2. Dry Microburst Environmental Precursors

Severe dry microbursts tend to occur most frequently during the mid-afternoon to early evening hours when solar insolation is at a maximum and sub-cloud lapse rates are the steepest. A typical "dry" microburst environment consists of ~500 J kg^-1 of CAPE (convective available potential energy), weak winds, and a deep, well-mixed, "dry" sub-cloud layer (see e.g., Knapp 1996). However, the surface/boundary layer moisture values generally exceed those of the sub-cloud layer. Cloud bases can be as high as 450 mb and the classic sounding has been referred to as the "inverted-V" sounding (Fig. 1). Although heating of elevated terrain can produce shallow, cumuliform clouds which are capable of producing microbursts, case studies performed by the authors indicate that the lifting of surface-based parcels due to intense surface heating generally results in higher observed peak wind gusts and a greater frequency of severe wind gusts. In some instances, "warm" microbursts (the term "heat burst" is generally reserved for the larger-scale warm macrobursts) have even occurred.

The moist layer aloft plays an important role in determining the vertical extent and diameter of the updraft and, subsequently, the amount and size of particles that can be suspended above cloud base and the freezing level. Once a parcel reaches the LFC, lateral entrainment of environmental air begins. If the air is too dry, significant evaporation of cloud material will occur, resulting in rapid weakening or even complete collapse of the updraft. Forecasters should play close attention the amount and depth of the mid-level moist layer since this may be the determining factor as to whether or not significant microbursts will develop.

In Figure 2, a more specific breakdown of the various environmental parameters can be seen -- (1) A steep, nearly dry adiabatic lapse rate is present due to afternoon surface heating; (2) low relative humidity below cloud base develops in response to deep convective boundary layer mixing; (3) local moisture variations can increase the surface dewpoint/humidity; and (4) CAPE (red-shaded area) values are generally low (<500 J kg^-1) due to the lack of significant low-level moisture. Note that the cloud bases typically develop above the height of the freezing/melting level which indicates ice processes may play an important role in the initiation of downdrafts.