Climatology of Millimeter Wavelength Radar Doppler Spectra Shape

The DOE Atmospheric Radiation Measurement (ARM) program operates a number of profiling millimeter wavelength (cloud) radars at 35- and 94-GHz. These radars employ a very short pulse length (30-60 m), high temporal resolution (1-2 sec), narrow antenna beamwidth (0.3-0.5^o) and short Nyquist velocity (4-8 ms^-1), resulting in the recording of 256 and 512 point FFT radar Doppler spectra for which kinematical broadening is minimized. Subsequently, these Doppler spectra generally depart from a Gaussian shape and are often multimodal, providing information about cloud and precipitation microphysics. Here, a large dataset (6 years record) of ARM radar Doppler spectra collected at two different sites (Barrow, Alaska and Graciosa Island, Azores) is analyzed using the ARM MicroARSCL radar Doppler spectra post-processing algorithm. The dataset includes liquid, ice and mixed-phase clouds, covering a wide range of cloud and precipitation conditions. Statistics of radar Doppler spectra shape (e.g. skewness, right and left slope) for liquid and ice/snow precipitating conditions and their relationship to the particle size distribution shape will be presented. The frequency of occurrence of multi-modal spectra as a function temperature and phase in cold temperatures (T < -5^oC) will also be discussed. The climatology confirms the initial argument put forward more than 10 years ago for continuously recording the radar Doppler spectrum: if the radar is operated in a way that limits the role of dynamics in smearing the radar Doppler spectra shape, then the resulting spectra contain rich information regarding microphysical processes.