation. Using the sequence of observed changes of the circulation and of convective activity in and around the CDO of Kathy, a mathematical model has been developed to forecast the intensity of a tropical cyclone. The model and its application to three tropical cyclones in the Australian region are described in Part II of this paper.
... As the Dvorak technique is subjective, time-intensive, and relies on the expertise of the analyst, Velden et al. [4] and Olander and Velden [5] developed the objective and advanced Dvorak techniques (ODT, ADT), respectively, which are automated alternatives to estimate the intensity of TCs. Subsequently, measurements from microwave satellite sounders such as the Advanced Microwave Sounding Unit (AMSU) have also been used to estimate the intensity of TCs [6], [7], and some multisatellite consensus techniques such as the CIMSS Satellite Consensus (SATCON) have been developed to enhance the TC intensity estimation [8]. ...
Article
Full-text available
The deviation angle variance (DAV) method was developed to objectively estimate tropical cyclone (TC) intensity from geostationary infrared (IR) brightness temperature data. Here, we demonstrate that improvements of 25% root mean square error (RMSE) in major hurricane intensity estimation (relative to best track) can be obtained by considering the pixel-by-pixel satellite view angle in the estimation. Using data from the Chinese Fengyun 2E and 2F satellites for Super Typhoon Soudelor (2015), we demonstrate how the satellite observation angle can reduce the accuracy of intensity estimation, especially for the strongest TCs. Based on these results, an improved DAV estimator is developed using 12 years (2004-2015) of Geostationary Operational Environmental Satellite (GOES)-East satellite IR images over the North Atlantic basin.
... While the ADT currently provides forecasters with an objective tool based on IR imagery, the use of supplementary spectral information has the potential to advance satellite-based intensity estimation considerably further than can be achieved with the IR band alone. For example, polar-orbiting microwave sensors are being used to denote TC structure and infer intensity (Herndon and Velden 2004;Demuth et al. 2004;Edson and Lander 2002;Spencer and Braswell 2001;Hawkins et al. 2001;Bankert and Tag 2002). Employment of these instruments and methods in conjunction with the existing ADT into an integrated algorithm should provide TC analysts with an even more powerful tool for estimating tropical cyclone intensity. ...
Article
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Tropical cyclones are becoming an increasing menace to society as populations grow in coastal regions. Forecasting the intensity of these often-temperamental weather systems can be a real challenge, especially if the true intensity at the forecast time is not well known. To address this issue, techniques to accurately estimate tropical cyclone intensity from satellites are a natural goal because in situ observations over the vast oceanic basins are scarce. The most widely utilized satellite-based method to estimate tropical cyclone intensity is the Dvorak technique, a partially subjective scheme that has been employed operationally at tropical forecast centers around the world for over 30 yr. With the recent advent of improved satellite sensors, the rapid advances in computing capacity, and accumulated experience with the behavioral char-acteristics of the Dvorak technique, the development of a fully automated, computer-based objective scheme to derive tropical cyclone intensity has become possible. In this paper the advanced Dvorak technique is introduced, which, as its name implies, is a derivative of the original Dvorak technique. The advanced Dvorak technique builds on the basic conceptual model and empirically derived rules of the original Dvorak technique, but advances the science and applicability in an automated environment that does not require human intervention. The algorithm is the culmination of a body of research that includes the objective Dvorak technique (ODT) and advanced objective Dvorak technique (AODT) developed at the University of Wisconsin—Madison's Cooperative Institute for Me-teorological Satellite Studies. The ODT could only be applied to storms that possessed a minimum intensity of hurricane/typhoon strength. In addition, the ODT still required a storm center location to be manually selected by an analyst prior to algorithm execution. These issues were the primary motivations for the continued advancement of the algorithm (AODT). While these two objective schemes had as their primary goal to simply achieve the basic functionality and performance of the Dvorak technique in a computer-driven environment, the advanced Dvorak technique exceeds the boundaries of the original Dvorak tech-nique through modifications based on rigorous statistical and empirical analysis. It is shown that the accuracy of the advanced Dvorak technique is statistically competitive with the original Dvorak technique, and can provide objective tropical cyclone intensity guidance for systems in all global basins.
... The technique estimates the two-dimensional wind field from the IR imagery and then validates the field against aircraft wind observations. Furthermore, techniques for estimating the intensity of a tropical cyclone have also been developed using measurements from the Advanced Microwave Sounding Unit (AMSU; Spencer and Braswell 2001;Demuth et al. 2004). Some of these techniques have been combined to enhance the tropical cyclone intensity estimation (e.g., Velden et al. 2006a). ...
