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Dennis Chesters
,
Anthony Mostek
, and
Dennis A. Keyser

Abstract

Local forecasts often rely upon the extrapolation of trends seen in images of clouds from the GOES satellite. This work presents correspondingly high resolution images of atmospheric soundings calculated from the VAS radiometer on GOES. These VAS sounding images vividly depict moisture and stability conditions in preconvective regions, as though GOES were observing the United States with “stability detectors” instead of infrared detectors at 1–3 h intervals and 60 km horizontal resolution. False color images are presented for VAS-derived precipitable water and lifted index fields during two midsummer days that contain a wide variety of preconvective and convective conditions. Since each sounding image requires only 5 min to calculate with an automated regression algorithm on a minicomputer, it should be possible to process VAS data operationally for real-time objective analysis of potential convective instabilities.

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Eric P. Salathé Jr.
and
Dennis Chesters

Abstract

Large-scale variability of moisture in the upper troposphere is examined using TIROS (Television Infrared Observation Satellite) Operational Vertical Sounder (TOYS) satellite observations and ECMWF model analyses from 1989 in the latitude band from 40° to 40°S. To compare these dissimilar datasets, upwelling radiances were computed for the 6-7-μm water vapor band from the ECMWF temperature and moisture analyses, and these computed radiances were compared to the corresponding TOVS satellite observations. The ECMWF-based radiances reproduce the general locations and seasonal cycle of the TOVS-observed moisture features, particularly after an improved convective parameterization scheme was adopted by ECMWF in May 1989. However, the ECMWF analysis scheme still results in much milder lateral moisture gradients and seasonal contrasts than indicated by the TOYS observations. Seasonally, the upper troposphere in each hemisphere dries in winter and moistens in summer, but there are regions in each hemisphere that run counter to this seasonal trend, apparently depending on continental- and monsoon-scale dynamics. Dynamically, the TOVS-observed regions of significant subtropical dryness are correlated with persistent subsidence indicated by ECMWF 300-mb vertical velocity analyses. The TOVS radiance observations indicate large variations in space and time of the upper-tropospheric moisture field, which are not fully captured by the ECMWF analyses.

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Dennis Chesters
,
Louis W. Uccellini
, and
Anthony Mostek

Abstract

The GOES satellites launched in the 1980's are carrying an instrument called the VISSR Atmospheric Sounder (VAS), which is designed to provide temperature and moisture profile-sounding capability for mesoscale weather systems. As a controlled study of this capability, VAS radiance fields are simulated for pre-thunderstorm environments in Oklahoma to demonstrate three points: 1) significant moisture gradients can be seen directly in images of the VAS channels, 2) temperature and moisture profiles can be retrieved from VAS radiances with sufficient accuracy to delineate the major features of a severe storm environment, and 3) the quality of VAS mesoscale soundings improve with conditioning by local weather statistics.

Even though the simulated VAS soundings have the usual limitations in absolute accuracy, gradient strength and vertical resolution (especially in the lower tropospheric moisture retrievals), it is still possible to retrieve mesoscale regions of potential instability from the synthetic VAS radiances for a mostly clear pre-thunderstorm environment. The rms tropospheric profile errors are ±1°C and ±25% in temperature and mixing ratio, respectively. The results represent the optimum retrievability of mesoscale information from VAS radiances without the use of ancillary data. The simulations suggest that VAS data will yield the best soundings when a human being classifies the scene, picks relatively clear areas for retrieval, and applies a “local” statistical database to resolve the ambiguities of satellite observations in favor of the most probable atmospheric structure.

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Tay-How Lee
,
Dennis Chesters
, and
Anthony Mostek

Abstract

Surface temperature and dewpoint reports are added to the infrared radiances from the VISSR Atmospheric Sounder (VAS) in order to improve the retrieval of temperature and moisture profiles in the lower troposphere. The conventional (airways) surface data are combined with the 12 VAS channels as additional predictors in a ridge regression retrieval scheme, with the aim of using all available data to make high resolution space-time interpolations of the radiosonde network. For one day of VAS observations, retrievals using only VAS radiances are compared with retrievals using VAS radiances plus surface data. Temperature retrieval accuracy evaluated at coincident radiosonde sites shows a significant impact within the boundary layer. Dewpoint retrieval accuracy shows a broader improvement within the lowest tropospheric layers. The most dramatic impact of surface data is observer in the spatial and temporal continuity of low-level fields retrieved over the Midwestern United States.

