Article

Role of the cloud adjustment time scale in simulation of the interannual variability of Indian summer monsoon

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Abstract

The simulation of precipitation in a general circulation model relying on relaxed mass flux cumulus parameterization scheme is sensitive to cloud adjustment time scale (CATS). In this study, the frequency of the dominant intra-seasonal mode and interannual variability of Indian summer monsoon rainfall (ISMR) simulated by an atmospheric general circulation model is shown to be sensitive to the CATS. It has been shown that a longer CATS of about 5 h simulates the spatial distribution of the ISMR better. El Nin˜o Southern Oscillation–ISMR relationship is also sensitive to CATS. The equatorial Indian Ocean rainfall and ISMR coupling is sensitive to CATS. Our study suggests that a careful choice of CATS is necessary for adequate simulation of spatial pattern as well as interannual variation of Indian summer monsoon precipitation.

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Chapter
Relationship between convective available potential energy (CAPE) and precipitation is explored in a season-long cloud-resolving model (CRM) simulation of Indian summer monsoon. The location of maximum precipitation and CAPE does not always coincide in a CRM simulation. The diurnal land surface heating is shown to have an effect on CAPE and precipitation over ocean. Convective inhibition energy is shown to have a significant effect on the location of precipitation. It is shown that mass flux parameterizations which depend on CAPE consumption do not get the location or magnitude of precipitation right at CRM resolution. It is emphasized that once the model resolution starts approaching cloud scale, the basic assumption of convective quasi-equilibrium is not sufficient and representation of organized mesoscale convective systems becomes imperative. Present-day cumulus parameterizations do not include any representation of organized mesoscale convective systems. We show that CAPE consumed by these systems not only triggers vertical motion but also contributes to horizontal motion of the system.
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Ph.D Thesis, Indian Institute of Science, Bangalore, India; Submitted November 2001
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A short record of year-to-year variations of summer monsoon rainfall over India is compared with that of winter snow cover over Eurasia as derived from satellite data. An inverse relationship between these two quantities is indicated, i.e., winters with extensive (little) snow cover over Eurasia tend to be followed by summers with less (more) rainfall over India.
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Changes in temporal and spatial relationships between rainfall of Sri Lanka and the Southern Oscillation Index (SOI) during the 110 year period (1881–1990) are presented. The relationships during the first intermonsoon (FIM) and south-west monsoon (SWM) seasons indicate periods of weak positive and negative correlations.On the basis of major turning points in the summer rainfall over India and Sri Lanka, four distinct epochs were identified which cover the years 1881–1900, 1901–1930, 1931–1960 and 1961–1990. Indian and Sri Lankan summer rainfall shows a weak in-phase relationship from 1881 to 1900, a strong in-phase relationship from 1961 to 1990, and an out-of-phase relationship from 1901 to 1960. The correlation between the SOI and SWM season's rainfall is primarily negative from 1931 to 1960 and positive during other epochs. Strong negative correlations are generally present for the SIM season and strengthening and weakening of negative correlations during this season coincide with changes in the SWM season rainfall and its relationships with the SOI. The north-east monsoon (NEM) season correlations are negative and insignificant.Major changes in spatial patterns of correlations between seasonal rainfall and the SOI have occurred in Sri Lanka during SWM and SIM seasons. The periods of strong positive (negative) correlations during the SWM season coincide with weak (strong) negative correlations during the SIM season. This contrasting pattern is clear when the Indian and Sri Lankan summer monsoon rainfalls were out of phase between 1900 and 1960, but not before 1900, or after 1960. The sudden change in correlations around 1960 suggests a change in the coupled ocean–atmosphere system that dominates the climate of these regions. Changes in temporal and spatial patterns of correlations between the SOI and rainfall have been linked to changes in the location of the active intertropical convergence zone.
Article
This paper describes the development of a fractional cloud cover scheme which was implemented operationally in the ECMWF medium range forecast model in May 1985. the scheme is based on a diagnostic approach in which cloudiness is related empirically to the large-scale model variables, including convective activity. an example of the performance of the scheme is given, showing that a fair degree of skill is achieved in forecasting tropical and extratropical cloudiness. an attempt to verify the results using retrieved cloudiness from Nimbus 7 is described. Considerable difficulties were experienced due to the different height classifications of clouds in the model and in the satellite data. the importance of cloud radiative properties as well as cloud amount is considered briefly, based on a comparison of model earth radiation budget diagnostics with similar data from the NOAA polar-orbiting satellites. the sensitivity of the simulated outgoing long-wave radiance to changes in the prescribed cloud liquid water content is discussed. Two examples of the effects of cloudradiation interaction on boundary layer processes are described which demonstrate the importance of an integrated approach to the treatment of clouds, radiation and turbulent fluxes.
