The effect of radar pulse length on cloud reflectivity statistics

When observing clouds with radars, there are a number of design parameters, such as transmitted power, antenna size, and wavelength, that can affect t he detection threshold. In making calculations of radar thresholds, also kn own as minimum sensitivities, it is usually assumed that the radar pulse vo lume is completely filled with targets. In this paper, the issue of partial beam filling, which results, for instance, if a cloud is thin with respect to the pulse length, or measurements are being made near cloud edges, is i nvestigated. This study pursues this question by using measurements of rada r reflectivities made with a 35-GHz, surface-based radar with 37.5-m pulse lengths, and computing how reflectivity statistics would be affected if the same clouds and/or precipitation had been observed with a radar with a 450 -m pulse length. In a dataset measured during winter over a midcontinental site, partial beamfilling degraded the percentage of clouds detected by abo ut 22% if it was assumed that the minimum detection threshold was -30 dBZ. In a second dataset collected during summer over a summertime subtropical s ite that was dominated by thin, boundary layer stratus, partial beam fillin g degraded the percentage of clouds detected by 38%, again assuming a minim um detection threshold of -30 dBZ. This study provides a preliminary indica tion of how radar reflectivity statistics from a spaceborne cloud radar may be impacted by design constraints, which would mandate a pulse length of a round 500 m and a minimum detection threshold of around -30 dBZ.