Humidity and cloud liquid water content estimates using simultaneous S and Ka-band radar measurements.

摘要

Water vapor and total liquid water content (LWC) are very important for many atmospheric processes, however observations of both quantities are under-sampled temporally and spatially for many research and operational applications in the atmospheric sciences. Both water vapor and LWC are critical quantities for high societal impact cloud and precipitation systems such as thunderstorms causing locally severe weather and stratocumulus clouds with important global climate feedback mechanisms. Two new dual-wavelength radar techniques to estimate humidity in the lower troposphere and total cloud liquid water content (LWC) are proposed and tested using the National Center for Atmospheric Research (NCAR) S-band/Ka-band dual-polarimetric (S-PolKa) radar. The proposed techniques can augment existing measurements and improve the spatial and temporal sampling of water vapor and LWC. Also, both techniques are unique radar measurements, but can be used with dual-wavelength radars utilizing different wavelength pairs.;The humidity estimation method compares the reflectivity from clouds and precipitation of a non-attenuated wavelength (S-band, 10 cm) and an attenuated wavelength (Ka-band, 8 mm) to compute the clear-air gaseous attenuation at the attenuated wavelength. These estimates are of total gaseous attenuation on radar ray segments that extend from the radar to a cloud/precipitation echo or from one echo to another. The attenuation estimates are then used to compute the path-integrated humidity, which are plotted at the midpoint of the ray segments. Using estimates at several elevation angles and different ranges, a profile of humidity through the lower troposphere can be retrieved. The retrieved humidity compared favorably to proximity in situ soundings with root mean square difference values between the retrieval and sounding ranging from 0.14 to 0.85 g m-3 (approximately 2% to 6% relative error, respectively).;Using scanning simultaneous S- and Ka-band radar observations to estimate LWC have not previously been demonstrated. The sources of error for this wavelength pair are evaluated and the methods to mitigate them discussed. The results are LWC estimates at each radar volume that are equivalent to specifying a reflectivity (Z) -- LWC relation constrained by the measured attenuation over 2 km radar ray segments. Because the radars are scanning, the LWC can be mapped out over the spatial volume and temporal evolution of the clouds. The results are reasonable and compare well with the in situ observations.