Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 126

column of ice particles falling which, when descending, turns into a region of high
reflectivity and high differential reflectivity, and is thus suggestive of a developing
microburst.
The vertical dynamic pressure gradient force can also play a role in microburst
dynamics. At the ground, where there are strong gradients in the wind the squared
terms in the diagnostic equation for perturbation pressure (the divergence equa-
tion for the Boussinesq system (2.62)) are substantial. From (2.62), we see that
this term is associated with a positive perturbation pressure. Above the ground,
where the flow is not as divergent or non-divergent and vertical velocity does not
change as much with height, the perturbation pressure is much smaller in
magnitude. Thus, there is an upward perturbation pressure gradient force that
counteracts the downward negative buoyancy force in the microburst near the
ground.
Microbursts/downbursts sometimes rotate when there is vertical shear in the
environment. A discussion of this topic is delayed until the next chapter, when
supercell dynamics is discussed.
3.2.1.2 Heat bursts
When a dry microburst produces sinking air that contains enough momentum to
break through a shallow surface inversion, a gust of wind at the surface will be
warmer than the surrounding air and in some instances much warmer, by as much
as 4-10 C or slightly more ( Figure 3.27 ) . Such events, during which relative
humidity drops, temperature rises, wind speed increases and becomes gusty, and
pressure falls ( Figure 3.27 ), are called ''heat bursts''; changes in temperature, dew
point, wind, and pressure do not necessarily occur simultaneously. Once
thought to be very rare because the chance of one striking a station from the
operational synoptic-scale network is very small, they have been documented in
the last few decades much more frequently in more closely meshed surface meso-
networks.
Since surface inversions are common at night when surface winds are not too
strong, heat bursts occur most often at night, especially during the convective
season (i.e., spring through summer and early autumn). It can be very startling to
experience the temperature at night suddenly rising to daytime levels or even
higher. Heat bursts can occur during the day when a surface stable layer is
present, but not one due to radiation (e.g., a microburst may hit a shallow cold
pool or the top of a shallow front and instigate a heat burst). It is also possible
that when a microburst hits a stable layer, gravity waves are triggered. The most
dangerous effect of heat bursts is their strong surface winds—not the warm
temperatures. Heat bursts tend to be driven by convective systems (but not in
every case) and not always as a result of microbursts emanating from them. In
Oklahoma, where there has been a long-operating surface mesonetwork, heat
bursts have been found underneath or adjacent to weak radar echoes, mostly less
than 30 dBZ in reflectivity factor.