The Interaction of Clouds and Dry Air in the Eastern Tropical Pacific

Cloud radar observations of eastern Pacific Intertropical Convergence Zone cloud vertical structure are interpreted in light of soundings, 100 km-scale divergence profiles calculated from precipitation radar Doppler velocities, and surface rain gauge data, from a ship at 10N, 95W during the 2001 East Pacific Investigation of Climate (EPIC) experiment. The transition from convective to stratiform rain is clear in all 4 datasets, indicating a coherence from local to 100km scale. A novel finding is dry air intrusions at altitudes of 6-8 km, often undercutting upper-level ice clouds. Two distinct dry-air source regions are identified. One is a relatively dry area overlying the cooler waters of the Costa Rica oceanic thermocline dome, centered approximately 400 km east-north-east of the ship site. The other is the even drier near-equatorial subsidence zone south of 6-7N. The former source is somewhat peculiar to this specific shiplocation, so that the ship sample is not entirely representative of the region.

The Sept. 20-25 period is studied in detail, as it depicts two influences of the dry air on cloud vertical structure. One is the modulation of small-scale surface-based convection, evident as a weakening and narrowing of cloud radar reflectivity features. The other springs from intense sublimation cooling as differential advection brought snowing anvil clouds over the dry layers.  During one half-day period of strong sublimation, the cooling rate is inferred to be several 10s of degrees per day over a ~100 hPa layer, based on a heat budget estimate at 100km scale involving the horizontal wind divergence data. This is consistent with fluxing ice water contents of 0.05-0.10 g/m^3 derived from the cloud-radar reflectivities. The temperature profile shows the dynamically expected response to this cooling, a positive-negative-positive temperature anomaly pattern centered on the sublimating layer. A buoyancy-sorting diagnostic model of convection indicates that these upper-troposphere temperature anomalies can cause premature detrainment of updrafts into the lower part of the cloudy layer, a feedback which may actively maintain these long-lasting dense anvils.

Middle troposphere southerly dry air inflow is also evident in large-scale analysis, a feature thought to be related to the tropical moist adiabatic temperature profile. Given the proximity of the dry equatorial subsidence zone to the eastern tropical Pacific, the differential advection of dry and cloudy air and its after-effects may play a role in establishing the region's climate, although the extent of the significance needs to be further established.