Article
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This paper describes results from an improvement to the objective deviation angle variance technique to estimate the intensity of tropical cyclones from satellite infrared imagery in the North Atlantic basin. The technique quantifies the level of organization of the infrared cloud signature of a tropical cyclone as an indirect measurement of its maximum wind speed. The major change described here is to use the National Hurricane Center's best-track database to constrain the technique. Results are shown for the 2004-10 North Atlantic hurricane seasons and include an overall root-mean-square intensity error of 12.9 kt (6.6 m s -1, where 1 kt50.514ms -1) and annual root-mean-square intensity errors ranging from 10.3 to 14.1 kt. Adirect comparison between the previous version and the one reported here shows root-mean-square intensity error improvements in all years with a best improvement in 2009 from 17.9 to 10.6 kt and an overall improvement from 14.8 to 12.9 kt. In addition, samples from the 7-yr period are binned based on level of intensity and on the strength of environmental vertical wind shear as extracted from Statistical Hurricane Intensity Prediction Scheme (SHIPS) data. Preliminary results suggest that the deviation angle variance technique performs best at the weakest intensity categories of tropical storm through hurricane category 3, representing 90%of the samples, and then degrades in performance for hurricane categories 4 and 5. For environmental vertical wind shear, there is far less spread in the results with the technique performing better with increasing vertical wind shear.
... Knaff et al. [2000] and Zhu et al. [2002] retrieved and analyzed atmospheric temperatures in hurricane systems with AMSU-A observations. Spencer and Braswell [2001] estimated TC maximum sustained wind (MSW) using the temperature gradient derived from AMSU-A measurement. Demuth et al. [2004Demuth et al. [ , 2006 developed regression algorithms to estimate TC MSW, minimum sea level pressure (MSLP), and TC size (radii of winds). ...
Article
The warm-core structures of Hurricane Sandy and other nine tropical cyclones (TCs) are studied using the temperatures retrieved from Advanced Technology Microwave Sounder (ATMS). A new algorithm is developed for the retrieval of atmospheric temperature profiles from the ATMS radiances. Since ATMS observation has a higher spatial resolution and better coverage than its predecessor, Advanced Microwave Sounding Unit-A, the retrieved temperature field explicitly resolves TC warm core throughout troposphere and depicts the cold temperature anomalies in the eyewall and spiral rainbands. Unlike a typical TC, the height of maximum warm core of Hurricane Sandy is very low, but the storm size is quite large. Based on the analysis of 10 TCs in 2012, close correlations are found between ATMS-derived warm core and the TC maximum sustained wind (MSW) or minimum sea level pressure (MSLP). The estimation errors of MSW and MSLP from ATMS-retrieved warm core are 13.5 mph and 13.1 hPa, respectively.
... The National Hurricane Center (NHC) best track was used for the ground truth values of intensity (maximum 1-min-sustained surface winds and minimum sea level pressure), and the wind structure was measured by the radii of 34-, 50-, and 64-kt winds in four quadrants relative to the TC center (1 kt 0.5144 m s 1 ). Spencer and Braswell (2001) used a somewhat similar statistical approach to estimate the TC maximum wind from several AMSU channels. ...
Article
Passive microwave observations from the current NOAA series of polar-orbiting satellites of a large sample of North Atlantic tropical cyclones are qualitatively and quantitatively analyzed. Microwave observations can penetrate the cloud cover associated with tropical cyclones and capture the upper-level warm temperature anomaly, which is characteristic of these storms. The data are used to develop a statistical algorithm for estimating surface intensity. Based upon hydrostatic assumptions, linear regression relationships are developed between the satellite-depicted horizontal temperature gradient of the upper-level warm core (T250), and the surface intensity (PSFC) as measured by reconnaissance reports. A good correlation is found to exist. Results indicate that standard errors of estimate of 8 mb and 13 kts are found for surface pressure and maximum winds, respectively. These errors are reduced when the effects of storm latitude, eye size, and surface-pressure tendency on the relationship are included. Knowledge gained in examining the accuracies and limitations of the current microwave sounders in tropical cyclone applications will be helpful in setting quantitative observational guidelines for future instruments.
... D. Hawkins 2005, personal communication). Several groups have utilized the Advanced Microwave Sounding Unit (AMSU) flown on NOAA polar-orbiting satellites to develop algorithms to estimate TC intensity (Brueske and Velden 2003;Herndon and Velden 2004;Demuth et al. 2004;Spencer and Braswell 2001). These tech- niques take advantage of the tropospheric profiling capability of the AMSU to depict TC warm cores, and statistically relate these measurements through hydrostatic assumptions to intensity. ...
Article
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The history of meteorology has taught us that weather analysis and prediction usually advances by a series of small, progressive studies. Occasionally, however, a special body of work can accelerate this process. When that work pertains to high-impact weather events that can affect large populations, it is especially notable. In this paper we review the contributions by Vernon F. Dvorak, whose innovations using satellite observations of cloud patterns fundamentally enhanced the ability to monitor tropical cyclones on a global scale. We discuss how this original technique has progressed, and the ways in which new spaceborne instruments are being employed to complement Dvorak's original vision.
