Relationships between water mass, rain rate, and radar reflectivity in marine stratus

Qing Yang

University of Wyoming

Accurate measurement of rainfall is of great importance both for hydrology and for cloud physics. In cloud physics, estimation of the water budget of clouds (precipitation efficiency) remains a major problem. Rainfall intensity is known to have high spatial variability, yet common measurements of rainfall rely on instruments which provide point measurements only; hence, accurate and detailed area rainfall information is difficult to obtain with a limited numbers of gauges.

Meteorological radars detect rain and readily yield area (or volume) measurements, but radar reflectivity is not equal to rain rate. Radar reflectivity factor (Z) is roughly proportional to the sixth moment of the raindrop size spectrum, while rain rate (R) is roughly proportional to the fourth moment of the spectrum. Without information about the drop spectra, it is difficult to predict Z-R relationships. Hence, comparisons of rain gauge and radar measurements have been used to produce empirical relationships, of the form Z=cR^d, where c and d are constants. However, c and d have been found to vary widely with rainfall type and location.

In this work, Z-R relationships were derived for marine/coastal stratus clouds, using data from the CS99 and DYCOMS projects. R and Z are calculated from size distributions measured by aircraft instruments instead of direct measurements of these parameters. The relationships so obtained are used to interpret reflectivity measured by an airborne radar in the proximity of the drop size measurements.