3. Conceptual Model of a Dry Microburst

Assuming a sufficient sub-cloud, thermodynamic profile exists, then the first step (Fig. 3) in "dry" microburst development is the formation of a core of relatively (i.e., relative to surrounding cloud water) high liquid water/ice content (LWIC) above but within 5,000 to 8,000 ft of the cloud base. Development of this region of high LWIC can result from either (a) the lifting of surface parcels to the LFC/cloud base or (b) development over higher terrain (e.g., mountain peaks) with the cloud elements propagating away from these elevated heat sources and over a nearly dry, adiabatic boundary layer. However, numerous case studies by the authors reveal that the former case is the preferred mode for the development of severe microbursts (gusts > 50kt). This is likely due to the existence of higher mixing ratios near than ground than what generally occurs aloft.

Owing to the increased size and mixed phase of the water/ice particles, radar reflectivity values above cloud base are usually a maximum at this time. Also, the increased weight of the LWIC core suggests that a downdraft is initiated within cloud by precipitation drag (Roberts and Wilson, 1989). Depending on the amount of CAPE and environmental forcing that may exist, reflectivity values are frequently between 30 and 40 dBZ. Microbursts associated with larger CAPE and resulting deeper convection are more toward the "wet" end of the spectrum of microbursts and can be accompanied by small hail.

In step two (Fig. 4), descent of the high reflectivity/LWIC core below cloud base and below the freezing level begins. In using the term "cloud base," we are referring to the level of initial cloud development as determined from a sounding or from aviation surface observations. As the combination of water and ice particles fall into the dry sub-cloud air, evaporational cooling/sublimation begins which creates the negative buoyancy that drives the downdraft. Notice that at this stage, only weak cloud base convergence is occurring.

By the time step three is reached (Fig. 5), the downdraft has reached approximately 75% of its maximum penetrative depth. Model results by Srivastava (1985) indicate that the downdraft has also reached more than 60% of the peak downdraft velocity. Numerous case studies  performed by the authors revealed that cloud base  convergence is  still  relatively weak (< .001 s^-1) at this time. As a result, the usefulness of that radar velocity signature as a predictive tool when attempting to apply mass continuity arguments is minimal at best.

In step four (Fig. 6), the downdraft core has reached the ground and horizontal divergence begins. Maximum cloud base convergence along with maximum downdraft velocity is occurring at this time which is approximately 15 to 20 minutes after downdraft descent first began (Srivastava, 1985). Whether the surface temperature decreases or increases in association with the downdraft core is directly related to the amount of liquid water and ice that is available for evaporation/sublimation into the sub-cloud air. In cases where the LWIC is exhausted well above ground-level, the surface temperature can rise several degrees above the ambient conditions resulting in a somewhat rare phenomenon known as a "warm" microburst.

The fifth step (Fig. 7) in the dry microburst development process is the time when maximum horizontal surface winds are occurring. When the sub-cloud lapse rate is dry adiabatic or slightly superadiabatic, wind gusts in excess of 50 kt are possible when high concentrations of liquid water and ice particles (e.g., > 30 dBZ) were present above cloud base in step one. However, even low LWIC's which only create virga can still produce intense downdrafts in the presence of such steep lapse rates (Srivastava, 1985). For this reason, forecasters must closely monitor the sub-cloud temperature profile since a change of only 0.5° C km^-1 can result in a difference in peak gusts by as much as 10 kt.

The sixth and final step (Fig. 8) occurs during dissipation of the downdraft and spreading out of the horizontal wind field. Although wind gusts may still be strong during this stage, rapid weakening of the outflow begins due to loss of downdraft momentum. Also, the parent cloud will completely dissipate in some instances.