... Based on this observed structure, two approaches have been taken to estimate TC intensity from AMSU. In the first approach, brightness temperatures from AMSU channels that sense the upper troposphere are used directly to estimate the warm core (Spencer and Braswell 2001;Brueske and Velden 2003). The characteristics of the warm core are then related to the TC intensity. ...
Article
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Tropical cyclones spend most of their life cycle over the tropical and subtropical oceans. Because of the lack of in situ data in these regions, satellite observations are fundamental for tracking and estimating the intensity of these storms for real-time forecasting and monitoring climate trends. This chapter reviews methods for estimating tropical cyclone intensity from satellites, including those based on visible, infrared, and microwave instruments. Satellite intensity estimates are transitioning from subjective to objective methods, and new instruments on the next generation of NOAA low-earth orbiting and geostationary satellites hold promise for continued improvement in satellite analysis of tropical cyclones. © 2013 Springer Science+Business Media Dordrecht. All rights are reserved.
... • Tropical Cyclone Detection and Wind Speed Application -The Advanced Microwave Sounding Unit is a microwave radiometer that can be used to detect temperature at different levels of the atmosphere. Based on gradients in temperature measurements in a given area, it is possible to estimate maximal sustained radial wind speed [6] ...
... Since NOAA launched the Advanced Microwave Sounding Unit (AMSU) aboard their polar-orbiting satellite series in May 1998, passive microwave warm-core measurements of TCs have been made. The science behind the measurement is that vertical temperature soundings yield information about the mean sea level pressure and wind fields within a TC through thermodynamic and dynamic constraints (Spencer and Braswell 2001;Brueske and Velden 2003). Demuth et al. (2004) derived a method for estimating TC wind radii via a statistical procedure utilizing AMSU-derived parameters. ...
Article
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A new algorithm to quantitatively estimate the wind structure of tropical cyclones from infrared satellite imagery was developed. The algorithm development used 405 cases of IR imagery with corresponding flight-level (5,000 to 10,000 ft) wind analyses from 1995-2003 aircraft reconnaissance flights into 87 Atlantic and eastern Pacific tropical cyclones. The wind analyses cover the inner 200 km of the storm. The resultant algorithm represents a major advance in the ability to monitor tropical cyclones from satellites.
... The AMSU-A antenna provides a cross-track scan, scanning ±48.3°from nadir with a total of 30 Earth (Bennartz 2000;Kramer 2002). AMSU applicability to retrieval of the oceanic and atmospheric parameters in the tropical zone and to TC analysis was shown by Knaff et al. (2000), Staelin and Chen (2000), Spencer and Braswell (2001), Demuth et al. (2004Demuth et al. ( , 2006, and Bessho et al. (2006). ...
Chapter
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Development of microwave technologies and our ability to penetrate into Tropical Cyclones (TCs) by instrumented aircraft and observe from satellites have contributed much of the knowledge and understanding that exist today. We can now follow the structure and development of a storm from inception through the many stages towards a dangerous typhoon, hurricane or cyclone, as they are variously called around the globe. The data from satellite microwave radiometers, scatterometers, altimeters, Synthetic Aperture Radars (SARs), microwave sounders, a rain radar and a cloud profiling radar as well as coastal radars, airborne radars and microwave radiometers have all contributed to changing the fields of both TC research and TC operational forecasting.
... The AMSU has a resolution of approximately 48 km at nadir and can be used to retrieve a coarse warm-core structure. After using a variety of transformations of the original AMSU-derived parameters, the surface wind radii and other wind fields may be retrieved Knaff et al. 2000;Spencer and Braswell 2001;Brueske and Velden 2003;Knaff et al. 2004;Demuth et al. 2004;Bessho et al. 2006;Demuth et al. 2006). Demuth et al. (2006) used a multiple regression technique where the dependent data (i.e., maximum sustained winds, minimum pressure, wind radii) are from the Cooperative Institute for Research in the Atmosphere (CIRA) extended best-track and the independent data rely on AMSU-derived parameters of the retrieved pressure, winds, temperature, moisture, and satellite resolution. ...
Article
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This study extends past research based on the deviation angle variance (DAV) technique that utilizes digital brightness temperatures from longwave infrared satellite images to objectively measure the symmetry of a tropical cyclone (TC). In previous work, the single-pixel DAV values were used as an objective estimator of storm intensity while maps of the DAV values indicated areas where tropical cyclogenesis was occurring. In this study the spatial information in the DAV maps is utilized along with information from the Cooperative Institute for Research in the Atmosphere's extended best-track archive and the Statistical Hurricane Intensity Prediction Scheme model to create multiple linear regression models of wind radii parameters for TCs in the North Atlantic basin. These models are used to estimate both symmetric, and by quadrant, 34-, 50-, and 64-kt wind radii (where 1 kt = 0.51 m s⁻¹ 1) on a half-hourly time scale. The symmetric model assumes azimuthal symmetry and has mean absolute errors of 38.5, 23.2, and 13.5 km (20.8, 12.5, and 7.3 n mi) for the 34-, 50-, and 64-kt wind radii, respectively, which are lower than results for most other techniques except for those based on AMSU. The asymmetric model independently estimates radii in each quadrant and produces mean absolute errors for the wind radii that are generally highest in the northwest quadrant and lowest in the southwest quadrant similar to other techniques. However, as a percentage of the average wind radii from aircraft reconnaissance, all quadrants have similar errors.
