Retrieval and Use of High-Resolution Moisture and Stability Fields from Nimbus 6 HIRS Radiances in Pre-Convective Situations

Abstract

This is a study of environmental conditions prior to convective development on the Great Plains of the United States on four case study days in August 1975. The tool used was the High-resolution Infrared Radiation Sounder (HIRS) on Nimbus 6. A moisture-temperature retrieval scheme was developed to retrieve various lower tropospheric analysis and forecasting parameters from the HIRS radiances. Specifically, dew points and temperatures and other secondary parameters such as total precipitable water and static stability indices were derived and analyzed at a horizontal resolution of up to 30 km on these days. For the moisture parameters the comparisons to time-interpolated NWS rawinsonde values were especially good in spite of time and resolution differences. Comparisons with higher resolution synoptic surface observations of dew point and temperature were also good. The true quality of the mesoscale analyses, however, is only seen by examining the individual case study days. Small features at a scale of ?100 km, below the resolution of upper air and surface observations, were detected by the high-resolution satellite data. For example, perturbations on the dry line usually seen at this time of the year were apparent in the satellite data, although only the general dry line position was picked up by synoptic surface observations. The time lead also was important. Convective development starting from 2?2.5 h after the satellite pass at local noon did correlate well with the local maxima of moisture and instability seen in the satellite-derived analyses. A statistical structure analysis of the satellite-derived parameters also gave the highest signal-to-noise values for the moisture and stability parameters, whereas the temperature parameters showed much less signal-to-noise content. Results from these case study days, therefore, show the quality of high-resolution satellite-derived parameters and the applicability of this method of retrieving and using satellite soundings at the mesoscale.