The WSR-88D observes non precipitating clouds

               

           

The WSR-88D radar network in the US is used to monitor severe weather and to measure precipitation. Existing radar volume coverage patterns, VCPs, have been design for those purposes. The maximum elevation angle of the VCPs is about 20^o and the signal to noise (SNR) threshold to display reflectivity is typically set to 2 dB.  Although there is a “clear air mode” whereby a long pulse is transmitted to increase the SNR, it is seldom used.  All these factors constructively combine to inhibit display of echoes from non precipitating clouds.

To study these clouds remotely, “cloud” radars at much higher frequencies than 3 GHz of the WSR-88D are used. Our radar observations and pictures of the sky confirm that almost all types of non precipitating clouds that can be seen by a naked eye are detected with the 10 cm wavelength WSR-88D. With this radar, important cloud characteristics such as cloud tops and bottoms, horizontal extend, layer structures, etc. can be obtained (panels a,c below). Enhancements in signal processing increase the sensitivity of the WSR-88D and use of capability to point antenna up to 60^o in elevation is sufficient to observe practically all clouds in all but a small area within a cone of silence.

More radar variables can be measured with polarimetric radar. Observations in non precipitating clouds conducted on the research polarimetric WSR-88D (KOUN in Norman OK) have shown tremendous variety of polarimetric properties: differential reflectivity, Z_DR, spreads a wide interval of 0 to 5 dB and frequently exhibits patterns with “pockets” of high and low Z_DR (panels b,d below). Fields of the copolar correlation coefficients also exhibit an abundance of information that can be useful to characterize clouds. The specific differential phase can gauge water content in clouds where it is high. Dense sampling in elevation, longer dwell time, and lag one estimators of polarimetric variables have been applied at SNR as low as -10 dB for observations of clouds to distances beyond 150 km. These observations are important for studies of initiation and evolution of precipitation, cloud model parameterization development, application to climate effects and radiation transfer in atmosphere and others.