Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 111

of the radar beams with increasing range, which is different for identical radars
when the storm sampled is at different ranges to each radar; thus, error is intro-
duced, which makes accurate estimation of vertical velocity even more dicult.
Downward-looking Doppler radars mounted on an aircraft can be used to
measure vertical velocities by flying over the tops of convective storms. Gerry
Heymsfield et al. at NASA have used downward-looking Doppler radars mounted
in a high-altitude (20 km) aircraft. The advantage of downward-looking radars is
that vertical air motion can be estimated directly (not kinematically), while two-
dimensional motions in the plane of the aircraft track can be resolved using
pseudo dual-Doppler techniques.
Vertically pointing Doppler radars may be used to measure vertical velocities
if they can be placed underneath a convective storm updraft. In any case, the
terminal fall speed of hydrometeors backscattering the radar beam must be known
so that it can be removed. To do so, simplified models relating the radar
reflectivity factor to terminal fall size are used. A better method would be to use
polarimetric data to classify hydrometeors based on fuzzy logic and then use a
terminal fall velocity relationship between hydrometeor type and characteristics
and fall velocity. In the latter case, however, the use of polarimetric radar data is
optimum at low elevation angles and becomes more di cult, if not impossible, at
higher elevation angles because typically used parameters such as differential
reflectivity Z DR and specific differential phase K DP are defined from horizontally
and vertically polarized signals; at high elevation angles, the polarimetric param-
eters do not measure Z DR or K DP well, owing to the geometry. In summary,
obtaining accurate estimates of vertical velocity in convective storms using multi-
ple Doppler radar analyses is challenging because identical radar-sampling
volumes at the exact same time are usually not available and there is uncertainty
in the terminal fall speed of the scatterers in the volume. It is possible that electro-
nically scanning (rapid scan) radars with very narrow beams could improve our
ability to estimate vertical velocity because they could sample at high elevation
angle and decrease errors due to non-simultaneity.
Before the updraft reaches the equilibrium level, precipitation may form. At
this stage, radars detect precipitation suspended aloft. If there is substantial liquid
water loading, owing to suspended, supercooled drops whose terminal velocity just
equals the updraft speed, then the equilibrium level is lower, near or below the
tropopause. The stronger the updraft, the less likely precipitation will have had
enough time to form and the suspended supercooled drops are higher up, along
with the EL. The appearance of radar echoes aloft is a good indicator of the
beginning of a convective storm. For this reason, it is necessary for operational
radars to scan up to a number of relatively high elevation angles.
When the terminal fall speed exceeds the updraft speed, then precipitation
begins to fall out into the updraft if there is no vertical shear and the updraft is
destroyed, and a downdraft is produced as buoyancy becomes negative. The
physics of downdrafts are different from the physics of updrafts, in large part
because microphysics becomes a more important consideration. Also, buoyant
updrafts are saturated so that the lapse rate is moist adiabatic. If precipitation