The signature of aerosols and meteorology on the development of precipitation from shallow cumuli in the trades is investigated with ground-based lidar and radar remote sensing. The measurements are taken from a cloud observatory recently established on the windward shore of Barbados. Cloud microphysical development is explored through an analysis of the radar echo of shallow cumuli before the development of active precipitation. The increase of reflectivity with height (Z gradient) depends on the amount of cloud water, which varies with meteorology, and cloud droplet number concentration N, which varies with the aerosol. Clouds with a large Z gradient have a higher tendency to form precipitation than clouds with a small Z gradient. Under similar meteorological conditions, the Z gradient is expected to be large in an environment with few aerosols and small in an environment with many aerosols. The aerosol environment is defined using three methods, but only one (based on the Raman lidar linear-depolarization ratio) to measure dusty conditions correlates significantly with the Z gradient. On average, nondusty days are characterized by a larger Z gradient. However, the dust concentration varies seasonally and covaries with relative humidity. Large-eddy simulations show that small changes in the relative humidity can have as much influence on the development of precipitation within the cloud layer as large changes in N. When clouds are conditioned on their ambient relative humidity, the sensitivity of the Z gradient to dust vanishes.

Corresponding author address: Katrin Lonitz, ECMWF, Shinfield Park, Reading RG2 9AX, United Kingdom. E-mail:

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