An evaluation of various forms of VAS retrievals in the analysis of a preconvective environment

Abstract

Quantitative retrievals derived from VISSR Atmospheric Sounder (VAS) radiances are combined with conventional surface and radiosonde data to evaluate the impact of the higher time and space resolution geosynchronous satellite soundings on the diagnosis of a preconvective environment over the central United States on 20 July 1981. Retrievals of temperature, dewpoint temperature, equivalent potential temperature, total column precipitable water, and lifted index, all derived at 60 km resolution over approximately three-hourly intervals, are shown to be physically consistent in space and time and to compare well with available radiosonde data. When VAS fields are used to augment qualitative VAS imagery and analyses from conventional data sources mesoscale regions with convective instability are more clearly delineated prior to the development of convection. The analysis of the VAS retrievals identifies significant spatial gradients and temporal changes in the thermal and moisture fields, especially at times and locations between radiosonde observations. Direct retrievals of vertically integrated “bulk” precipitable water and lifted index are particularly useful in this case since they correspond well to features in the VAS imagery and provide a strong preconvective signature. The nature of the VAS instrument dictates that these bulk parameters should display more consistency in space and time than the level-specific parameters due to the poor vertical resolution of the VAS instrument. The detailed analyses also point to limitations in using VAS data. Even with nearly optimal conditions for passive remote sounding (generally clear skies, minimal orographic effects, and a rapidly changing moisture field), the VAS retrievals were still degraded in some regions by VAS instrument noise and calibration errors and unresolved cloud contamination. Another problem is the cloud-free nature of the instrument data set, which biases the results toward a drier environment. In spite of these and other limitations, the analyses demonstrate that the geosynchronous VAS can be used in a case study mode to produce high-resolution spatial and temporal measurements which are useful for the quantitative analysis of a cloud-free preconvective environment.