... Kidder et al. [2000] gave a comprehensive overview of applying AMSU data in estimating TC intensities, retrieving upper tropospheric temperature anomalies, and determining TC precipitation potentials. Spencer and Braswell [2001] estimated TC maximum sustained wind (MSW) using the temperature gradient derived from AMSU-A measurement. Demuth et al. [2004Demuth et al. [ , 2006 developed algorithms to apply AMSU observations in evaluating the maximum sustained wind (MSW), minimum sea level pressure, and radii of winds of TCs. ...
Article
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The Advanced Technology Microwave Sounder (ATMS) is a cross-track microwave radiometer. Its temperature sounding channels 5-15 can provide measurements of thermal radiation emitted from different layers of the atmosphere. In this study, a traditional Advanced Microwave Sounding Unit-A (AMSU-A) temperature retrieval algorithm is modified to remove the scan biases in the temperature retrieval and to include only those ATMS sounding channels that are correlated with the atmospheric temperatures on the pressure level of the retrieval. The warm core structures derived for Hurricane Sandy when it moved from tropics to middle latitudes are examined. It is shown that scan biases that are present in the traditional retrieval are adequately removed using the modified algorithm. In addition, temperature retrievals in the upper troposphere (~250 hPa) obtained by using the modified algorithm have more homogeneous warm core structures and those from the traditional retrieval are affected by small-scale features from the low troposphere such as precipitation. Based on ATMS observations, Hurricane Sandy's warm core was confined to the upper troposphere during its intensifying stage and when it was located in the tropics, but extended to the entire troposphere when it moved into subtropics and middle latitudes and stopped its further intensification. The modified algorithm was also applied to AMSU-A observation data to retrieve the warm core structures of Hurricane Michael. The retrieved warm core features are more realistic when compared with those from the operational Microwave Integrated Retrieval System (MIRS).
... There has now been a number of studies aiming at showing the potential of the AMSU onboard the NOAA polar-orbiting satellites for tropical and mid-latitude storms (e.g. Kidder et al., 2000;Spencer and Braswell, 2001;Zhu et al., 2002;Brueske and Velden, 2003;Demuth et al., 2004;Funatsu et al., 2007, Yao et al., 2008. This study concentrates on polar oceanic regions where conventional observations are scarce and for which a single study has been conducted (Moore and Von der Haar, 2003). ...
Article
The potential of the Advanced Microwave Sounding Unit (AMSU) observations for the depiction and tracking of intense high-latitude mesoscale maritime weather systems, called polar lows, is explored. Since a variety of mechanisms are important for their development and maintenance, this investigation is based on three polar low cases of different types. The AMSU-B channels at 183 GHz are able to locate convective polar lows (PL) even in their incipient stage, at a time when there is considerable uncertainty as to the nature of the cloud structures seen in the visible or infrared imagery. This detection is based on temperature depression due to scattering by hydrometeors, as confirmed by comparison with radar data. These same channels will, however, fail to unambiguously detect weakly convective and mainly baroclinic PL. The AMSU-A channels help documenting the large-scale environment in which PL develop. Channel 5 clearly shows the cold air outbreaks associated with these developments, whereas the difference between channels 7 and 5 can be used to detect and locate positive upper-level potential vorticity anomalies. Because of the high temporal availability of AMSU observations and despite some limitations pointed out in this study, these results are relevant for PL forecasting and monitoring.
... In addition to geostationary satellite infrared imagery, temperature anomaly profiles associated with the TC warm core, observed by the Advanced Microwave Sounding Unit (AMSU), have been applied for intensity estimation (Spencer and Braswell 2001;Demuth et al. 2004). The AMSU, which is in low Earth orbit (;810 km above the surface vs ;36 000 km for geostationary satellites), detects earth/atmosphere emitted radiation in the microwave portion of the electromagnetic spectrum. ...