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Anthony Mostek
,
Louis W. Uccellini
,
Ralph A. Petersen
, and
Dennis Chesters

Abstract

Retrievals from the VISSR Atmospheric Sounder (VAS) an combined with conventional data to assess the impact of geosynchronous satellite soundings upon the analysis of a pre-convective environment over the central United States on 13 July 1981. VAS retrievals of temperature, dewpoint, equivalent potential temperature, precipitable water, and lifted index are derived with 30 km resolution at 3 hour intervals. When VAS fields are combined with analyses from conventional data sources regions with convective instability are more clearly delineated prior to the rapid development of the thunderstorms. The retrievals differentiate isolated areas in which most air extends throughout the lower troposphere (and are therefore more conducive for the development of deep convective storms) from those regions where moisture is confined to a thin layer near the earth's surface (where convection is less likely to occur). The analyses of the VAS retrievals identify significant spatial gradients and temporal changes in the thermal and moisture fields, especially in the regions between radiosonde observations. The detailed analyses also point to limitations in using VAS data. Even with nearly optimal conditions for passive remote sounding (generally clew skies, minimal orographic effects, and a rapidly changing moisture field), the VAS retrievals were still degraded in some regions by small clouds which are unresolved in the infrared imagery. These analyses, however, demonstrate that the geosynchronous VAS can be used in a case study mode to produce high-resolution spatial and temporal measurements that are useful for the quantitative analysis of a cloud-free pre-convective environment.

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Dennis Chesters
,
Wayne D. Robinson
, and
Louis W. Uccellini

Abstract

Precipitable water fields have been retrieved from the VISSR Atmospheric Sounder (VAS) using a radiation transfer model for the differential water vapor absorption between the 11 and 12 μm “split window” channels. Previous moisture retrievals using only the split window channels provided very good space-time continuity but poor absolute accuracy. This note describes how retrieval errors can be significantly reduced from ±0.9 to ±0.6 gm cm−2 by empirically optimizing the effective air temperature and absorption coefficients used in the two-channel model. The differential absorption between the VAS 11 and 12 μm channels, empirically estimated from 135 colocated VAS-RAOB observations, is found to be approximately 50% smaller than the theoretical estimates. Similar discrepancies have been noted previously between theoretical and empirical absorption coefficients applied to the retrieval of sea surface temperatures using radiances observed by VAS and polar-orbiting satellites. These discrepancies indicate that radiation transfer models for the 11 μm window appear to be less accurate than the satellite observations.

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Dennis Chesters
,
Louis W. Uccellini
, and
Wayne D. Robinson

Abstract

A simple physical algorithm is developed which calculates the water vapor content of the lower troposphere from the 11 and 12 μm (split window) channels on the VISSR Atmospheric Sounder (VAS) on the Geostationary Operational Environmental Satellites (GOES). The algorithm is applied to a time series of VAS split window radiances observed at 15 km horizontal resolution over eastern North America during a twelve hour period on 13 July 1981. Color coded images of the derived precipitable water (g cm−2) fields show vivid water vapor features whose broad structure and evolution are verified by the radiosonde and surface networks. The satellite moisture fields also reveal significant mesoscale features and rapid developments which are not resolved by the conventional networks. The VAS split window clearly differentiates those areas in which water vapor extends over a deep layer and is more able to support convective cells from those arms in which water vapor is confined to a shallow layer and is therefore less able to support convection. The spatial and temporal continuity of the water vapor features indicates very good relative accuracy, and point verification at radiosonde sites indicates fair absolute accuracy. Surface temperature variations are very effectively removed by the algorithm. Consequently, the VAS split window could be used operationally to monitor mesoscale developments in the low-level moisture fields over relatively cloud-free areas of the United States.

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Eric P. Salathé Jr.
,
Dennis Chesters
, and
Y. C. Sud

Abstract

TOVS satellite observations are used to evaluate the upper-tropospheric (400–200 mb) moisture distribution simulated by the GLA GCM in a 10-yr (1979–1988) integration produced for the Atmospheric Model Inter-comparison Project, in which several models participated worldwide. The simulated moisture fields show remarkable success in duplicating the large-scale structure and seasonal features in the observations, but they show insufficient contrast between very dry and very moist regions. The simulation generally does well in northern summer (June–July–August) but worse in northern winter. This is consistent with deficiencies in the annual cycle of moist convection so that convective rain stays too close to the equator in northern winter. The related misplacement of convective activity associated with the Asian monsoon produces discrepancies in the moisture over much of the Eastern Hemisphere. The simulation also shows a too weak moisture response to interannual fluctuations in the sea surface temperature, even for the large El Niño episode of 1983. These results indicate that deficiencies in modeling oceanic convection may be in part responsible for errors in the simulated upper-tropospheric moisture patterns.

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