Delineating Mid- and Low-Level Water Vapor Patterns in Pre-Convective Environments Using VAS Moisture Channels

Abstract

Infrared and visible imagery from VAS are used to delineate mid- and lower-tropospheric moisture fields for a variety of severe storm cases in the southern and central United States. The ability of sequences of images to isolate areas of large negative vertical moisture gradients and apparent convective instability prior to the onset of convective storms is assessed. Midlevel dryness is diagnosed directly from the VAS 6.7 channel observations, while low-level water vapor is either inferred from the presence of clouds in visible and infrared imagery or, in cloud-free areas, calculated from VAS "split window" channels. A variety ofimage combination procedures are used to deduce the stability fields which are then compared with the available radiosonde data. The results for several severe storm cases indicate that VAS can detect mid- and low-level mesoscale water vapor fields as distinct radiometric signals. The VAS imagery shows a strong tendency for thunderstorms to develop along the edges of bands of midlevel dryness as they overtake either pre-existing or developing low-level moisture maxima. Image sequences depict the speed with which deep moist and dry layers can develop and move, often at scales not resolvable using conventional radiosonde data. The images thus demonstrate the ability of VAS radiance data to detect differential moisture advectionsin rapidly changing pre-convective environments.