Article
Accurately estimating tropical cyclone (TC) intensity is one of the most critical steps in TC forecasting and disaster warning/management. For over 40 years, the Dvorak technique (and several improved versions) has been applied for estimating TC intensity by forecasters worldwide. However, the operational Dvorak techniques primarily used in various agencies have several deficiencies, such as inherent subjectivity leading to inconsistent intensity estimates within various basins. This collaborative study between meteorologists and data scientists has developed a deep-learning model using satellite imagery to estimate TC intensity. The conventional convolutional neural network (CNN), which is a mature technology for object classification, requires several modifications when being used for directly estimating TC intensity (a regression task). Compared to the Dvorak technique, the CNN model proposed here is objective and consistent among various basins; it has been trained with satellite infrared brightness temperature and microwave rain-rate data from 1097 global TCs during 2003–14 and optimized with data from 188 TCs during 2015–16. This paper also introduces an upgraded version that further improves the accuracy by using additional TC information (i.e., basin, day of year, local time, longitude, and latitude) and applying a postsmoothing procedure. An independent testing dataset of 94 global TCs during 2017 has been used to evaluate the model performance. A root-mean-square intensity difference of 8.39 kt (1 kt ≈ 0.51 m s−1) is achieved relative to the best track intensities. For a subset of 482 samples analyzed with reconnaissance observations, a root-mean-square intensity difference of 8.79 kt is achieved.
... However, if one restricts the AMSU cases to only Atlantic storms as Velden et al. did, the resultant rmse of 8.2 hPa is comparable to their findings. The standard deviation of the MSW jackknife residuals is 14.1 kt, akin to 14.6 kt from Spencer and Braswell (2001). Therefore, the intensity estimation equations developed in this study have errors comparable to those from other methods, with the advantage that they estimate MSW and MSLP for tropical disturbances of all strengths. ...
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Advanced Microwave Sounding Unit (AMSU) data are used to provide objective estimates of 1-min maximum sustained surface winds, minimum sea level pressure, and the radii of 34-, 50-, and 64-kt (1 kt 0.5144 m s-1) winds in the northeast, southeast, southwest, and northwest quadrants of tropical cyclones. The algorithms are derived from AMSU temperature, pressure, and wind retrievals from all tropical cyclones in the Atlantic and east Pacific basins during 1999 2001. National Hurricane Center best-track intensity and operational radii estimates are used as dependent variables in a multiple-regression approach. The intensity algorithms are evaluated for the developmental sample using a jackknife procedure and independent cases from the 2002 hurricane season. Jackknife results for the maximum winds and minimum sea level pressure estimates are mean absolute errors (MAE) of 11.0 kt and 6.7 hPa, respectively, and rmse of 14.1 kt and 9.3 hPa, respectively. For cases with corresponding reconnaissance data, the MAE are 10.7 kt and 6.1 hPa, and the rmse are 14.9 kt and 9.2 hPa. The independent cases for 2002 have errors that are only slightly larger than those from the developmental sample. Results from the jackknife evaluation of the 34-, 50-, and 64-kt radii show mean errors of 30, 24, and 14 n mi, respectively. The results for the independent sample from 2002 are generally comparable to the developmental sample, except for the 64-kt wind radii, which have larger errors. The radii errors for the 2002 sample with aircraft reconnaissance data available are all comparable to the errors from the jackknife sample, including the 64-kt radii.
Article
In tropical cyclones, a strong inverse relationship exists between the magnitude of the upper-tropospheric warm anomaly (UTWA) and minimum sea level pressure (MSLP). Uniquely poised to capture this warming aloft, the Advanced Microwave Sounding Unit (AMSU) flown aboard current National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites is capable of observing Tropical Cyclones (TC's) worldwide. A physical/statistical MSLP estimation algorithm based on AMSU brightness temperature anomalies (dTbs) has been operating in an experimental mode at the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS) for two years. The algorithm relies on a single AMSU channel (54.9 GHz) and shows great promise as a viable TC analysis tool. However, the radiances can be susceptible to environmental variability leading to sub-sampling and errors in MSLP. The goal of this research is to improve the existing single-channel algorithm by introducing an additional channel (55.5 GHz) that seeks to capture the true magnitude of the UTWA in instances when the single channel fails. By implementing the multi-channel approach, the goal is to create an operationally viable satellite-based guidance tool to help support tropical forecast and analysis centers worldwide.
Article
Brightness temperature anomalies measured by the Advanced Microwave Sounding Unit (AMSU) on the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting series are suited to estimate tropical cyclone (TC) intensity by virtue of their ability to assess changes in tropospheric warm core structure in the presence of clouds. Analysis of the measurements from different satellites shows that the variable horizontal resolution of the instrument has significant effects on the observed brightness temperature anomalies. With the aim to decrease these effects on TC intensity estimation more easily and effectively, a new simple correction algorithm, which is related to the product of the brightness temperature gradient near the TC center and the size of the field-of-view (FOV) observing the TC center, is proposed to modify the observed anomalies. Without other measurements, the comparison shows that the performance of the new algorithm is better than that of the traditional, physically-based algorithm. Furthermore, based on the correction algorithm, a new scheme, in which the brightness temperature anomalies at 31.4 GHz and 89 GHz accounting for precipitation effects are directly used as the predictors with those at 54.94 GHz and 55.5 GHz, is developed to estimate TC intensity in the western North Pacific basin. The collocated AMSU-A observations from NOAA-16 with the best track (BT) intensity data from the Japan Meteorological Agency (JMA) in 2002–2003 and in 2004 are used respectively to develop and validate regression coefficients. For the independent validation dataset, the scheme yields 8.4 hPa of the root mean square error and 6.6 hPa of the mean absolute error. For the 81 collocated cases in the western North Pacific basin and for the 24 collocated cases in the Atlantic basin, compared to the BT data, the standard deviations of the estimation differences of the results are 15% and 11% less than those of the CIMSS (Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison) TC intensity AMSU estimation products.