Article
In this study we used the National Center for Environmental Prediction (NCEP) T170L42 Global Forecast System model for simulating the monsoon of 2006 using two different cumulus parameterization schemes viz. the Simple Arakawa–Schubert (SAS) scheme and the Relaxed Arakawa–Schubert (RAS) scheme. The NCEP Coupled Forecast System (CFS) monthly forecast sea surface temperatures (SST) were used for simulating the monsoon season. It is seen that both the schemes forecast the mean seasonal rainfall close to the observed. However, there were differences in the spatial and temporal distribution of the rainfall. The spatial distribution of the precipitation simulated by the RAS scheme was comparable to the observed. The SAS scheme simulated realistic distribution of the rainfall in the months of June and July. In the months of August and September, the SAS scheme could not simulate the convective regions over the south and central Bay of Bengal, Arabian Sea and the Indian landmass. The SAS scheme simulated withdrawal-like symptoms of the monsoon in the month of August itself. However, the same SAS scheme simulated more realistic rainfall distribution over the Bay of Bengal when the model was run with climatological SST and also when the model was run at a resolution of T62 and with CFS-forecast SST. The onset of the monsoon over the Kerala coast of India simulated with both the schemes was within one pentad of the observed date of onset. Copyright © 2008 Royal Meteorological Society
Article
Despite advances in the capacity to predict the evolution of the El Niño–southern oscillation (ENSO) phenomenon and advances in understanding the influence of ENSO on rainfall in tropical regions such as Sri Lanka, there has been limited use of climate predictions for agricultural decision-making. Climatic fluctuations have a profound influence on the cultivation of crops such as rice, which is the staple food in Sri Lanka. Here, the relationship between the sea-surface temperature-based ENSO index of NINO3.4, rainfall and the departure of Sri Lankan rice production from long-term trends, is analysed for the ‘Maha’ (October to March) and ‘Yala’ (April to September) cultivation seasons between 1952 and 1997. During the El Niño phase, the Maha rice production frequently increased (10 out of 15 seasons) and the Yala production frequently decreased (10 out of 14 seasons). Conversely, during the La Niña phase, the Maha production decreased (seven out of ten seasons) and Yala production increased (six out of eight seasons). Floods, state interventions, civil disturbances, fertilizer price hikes and extreme anomalies in the previous season were noted in the majority of seasons in which these ENSO–production linkages were violated. The correlation of the Maha rice production anomaly with the average NINO3.4 from October to December was significant at the 5% level and that with the aggregate October to December rainfall was significant at the 1% level. Yala rice production showed a significant relationship with concurrent NINO3.4 and a strong correlation (r = 0.60) with the previous season's rainfall. Yala cultivation is water constrained, and carryover storage from the previous season is often used to determine the extent of planting. The relationships between ENSO and seasonal rice production and the relationship between Yala rice production and previous Maha rainfall could be used for agricultural management and policy formulation. Copyright
Article
We examine the relationship between the state of the equatorial Indian Ocean, ENSO, and the Indian summer monsoon rainfall using data from 1881 to 1998. The zonal wind anomalies and SST anomaly gradient over the equatorial Indian Ocean are used as indices that represent the condition of the Indian Ocean. Although the index defined by the zonal wind anomalies correlates poorly with Indian summer monsoon rainfall, the linear reconstruction of Indian summer monsoon rainfall on the basis of a multiple regression from the NINO3 and this wind index better specifies the Indian summer monsoon rainfall than the regression with only NINO3. Using contingency tables, we find that the negative association between the categories of Indian summer monsoon rainfall and the wind index is significant during warm years (El Niño) but not during cold years (La Niña). Composite maps of land precipitation also indicate that this relationship is significant during El Niño events. We conclude that there is a significant negative association between Indian summer monsoon rainfall and the zonal wind anomalies over the equatorial Indian Ocean during El Niño events. A similar investigation of the relationship between the SST index and Indian summer monsoon rainfall does not reveal a significant association. Copyright © 2006 Royal Meteorological Society.
