Keynote Talk: New insights from millimeter-wave radars

These new radars opened the way in recent years to better appreciation of the structures of various phenomena and of the conditions that lead to them, provide fertile material for advancing LES and other detailed model descriptions, and raise many new questions.

The availability of radar systems operating at wavelengths of about 3 millimeters (94 GHz frequency; W-band) led to new opportunities for observations of clouds and precipitation over the past decade or so, mainly due to the greater sensitivity and smaller hardware bulk that goes with this short operating wavelength. This wavelength is in the next water vapor window below 8.6 mm which has been used for much longer. Greater attenuation is the major drawback at the 3 mm wavelengths.

As with other radars, reflectivity and Doppler velocity are the primary measurements. Both fixed-based and mobile W-band radars have been deployed. The former are operated mostly in a vertically pointing mode and yield time-height profiles similar to what is obtained with the longer wavelength radars. Excellent studies at Chilbolton and GKSS with such installations, often in combination with other measurements like radiometers, lidars or radars of longer wavelengths, have yielded new information on Ci and Sc frequencies, cloud overlaps, Ci, St and Sc microphysics, and more. This talk will focus on results obtained with the mobile systems.

The smaller sizes of W-band radars naturally lead to mobile applications, and at the same time mobile applications can largely overcome the disadvantages of greater attenuation and shorter useful operating range of these radars by getting close to the targets. Mobility also allows for the observations of shallow boundary layer phenomena, which fixed radars can only do over very small domains. Results from two radar systems – the University of Wyoming Cloud Radar (WCR) and the Univ Massachusetts tornado radar – will be used to illustrate what has been learned with these radars so far. The WCR has been operated principally on the Wyoming King Air, and also on the NSF/NCAR C130 and the Canadian NRC Convair 580. The in situ measurements of state parameters, hydrometeor size distributions and winds, plus lidar and radiation data, complement the radar observations, and vice versa.

In marine stratocumulus, studies with the WCR (in DYCOMS) revealed the ubiquity of drizzle, updraft characteristics, cellular organization, entrainment structures, unique pockets of open cells and much detail on cloud microphysics.

For continental, high-base cumulus, dual-Doppler analyses (HiCu03 experiment) show that sequences of thermals rise in close vicinity to one another. The thermals have toroidal structures. A major form of entrainment is seen as intrusions at the base of the thermal. Additional smaller mixing events were diagnosed near the tops of the thermals. Horizontal circulations are nearly as intense as the vertical ones.

The influence of cirrus generating cells on the vertical structure of nimbostratus was documented (WYICE00 experiment) but the additional variations within the clouds on scales of few hundred meters still have no clear explanation.

The near-instantaneous depictions of wave clouds with the airborne radar show the real magnitudes of the waves, the variations from multiply stratified cases to embedded small-scale convection, and document the evolution of ice crystals within them.

In the IHOP experiment, with insects providing the backscattered signal, the structure of a dry line was observed with the great detail, the statistical distribution and vertical extent of updrafts was quantified, and the influence of topography and soil moisture on boundary layer circulations was documented.

Using the UMass tornado radar Bluestein and colleagues documented a pair of counterrotating vortices and hook echoes, and the near-surface time history of a tornado, including its rapid changes.

The examples cited are also indicative of research directions to follow. With the launch of CloudSat, and with further capabilities added to the W-band radars already in operation, there is little doubt that the the years to come will bring great impetus to the fine-scale study of cloud and precipitation systems, boundary layer structures and the like.

The talk draws upon the work of many people. In many instances, other papers given during the conference will expand on the quick summaries assembled in this presentation.