Article
Temperatures retrieved from NOAA-15 AMSU-A during the period from July to September in 2001 and 2002 are used to develop an algorithm for estimating the intensity of tropical cyclones (TCs) over the western North Pacific. The variance R-2 explained by the algorithm is 76.9%, and the mean absolute error MAE (root-mean-square error, RMSE) is 5.6 (7.5) m s(-1), all of which are comparable to the published results for Atlantic and Eastern Pacific TCs. In addition to the maximum temperature anomaly over the TC center, the height of the warm core, represented by the uppermost position of a certain temperature anomaly contour (1.0 and 0.5 K), is found to be another important predictor in the final equation. Both the jackknife method, and an independent test, are applied to verify the algorithm. Results show that R-2 deteriorates slightly to 72.3% using the jackknife method, while the other three statistic, i.e., MAE, RMSE and standard deviation of residuals, increase by similar to 0.5-1 m s(-1) in both the jackknife and independent datasets. Most of large underestimations occur for small but strong TCs, while significant overestimations are either due to the lag of an upper-level warm core following the weakening of the surface circulation, or due to upper-level warming ahead of the surface circulation development.
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Satellite-borne passive microwave radiometers, such as the Advanced Microwave Sounding Unit (AMSU) on the NOAA polar-orbiting series, are well suited to monitor tropical cyclones (TCs) by virtue of their ability to assess changes in tropospheric warm core structure in the presence of clouds. The temporal variability in TC upper-tropospheric warm anomaly (UTWA) size, structure, and magnitude provides vital information on changes in kinematic structure and minimum sea level pressure (MSLP) through well-established thermodynamic and dynamic principles. This study outlines the aspects of several factors affecting the effective AMSU measurement accuracy of UTWAs, including the practical application of a previously developed maximum likelihood regression algorithm designed to explicity correct for TC scan geometry and UTWA-antenna gain pattern interaction issues (UTWA subsampling) unique to TC warm core applications. This single-channel AMSU approach (54.96 GHz) is the first step toward a more elaborate multichannel application that is currently under study. Independent application of the single-channel algorithm in the Atlantic and eastern Pacific basins in 2000 and 2001 demonstrates that AMSU-derived UTWAs are moderately correlated with coincident TC MSLP. In addition, further improvements in correlation, and MSLP estimate accuracy, are possible through application of the proposed corrective retrieval algorithm, provided that 1) accurate estimates of TC eye size (a proxy for the UTWA horizontal dimension) are available and 2) the peak upper-tropospheric warming represented by the AMSU-A 54.94-GHz radiances corresponds with the actual TC thermal structure. This study recommends potential remedies for both of these algorithm skill prerequisites that include the incorporation of improved eye size estimates from ancillary data sources and/or the utilization of additional AMSU-A upper-tropospheric sounding channels.
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Tropical cyclones form over the seas: a typical data-sparse region for conventional observations. Therefore, satellites, especially with microwave sensors, are ideal for cyclone studies. The advanced microwave sounding unit (AMSU), in addition to providing very valuable data over non-precipitating cloudy regions, can provide very high horizontal resolution of the temperature and humidity soundings. Such high-resolution microwave data can improve the poorly analysed cyclone. The objective of this study is to investigate the impact of ingesting and assimilating the AMSU data together with conventional upper air and surface meteorological observations over India on the prediction of a tropical cyclone which formed over the Arabian Sea during November 2003 using analysis nudging. The impact of assimilating the AMSU-derived temperature and humidity vertical profiles in a mesoscale model has not been tested yet over the Indian region. Such studies are important as most weather systems over India form over the seas. The present study is unique in the sense that it addresses the impact of ingesting and assimilating microwave sounding data ( together with conventional India Meteorological Department data) on the prediction of a tropical cyclone, which formed over the Arabian Sea during November 2003 using analysis nudging. Two sets of numerical experiments are designed in this study. While the first set utilizes the National Center for Environmental Prediction (NCEP) reanalysis (for the initial and lateral boundary conditions) only in the fifth-generation mesoscale model simulation, the second set utilized the AMSU satellite and conventional meteorological upper air and surface data to provide an improved analysis through analysis nudging. The results of the two sets of model simulations are compared with one another as well as with the NCEP reanalysis and the observations.
