Meridional Water Vapor Transport in Tropical and Sub-Tropical Regions

Abstract

Airborne differential absorption lidar (DIAL) measurements of tropospheric water vapor and aerosol from south Europe to Brazil in mid-March 2004 are presented and compared to operational analyses of the Euro-pean Center for Medium Range Weather Forecast (ECMWF). The two 2-D sections across the tropical and sub-tropical Atlantic (5°S–37°N) reveal two different meteorological regimes. South of 20°N the Hadley circulation is evident as a humid fountain near the equator with synoptic scale inflow below 4 km (16°N) to 8 km (0°N), upper tropospheric (UT) outflow till 15°N and a dry sub-tropical subsidence layer in between. The 2nd section along the north-west African coast is controlled by mid-latitude dynamics. A deep strato-spheric intrusion is associated with the cyclonic shear along a trough from the Canary Islands towards Gi-braltar. In the sub-tropical layer and the intrusion, H2O mixing ratios q  0.01-0.1 g/kg are measured while q > 0.5 g/kg is found in the lower tropospheric air having been lifted to UT levels. The dynamical features and the water vapor mixing ratios are quite accurately analysed by the ECMWF at T511/L60 operational resolution. Largest deviations arise from shifts of structures/gradients and occasionally from assimilation inaccuracies. The averaged q-bias between both data fields is below 10%. While at mid-latitudes low water vapor regions are associated with filaments of enhanced particle backscatter, the H2O signature of the tropi-cal circulation is not correlated with the aerosol.