Characteristics and Moistening Effects of Shallow Clouds in the Tropical Western Pacific

Shallow clouds in tropical regions of deep convection have recently gained attention because of their possible ‘preconditioning' role in the development of deep convection. There are, however, relatively few studies on shallow clouds and their effects in these regions compared to those on subtropical shallow clouds. We investigated the characteristics and environmental controls of shallow clouds using long-term observations from millimeter cloud radars, soundings, and rain gauges at the tropical western pacific (TWP) Atmospheric Radiation Measurement (ARM) sites. Bulk moistening effects of the shallow clouds in these regimes is analyzed through estimates of the evaporation of their condensed water. In a similar manner to the shallow clouds in subtropical regions, shallow clouds in the TWP largely vary coherently with lower tropospheric stability (LTS). In the deep tropics, however, variations in environmental moisture seem to be more important than temperature to explain shallow cloud variability. Also, perturbations in the macrophysical characteristics of these clouds tend to vary with their environment in periods longer than the synoptic time scale. On these time scales, there tend to be abundant precipitating shallow clouds during regimes of large LTS and a dry low tropospehric levels, and their moistening is relatively large (3-4 mm/day). The opposite case is observed during regimes of weak LTS. These results suggest that shallow cloud moistening do not directly dissipate large-scale dryness in the deep tropics, although it might help to dissipate it by promoting deep convection in short time scales.