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This paper describes results from a near-real-time objective technique for estimating the intensity of tropical cyclones from satellite infrared imagery in the North Atlantic Ocean basin. The technique quantifies the level of organization or axisymmetry of the infrared cloud signature of a tropical cyclone as an indirect measurement of its maximum wind speed. The final maximum wind speed calculated by the technique is an independent estimate of tropical cyclone intensity. Seventy-eight tropical cyclones from the 2004-09 seasons are used both to train and to test independently the intensity estimation technique. Two independent tests are performed to test the ability of the technique to estimate tropical cyclone intensity accurately. The best results from these tests have a root-mean-square intensity error of between 13 and 15 kt (where 1 kt'0.5m s21) for the two test sets.
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This study used the spectral features of the geostationary satellite infrared window channel and the water vapor channel data to calculate a new parameter, normalized difference convection index (NDCI), to help determine the overshooting areas in typhoon cloud systems and the centers and intensity of typhoons. The results showed that the two-dimensional NDCI analysis helped to identify typhoon convective cloud systems and the positions of overshooting areas. In addition, because the NDCI values near a typhoon eye were rather significant, if a typhoon eye was formed, the NDCI cross-section analysis could help to confirm its position. When the center of a typhoon was covered by the high anvils and cirrus layers, it could still be qualitatively found through two-dimensional analysis. As for determining the intensity of typhoons, this study also tried to perform correlation analyses with NDCI and maximum sustained wind speed. The result showed that in the ranges within circles of 200 to 250 km radii with a typhoon eye as the center, the correlation between the area with the NDCI values >0 and the maximum sustained wind speed is high with a coefficient 0.7. Thus, the NDCI value could be a referential index to determine the intensity of a typhoon.
Article
An objective index was proposed to determine the intensity of typhoons in this study. This was achieved using an image edge processing technique to examine meaningful discontinuity characteristics and thereby calculate the gradient of brightness temperature in satellite infrared images. By taking the typhoon centre as a reference point, the angle between the position vector and the gradient vector was defined as the deviation angle. Following this definition, the probability density and standard deviation of the deviation angle may be derived. After creating a scale from 1 to 0 (0–1) to, respectively, represent the maximum and minimum values of the probability density (standard deviation), this research proposed a non-dimensional typhoon intensity (TI) index. Analysis results reveal a high accuracy when the TI index was used to objectively measure TI. The bias, average error, root mean square error, and R 2 value reached 0.6, 3.5, 4.8 m s–1, and 0.89, respectively. Meanwhile, various evaluation parameters in assessing the forecasting skill were also employed, where a specific ‘yes’ and ‘no’ threshold for each typhoon stage was established. The ratio of the number of correct determination to the number of events for a specific typhoon stage was 0.74 (mild), 0.76 (moderate), and 0.89 (severe), respectively, for 557 infrared images of five validation typhoon cases in 2011. The results demonstrated that the TI index technique had good performance in assessing the TI even during typhoon stage changes.
Article
The deviation-angle variance technique (DAV-T), which was introduced in the North Atlantic basin for tropical cyclone (TC) intensity estimation, is adapted for use in the North Pacific Ocean using the "best-track center" application of the DAY. The adaptations include changes in preprocessing for different data sources [Geostationary Operational Environmental Satellite-East (GOES-E) in the Atlantic, stitched GOES-E Geostationary Operational Environmental Satellite-West (GOES-W) in the eastern North Pacific, and the Multifunctional Transport Satellite (MTSAT) in the western North Pacific], and retraining the algorithm parameters for different basins. Over the 2007-11 period, DAV-T intensity estimation in the western North Pacific results in a root-mean-square intensity error (RMSE, as measured by the maximum sustained surface winds) of 14.3 kt (1 kt approximate to 0.51 m s(-1)) when compared to the Joint Typhoon Warning Center best track, utilizing all TCs to train and test the algorithm. The RMSE obtained when testing on an individual year and training with the remaining set lies between 12.9 and 15.1 kt. In the eastern North Pacific the DAV-T produces an RMSE of 13.4 kt utilizing all TCs in 2005-11 when compared with the National Hurricane Center best track. The RMSE for individual years lies between 9.4 and 16.9 kt. The complex environment in the western North Pacific led to an extension to the DAV-T that includes two different radii of computation, producing a parametric surface that relates TC axisymmetry to intensity. The overall RMSE is reduced by an average of 1.3 kt in the western North Pacific and 0.8 kt in the eastern North Pacific. These results for the North Pacific are comparable with previously reported results using the DAV for the North Atlantic basin.
Article
Thesis (Ph. D.)--University of Wisconsin--Madison, 2005. Includes bibliographical references (p. 139-146). Photocopy.
