First radar echoes and the early Z_DR history of Florida cumulus

The early histories of radar echo and polarization differential reflectivity (Z_DR) from growing cumulus clouds observed in Florida with a 10-cm-wavelength radar are reported in detail. Raindrops 1 to several millimeters in diameter are present at about cloud-base level in most cases as soon as any identifiable precipitation echo is seen within cloud (distinct from Bragg scattering and echo from cloud droplets). This is in most cases by the time of the first 10-dBZ radar echo aloft. The very early occurrence of large drops is consistent with origination directly from coalescence on ultragiant aerosol. However, they appear to exist so early and so low in the clouds as to be unexpected if the cumulus were single, vigorous thermals. The explanation may lie in the presence of a more gradual, very early cloud stage that is generally not observed in any detail. Simultaneous Z_e and Z_DR measurements in the early stages of developing, warm cumulus will provide a powerful test of understanding the onset of drop growth by coalescence.