Wet Microburst Events Observed with Phased Array Radar

Though downbursts were thought to exist as early as the 1930’s (Fujita and Wakimoto 1981), it wasn’t until their dangerous impact on aviation was discovered that research on this phenomenon began to flourish (Fujita 1980). Fujita (1981) quantitatively defines a microburst as a downburst occurring on a horizontal scale from 0.4 – 4 km. Several years later, Wilson et. al. (1984) developed a radar-based definition of a microburst, using C and S-band Doppler radar data collected during the Joint Airport Weather Studies (JAWS) Project (McCarthy et al. 1982). Wilson et al. (1984) defined a microburst as a diverging Doppler velocity signature having a velocity differential of 10 m s within a 4-km horizontal distance. For real-time operational warnings to the aviation community, the automated microburst prediction algorithm (Wolfson et al. 1994) issues microburst alerts when a velocity differential of 15 m s or higher within a 4-km horizontal distance is identified (“wind-shear alerts” are issued if the velocity differential is between 7.5 – 15 m s). Microbursts are known to occur in a variety of atmospheric environments. However, the physical processes leading to the occurrence of a microburst can vary greatly depending on the character of the atmospheric environment. Results from the JAWS Project (McCarthy et al. 1982) reveal that some microbursts are accompanied by heavy precipitation from thunderstorms, whereas others are associated with virga shafts from a variety of high-based clouds. This distinction has led to microbursts being classified into two broad categories: “dry” and “wet. ” Dry microbursts are defined as microbursts accompanied by little or no precipitation at the ground between the onset and end of the high surface winds, and are most often observed in higher terrain regions (Fujita 1985; Wakimoto 1985). The typical environment favoring dry microbursts is characterized in the morning by a radiation inversion (40– 50 mb deep) capped by a dry adiabatic layer that extends up to a mid-altitude of ~500 mb (Wakimoto 1985). By afternoon, the inversion is replaced by a shallow, super-adiabatic layer (<10 mb deep) near the surface. Generally, the surface is relatively dry and a ─────────────────────────────────── *Corresponding author address: Arthur Witt, NSSL, 120 David L. Boren Blvd., Norman, OK 73072. E-mail: Arthur.Witt@noaa.gov. nearly saturated layer exists ~500 mb. The moisture profile from the surface to 500 mb is usually “wellmixed” with a constant or increasing mixing ratio. The clouds producing dry microbursts tend to have relatively high bases above ground level, with the strong surface winds resulting from negative buoyancy generated by evaporation, melting, and/or sublimation of precipitation below cloud base (Hjelmfelt 1987; Srivastava 1987; Proctor 1989). In contrast, “wet” microbursts predominate in environments typically characterized by shallow sub-cloud layers, warmer cloud bases, and vertical thermodynamic profiles that are more humid (Wakimoto and Bringi 1988; Kingsmill and Wakimoto 1991; Atkins and Wakimoto 1991). Atkins and Wakimoto (1991) found that wet microbursts were more likely on days when ∆θe > 20 K versus ∆θe < 13 K on days with thunderstorms and no microbursts. Proctor (1989) found via numerical model sensitivity studies that the intensity and likelihood of wet microbursts increase as the environmental lapse rate becomes steeper, the melting level becomes higher above ground level, and the humidity becomes drier near the melting level and more moist at low altitudes. This study focuses on wet microbursts, as these are the type that thus-far occurred when PAR data were collected. Microbursts are quickly evolving phenomena that usually form and dissipate within 20 min (Fujita 1981; Wilson et. al. 1984; Hjelmfelt 1988). Since microbursts can develop rapidly, often in less than 5 min, they are challenging to predict. Hence, several previous studies attempted to identify reliable precursors to microbursts in an effort to forecast these small-scale phenomena. In their analysis of multipleDoppler radar data of microburst-producing storms in Colorado, Roberts and Wilson (1989) found several precursors of microbursts related to the vertical profiles of reflectivity and convergence in the storm. Proctor (1989) showed that with an increase in the diameter of the downdraft resulting from increasing width of precipitation shafts, the peak outflow speeds of the microburst, the depth of the outflow and the height of the outflow peak all increase, but the mean horizontal wind shear decreases.