Satellite-Observed Location of Stratocumulus Cloud-Top Heights in the Presence of Strong Inversions
ABSTRACT Infrared channels on the Moderate Resolution Imaging Spectroradiometer (MODIS) are used to infer cloud-top pressure (CTP), temperature, and effective cloud amount or emissivity. For low clouds, those with tops at pressures greater than 700 hPa, the infrared window 11-mum channel brightness temperature is used to determine the CTP and the corresponding cloud-top temperature by comparison with the temperature profile obtained from the NCEP Global Data Assimilation System meteorological analysis. In the presence of strong inversions which are common for marine stratus and stratocumulus, this leads to the identification of an erroneously high cloud-top height (CTH). This discrepancy is illustrated by comparing MODIS CTHs with those inferred from the geometric method used by the Multiangle Imaging SpectroRadiometer on the same satellite platform, and field observations. The error in CTH is typically about 2 km and depends on the shape of the actual temperature profile. It is shown that column water vapor above cloud retrieved from the MODIS solar infrared channels in the vicinity of the 0.94-mum water vapor absorption band can be used to flag the error and that the location of the true CTH could possibly be obtained using lapse rate formulations for cloud-topped boundary layers.
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ABSTRACT: We report the improved detection of thin cirrus clouds over the Tropics using oblique camera stereo retrieval of cloud top heights from the Multiangle Imaging SpectroRadiometer (MISR) instrument on the Terra satellite. The MISR oblique stereo captures 10% of thin cirrus with mean height of 13 km over all scenes that the standard stereo misses completely, especially when they are over lower-level clouds that provide more contrast. To determine thin cirrus properties missed by MISR, differences between modeled and measured outgoing longwave radiation (OLR) were used to compute its fractional cover and optical depth. The oblique MISR measurements were used as inputs to the model and a merged data set from CERES, MODIS and MISR instruments on the Terra satellite provided the measured fluxes and the cloud properties. For the cases investigated including all clear and cloudy scenes in the Tropics, the difference between modeled and measured OLR (Cirrus Forcing) averaged ≈ 19 W m-2. This can be accounted for by the addition of thin cirrus of coverage 77%. However, oblique analysis only detects 10% of thin cirrus (0.1 < τ < 0.3) and misses 67% of cirrus with τ < 0.3. The missed cirrus coverage includes 32% of homogeneous cirrus (0.1 < τ < 0.3) and 35% of subvisual cirrus (τ < 0.1). To improve the detection of homogeneous cirrus with MISR, the current contrast threshold should be decreased. This will increase the number of pixels to be matched stereoscopically that were screened as noise initially.Journal of Geophysical Research 03/2012; 117(D6):6208-. · 3.17 Impact Factor
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ABSTRACT: Cloud microphysical observations collected in situ during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment within the Chile-Peru stratocumulus cloud deck during October-November 2008 were used to assess MODIS Level 2 cloud property retrievals. The in situ aircraft-derived cloud property values were constructed from the drop size distributions measured by the Cloud Droplet Probe (drop diameter <52 micron) and Two-Dimensional Cloud Probe (drop diameters up to 1600 micron) during 20 vertical profiles. Almost all of the MODIS cloud scenes were highly homogeneous. MODIS cloud optical thickness correlated well with the aircraft-derived value with a slight offset within instrumental/retrieval uncertainties. In contrast, the standard 2.1 micron-derived MODIS effective radius (re) systematically exceeded the in situ cloud top reby 15%-20%, for an absolute error that increased with droplet size. The individual effective radius retrievals at 1.6, 2.1, and 3.7 micron did not provide additional information on cloud vertical structure for our data sample. The secondarily derived MODIS liquid water path also exceeded the in situ value. A MODIS-derived cloud droplet number concentration (Nd) estimate agreed the best of the four MODIS variables with the aircraft observations. The analysis also highlighted a lack of agreement in published satellite-derivedNd values, despite drawing on the same sources. A best a priori formula choice for Nd is likely to vary regionally. Four sources of errors within the MODIS reretrieval were investigated further: the cloud mode droplet size distribution breadth, the presence of a drizzle mode, above-cloud water vapor absorption, and sensor viewing angles. These processes combined conspired to explain most of the observed bias. The above-cloud water vapor paths were poorly specified, primarily because the cloud top heights are placed too high, and secondarily because the water vapor paths are unrealistic. Improvement of the above-cloud water vapor path specification can most easily and systematically improve the MODIS effective radius and liquid water path retrievals.Journal of Geophysical Research 12/2011; 116(D24):24206-. · 3.17 Impact Factor
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ABSTRACT: 1] We compare cirrus presence and heights (CTHs) using oblique stereo by the Multiangle Imaging SpectroRadiometer (MISR) with measurements from ground-based cloud radar and lidar sensors at the Tropical Western Pacific (TWP) sites operated by the U.S. Department of Energy Atmospheric Radiation Measurement Program. Precise point-wise comparisons, limited to only 195 coincident cases, showed that the total number of cirrus retrieved using oblique-stereo analysis improved to 70% from 39% using the standard-stereo technique. The stereo technique detects cloud with the highest contrast, which is often at lower altitude. The oblique-stereo technique's efficiency depends on the thickness and number of underlying cloud layers. A histogram approach allowed similar regions to be compared statistically with many more samples and showed three distinct peaks at %13 km, 15 km, and 19 km related to deep convective clouds, tropical tropopause layer (TTL) cirrus, and overshooting convective clouds, respectively. Most differences between the satellite and ground-based measurements resulted from a number of cases of invalid cloud comparisons (14%), blunders from edges and broken clouds (7%), low contrast stereo mismatches (4%), and under-estimation of CTHs (3%). Overall, the oblique-stereo analysis detected a cirrus-top layer in 65% of all the valid coincident cases, mostly <1 km in thickness. The oblique-stereo derived cirrus CTHs differed from the heights of cirrus-top layers from ground-based cloud radar and lidar by À0.5 AE 1.0 km, validating the MISR retrievals. This suggests global thin cirrus retrievals are possible with the oblique-stereo technique after the screening of occasional blunders., An assessment of cirrus heights from MISR oblique stereo using ground-based radar and lidar at the Tropical Western Pacific ARM sites, J. Geophys. Res. Atmos., 118, 5588–5599, doi:10.1002/jgrd.50454.Journal of Geophysical Research 06/2013; · 3.17 Impact Factor