Article
Aim of this diagnostic study is to investigate the impact of intra-seasonal oscillations in terms of number, duration and intensity on rainfall during June through September, 1979–2006. Analysis of wavelet spectra for winds at 850 hPa field for monsoon period reveals number and duration of oscillations, which exercise profound influence on monsoon rainfall. Results indicate that four to six oscillations appear in normal rainfall or flood cases, while two or three oscillations are identified in the years of drought episodes. Though total duration of above oscillations is varied from 25 to 85 days, the duration is short (20 to 35 days) obviously in the years of less number of oscillations and also the number of oscillations are directly related to the monsoon rainfall. The coefficient of correlation between them is 0.56, which is significant at 1% level. To examine the strength of intra-seasonal oscillations in terms of different indices on seasonal rainfall is investigated. The Madden and Julian Oscillation Index shows an inverse relationship with rainfall, where as a direct relationship is noticed between Monsoon Shear Index and rainfall for the study period. Both results are significant at 5% level. To consolidate the above statistical relationships, seasonal circulation changes in the contrasting years of monsoon rainfall have been examined; present study reveals that anomaly negative outgoing longwave radiation is noticed over most of Arabian Sea, Indian sub-continent and the Bay of Bengal during June through September in flood year (1988). But opposite convective activity is true in drought year (2002). Similarly the spatial U-850 hPa field distribution showed much stronger monsoon winds in 1988, while zonal circulation was very weak in 2002. Such differences are observed in the anomaly zonal wind field at 200 hPa also. Over the monsoon region U-850 hPa field is almost a mirror image of U-200 hPa distribution of wind field. Finally annual cycles of U-850 and U-200 hPa fields reflect striking difference at 200 hPa level during the summer monsoon period in flood and drought years. Key wordsIntra-seasonal oscillation-monsoon rainfall-flood-drought-wind field
Article
The global spectral model of NCMRWF at T80 horizontal resolution and 18 vertical levels has been integrated for the summer season (July) using different cumulus parameterization schemes namely, the Simplified Arakawa-Schubert scheme (SAS), the Relaxed Arakawa-Schubert Scheme (RAS), and the Kuo-type cumulus parameterization scheme (KUO). The results have been compared with mean analysis of the operational NCMRWF model (ANA) and other available observations. Results indicate that, while the global distributions of basic fields such as the wind, temperature and moisture are fairly well simulated by all the three schemes, there are many differences seen in the simulation of the typical features of the Indian summer monsoon. The strength of the Low Level Westerly Jet (LLWJ), the Cross Equatorial Flow (CEF), and the Tropical Easterly Jet (TEJ) are better simulated by RAS and SAS as compared to ANA than the KUO scheme. RAS and SAS produce strong rising motion owing to strong intensity of convection produced by these two schemes. This in turn produces stronger Hadley cell by RAS and SAS than compared to the KUO scheme. Simulation of the 200 mb velocity potential and divergent wind by RAS and SAS produced two prominent centers, one in the Bay of Bengal and another in the Western Pacific, which correspond to the intense latent heating by cumulus convection during the active monsoon phase. The velocity potential and divergent winds were weaker in KUO, than compared to RAS and SAS. The simulation of OLR is improved by RAS as compared to observations. The cold bias produced by KUO at 200 mb is reduced by RAS and is substantially improved by SAS. Study of observed and simulated rainfall indicated that RAS and SAS produced better distribution of precipitation over the Western Ghat Mountains and the Arakan coast, where deep cumulus convection is produced due to orographic forcing of the warm moist air. The KUO scheme underestimated the rainfall over these two regions, but produced slightly better distribution of rainfall over the northwest and central India, where the intensity of convection is relatively weaker. Evaluation of overall dynamics, thermal structure and rainfall indicates that in general, SAS is able to provide relatively better results compared to other two schemes.
Article
The new NOAA operational global sea surface temperature (SST) analysis is described. The analyses use 7 days of in situ (ship and buoy) and satellite SST. These analyses are produced weekly and daily using optimum interpolation (OI) on a 1° grid. The OI technique requires the specification of data and analysis error statistics. These statistics are derived and show that the SST rms data errors from ships are almost twice as large as the data errors from buoys or satellites. In addition, the average e-folding spatial error scales have been found to be 850 km in the zonal direction and 615 km in the meridional direction. The analysis also includes a preliminary step that corrects any satellite biases relative to the in situ data using Poisson's equation. The importance of this correction is demonstrated using recent data following the 1991 eruptions of Mt. Pinatubo. The OI analysis has been computed using the in situ and bias-corrected satellite data for the period 1985 to present.
