Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 117

3.2 GUST FRONTS AND DOWNDRAFTS

3.2.1 Gust fronts in the absence of vertical wind shear

When a downdraft hits the ground, it is forced to spread out laterally, owing to
the decrease in height of the downdraft ( 0). Gusty winds mark the

leading edge of the cooler air, which is called a gust front. Observational studies
of gust fronts have made use of surface instruments, especially in networks, instru-
mented towers, and Doppler radars. Instrumented towers are valuable because
they provide information not only about the wind, but also thermodynamic
variables, as a function of height and time. However, they are useful only when
they are taller than the depth of the cold air mass. In addition, the vertical resolu-
tion of the wind and thermodynamic vertical profiles determined from tower data
is limited by the number of instruments that can be placed on the tower. Doppler
radars (both ordinary and wind profilers) can provide high spatial and temporal
resolution of wind variables, but not of thermodynamic variables, unless the
thermodynamic profiles are retrieved from wind observations or a radio acoustic
sounding system (RASS) is used.
Typically, the wind shifts after gust front passage, becomes gusty, and the
temperature falls and the pressure rises ( Figure 3.20 ). The amount of cooling
behind a gust front depends upon the dryness of the air, the depth over which
evaporation (or melting or sublimation) takes place, and the sizes of the water
drops and hailstones, if present. Numerical cloud models at the current time have
diculty reproducing temperature deficits behind gust fronts accurately, mainly
owing to uncertainties in the amount of and drop-size distributions of the precipi-
tation and cloud droplets. Drop-size distributions vary according to the origin of
the air; for example, drop-size distributions in the tropics are different from those
in continental regions of mid-latitudes. Our knowledge of drop-size distributions
in convective storms using direct measurements is limited, owing to the impossi-
bility of placing an instrument at all locations in a storm all the time.

@ w =@ z <

3.2.1.1 Microbursts

In mid-latitudes over land, when clouds build upward into relatively dry
environmental air, the potential for evaporative cooling is high and, hence, very
strong downdrafts and gust fronts are possible. When both the winds and the
vertical shear are weak, the downdraft may be circularly symmetric and regions of
very strong lateral gradients in wind can be produced near the ground, about the
center. Horizontal vorticity is created baroclinically along the leading edges of
the downdraft of negatively buoyant air (cf. (2.51)) so that a ring of horizontal
vorticity is produced ( Figure 3.21 ). A vortex ring may also be induced by the ver-
tical shear of the horizontal wind associated with surface divergent flow ( Figure
3.22 ).
Very strong downdrafts (of 10 to 20m s 1 or more) that reach the ground are
called ''microbursts'' ( Figure 3.20 bottom); they can be very hazardous to aircraft
landing or taking off: aircraft that enter a microburst experience a brief period