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Top-down height tendency reasoning is explained and examined. This approach uses the assumption of a stratospheric level of insignificant dynamics (LID)—where height and pressure tendencies are considered negligible—to simplify the understanding of cyclone-scale hydrostatic height (pressure) tendency in the troposphere. Quasigeostrophic analytic model results confirm the existence of such a LID for scales less than approximately 5000 km. An examination of a height tendency equation with the LID assumption shows that there must be net integrated local warming (cooling) between the LID and any level below the LID where heights are falling (rising). The local temperature tendency, which from the thermodynamic equation results from advection, diabatic heating, and the product of vertical motion and static stability, reflects the combined actions of all thermodynamic and dynamic processes that together promote hydrostatic height change in isobaric coordinates. In particular, the important dynamic effects of mass-diverging secondary circulations are implicitly contained in the local temperature tendency. New observational evidence and analytic model simulations supporting the top-down approach for understanding height tendency are also provided. The analytic model simulations show that isolated layers of equivalent diabatic heating and temperature advection do not produce equivalent dynamic responses in the vertical-motion field and height tendency fields. This result is used to explain observations that temperature advections in the upper troposphere /lower stratosphere are associated with larger lower-tropospheric height tendencies than equivalent temperature advections in the lower troposphere.
Article
Microwave radiometric measurements in the 60-GHz oxygen band are considered, to infer atmospheric wind fields associated with tropical storms. On the basis of the thermal wind equation the radial derivative of brightness temperature is related to the vertically weighted tangential wind through a wind weighting function. The theory is tested in an analysis of Nimbus 6 scanning microwave spectrometer (Scams) 55.45-GHz data through typhoon June in November 1975. Scams-derived winds are obtained for 3 days throughout the storm area and are compared with 700-mbar aircraft reconnaissance winds. Major differences with the reconnaissance winds occur primarily near the storm center presumably owing to Scams' insufficient horizontal resolution. Also discussed are the errors due to upper level wind contributions and instrumental noise.
Article
Passive microwave observations from the current NOAA series of polar-orbiting satellites of a large sample of North Atlantic tropical cyclones are qualitatively and quantitatively analyzed. Microwave observations can penetrate the cloud cover associated with tropical cyclones and capture the upper-level warm temperature anomaly, which is characteristic of these storms. The data are used to develop a statistical algorithm for estimating surface intensity. Based upon hydrostatic assumptions, linear regression relationships are developed between the satellite-depicted horizontal temperature gradient of the upper-level warm core (T250), and the surface intensity (PSFC) as measured by reconnaissance reports. A good correlation is found to exist. Results indicate that standard errors of estimate of 8 mb and 13 kts are found for surface pressure and maximum winds, respectively. These errors are reduced when the effects of storm latitude, eye size, and surface-pressure tendency on the relationship are included. Knowledge gained in examining the accuracies and limitations of the current microwave sounders in tropical cyclone applications will be helpful in setting quantitative observational guidelines for future instruments.
Article
A method for estimating the horizontal structure of the upper-tropospheric warm anomaly of tropical cyclones from 55-GHz microwave observations is presented. Because the peak warming occurs over an area smaller than that viewed by current and planned satellite antenna systems, it is necessary to model explicitly the interaction of the warm anomaly structure and antenna gain pattern. The anomaly is approximated by an analytic function whose parameters are estimated using a maximum-likelihood algorithm with constraints analogous to that used for thermodynamic sounding retrieval or optimal interpolation. Simulation studies demonstrate the overall soundness of the technique and its possible performance and limitations when applied to two different polar-orbiting microwave sensors, MSU (Microwave Sounding Unit) and SSM/T (Special Sensor Microwave/Temperature).
Article
Upper tropospheric temperature anomalies are detected in brightness temperature data from the Nimbus 6 Scanning Microwave Spectrometer (SCAMS). Brightness temperature anomalies are related to surface pressure anomalies through the radiative transfer and hydrostatic equation. Surface wind speeds at outer radii are then estimated using the gradient wind equation and a shearing parameter. The method is first tested using simulated satellite data constructed from temperature, pressure and height data recorded by aircraft reconnaissance of four hurricanes. Wind speeds in the 80-95 kPa region are estimated with 2-3 m/sec accuracy. Next, 55.45 GHz SCAMS data over eight typhoons during 1975 are used to estimate the radii of 15.4 m/sec (30 kt) and 27.5 m/sec (50 kt) winds. Accuracies of about + or - 80 and + or - 70 km, respectively, are found. It is suggested that the technique be further tested using data from the Microwave Sounding Unit on board the TIROS-N and NOAA 6 satellites.
Article
Brightness temperatures obtained through examination of microwave data from the Nimbus 7 satellite are noted to be much lower than those expected on the strength of radiation emanating from rain-producing clouds. Very cold brightness temperature cases all coincided with heavy thunderstorm rainfall, with the cold temperatures being attributable to scattering by a layer of ice hydrometeors in the upper parts of the storms. It is accordingly suggested that brightness temperatures observed by satellite microwave radiometers can sometimes distinguish heavy rain over land.