Article
A two-layer model of soil hydrology and thermodynamics is combined with a one-dimensional model of the planetary boundary layer to study various interactions between evolution of the boundary layer and soil moisture transport. Boundary-layer moistening through surface evaporation reduces the potential and actual surface evaporation as well as the boundary-layer growth. With more advanced stages of soil drying, the restricted surface evaporation allows greater sensible heat flux which enhances boundary-layer growth and entrainment drying. Special individual cases are studied where the wind speed is strong, solar radiation is reduced, transpiration is important, the soil is thin, or the soil is covered with organic debris.
Article
Thesis (Ph. D.)--University of Maryland, College Park, 1999. Thesis research directed by Dept. of Meteorology. Includes bibliographical references (leaves 163-172).
Article
Thesis--University of California, Los Angeles, 1973. Vita. Includes bibliographical references (leaves 152-157).
Article
A solar radiation routine is developed for use in climate studies that includes absorption and scattering due to ozone, water vapor, oxygen, carbon dioxide, clouds, and aerosols. Rayleigh scattering is also included. Broadband parameterization is used to compute the absorption by water vapor in a clear atmosphere, and the k-distribution method is applied to compute fluxes in a scattering atmosphere. The reflectivity and transmissivity of a scattering layer are computed analytically using the delta-four-stream discrete-ordinate approximation. The two-stream adding method is then applied to compute fluxes for a composite of clear and scattering layers. Compared to the results of high spectral resolution and detailed multiple-scattering calculations, fluxes and heating rate are accurately computed to within a few percent. The high accuracy of the flux and heating-rate calculations is achieved with a reasonable amount of computing time. With the UV and visible region grouped into four bands, this solar radiation routine is useful not only for climate studies but also for studies on photolysis in the upper atmosphere and photosynthesis in the biosphere.
Article
A theory of the interaction of a cumulus cloud ensemble with the large-scale environment is developed. In this theory, the large-scale environment is divided into the subcloud mixed layer and the region above. The time changes of the environment are governed by the heat and moisture budget equations for the subcloud mixed layer and for the region above, and by a prognostic equation for the depth of the mixed layer. In the environment above the mixed layer, the cumulus convection affects the temperature and moisture fields through cumulus-induced subsidence and detrainment of saturated air containing liquid water which evaporates in the environment. In the subcloud mixed layer, the cumulus convection does not act directly on the temperature and moisture fields, but it affects the depth of the mixed layer through cumulus-induced subsidence.
Article
Calculations with a numerical model of the atmosphere show that the global fields of rainfall, temperature, and motion strongly depend on the land- surface evapotranspiration. This confirms the long-held idea that the surface vegetation, which produces the evapotransporation, is an important factor in the earth's climate.
Article
A simple implementation of the Arakawa and Schubert (1974) cumulus parameterization is presented. The major simplification made is to 'relax' the state toward equilibrium each time the parameterization is invoked, rather than requiring that the final state be balanced, as in the original Arakawa-Schubert implementation. This relaxed Arakawa-Schubert (RAS) scheme is evaluated in off-line tests using the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) Phase III data. The results show that RAS is equivalent to the standard implementation of Arakawa-Schubert but is more economical and simpler to code. RAS also avoids the ill-posed problem that occurs in Arakawa-Schubert as a result of having to solve for a balanced state.
Article
A detailed description of a parameterization for thermal infrared radiative transfer designed specifically for use in global climate models is presented. The parameterization includes the effects of the main absorbers of terrestrial radiation: water vapor, carbon dioxide, and ozone. While being computationally efficient, the schemes compute very accurately the clear-sky fluxes and cooling rates from the earth's surface to 0.01 mb. Errors are ! 0.4 K day Gamma1 in the cooling rate and ! 1% in the fluxes. Since no transmittances are pre-computed, the atmospheric layers and the vertical distribution of the absorbers may be freely specified. The scheme can also account for any vertical distribution of fractional cloudiness with arbitrary optical thickness. In addition, the numerics and the FORTRAN implementation have been carefully designed to conserve both memory and computer time. This code should thus be particularly attractive to those contemplating long-term climate simulations, wi...
Interaction between soil hydrology and boundary-layer development Sensitivity of Indian monsoon rainfall to different convective parameters in the Relaxed Arakawa– Schubert scheme
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