Stacy R. Stewart
National Hurricane Center
Miami, FL

1. Introduction

Fujita was the first researcher to identify a strong downdraft that produced damage as a DOWNBURST (1976). In later work, Fujita (1985) subdivided this term into different-scale features known as MACROBURSTS (> 4 km or 2.5 mi horizontal dimension) and MICROBURSTS (< 4 km or 2.5 mi horizontal dimension). In addition, microbursts were sub-classified as either being "wet" (> 0.01 in rainfall) or "dry" (< 0.01 in rainfall) and can even be classified further as either "cold" or "warm" (Fujita, 1985). In this module, we will focus primarily on the development of cold, dry microbursts emanating from pulse-type showers and (on rare occasions) thunderstorms where some rain is allowed to reach the ground. Specific references to "dry" microbursts will be related to the absolute humidity profile of the sub-cloud environment and not due to the lack of measurable rainfall reaching the surface as defined by Fujita (1985).

There are several factors that affect the formation of microbursts including cloud microphysics and environmental conditions. Microburst detection is affected by the physical limitations of the WSR-88D radar. As a result, the prediction and detection of dry microbursts provide a significant forecast challenge to meteorologists in the western U.S. Listed below are just a few of the problems involved when dealing with dry microburst (DMB) prediction:

1. Microburst storms and their sub-cloud environment can evolve very quickly;

2. Microbursts typically form over relatively "dry" boundary layers (T-Td spreads > 50° F; mean mixing ratio < 5 g kg^-1);

3. Microburst storms are typically very small in scale both temporally (< 1 h) and spatially (< 7 km vertical cloud depth and < 12 km diameter);

4. Radar reflectivity values for storm cells frequently lie below the WSR-88D default cell identification and tracking algorithm thresholds (i.e., below 30 dBZ);

5. Severe wind gusts can occur in association with moderate to low radar reflectivity values < 40 dBZ;

6. Several NWS Western Region WSR-88Ds are located on mountain peaks where the lowest elevation slice (0.5°) is well above the surface.

Since not all microburst storms (dry or wet) produce severe wind gusts (> 50 kt), this module will also discuss the the formation and occurrence of near-severe (> 40 kt) microburst events which can pose a signifcant hazard to aviation operations.

Although the gust prediction considerations presented in this module could be assumed to provide specific wind gust values, reality dictates that these considerations should only be used as very general guidance to help identify when/where near-severe or severe winds might occur. Currently, no tested and verified techniques for dry microburst prediction/detection exist.