Sy. Matrosov et Aj. Heymsfield, Use of Doppler radar to assess ice cloud particle fall velocity-size relations for remote sensing and climate studies, J GEO RES-A, 105(D17), 2000, pp. 22427-22436
Knowledge of ice crystal terminal velocities, both for individual crystals
and for size distributions, is important for an adequate representation of
ice particle sedimentation in climate models. While the terminal velocities
(nu(t)) of individual crystals of simple shapes have been measured, theore
tical relations of the form nu(t) = AD(B) (where D is the maximum particle
dimension), obtained using expressions for the aerodynamic drag force, are
often more useful because they can be applied to a wide range of particle s
izes and heights and temperatures in the atmosphere. For high tropospheric
ice clouds the coefficient A has been found to vary over 1 order of magnitu
de; the exponent B is generally within the range 0.7-1.4. Aerodynamic drag
force calculations show that A and B are related. A and B can also be used
to characterize terminal-velocity-particle characteristic size relations fo
r size distributions. In this study we use collocated, vertically pointing
measurements of ice cloud radar reflectivity, Doppler velocity, and IR brig
htness temperatures to estimate the vertical profiles of cloud particle cha
racteristic size, cloud ice water content, and vertically averaged value of
the coefficient A, emphasizing cirrus clouds. We analyze variations in ter
minal-velocity-size relations for individual particles and corresponding va
riations for ensembles of particles: for example, in relations between the
reflectivity-weighted terminal velocity and the median volume size and betw
een the mass-weighted terminal velocity and the median volume size. The ret
rievals indicate that A ranges from similar to 250 to almost 4000 legs unit
s), similar to the range found from the theoretical calculations. The coeff
icient A tends to decrease as a characteristic particle size (e.g., median
size) increases. As a simplification for climate modeling efforts, we prese
nt an empirical relation between median size and A, although there is a fai
r amount of variability about this relation. Using the Doppler measurements
and retrieval data, we also derive relations between the mass-weighted ter
minal velocity and cloud ice water content. Such relations are useful for r
epresenting fallout of ice particles in climate and cloud-resolving models.