Maintenance of the free-tropospheric tropical water vapor distribution. Part I: Clear regime budget

Abstract

The water vapor budget of the free troposphere of the maritime Tropics is investigated using radiosonde observations, analyzed fields, and satellite observations, with particular attention paid to regions free of organized convection. In these arid regions, time-average drying by subsidence must be balanced by moistening via horizontal advection from convective areas and via vertical turbulent transport from below. It is found that for at least 25% of the maritime Tropics, 80% ± 10% of this source above 700 mb is by horizontal advection. The remainder comes from vertical convective transport (scales < 250 km), with a pronounced local maximum at 500 mb. The regions for which this is true are characterized by pentad outgoing longwave radiation > 270 W m-2 and may be said to exist out of equilibrium with the surface as regards moisture. Transport from below makes a significant contribution between 700 and 800 mb, despite the usual presence of an inversion below these levels, but is difficult to quantify accurately. The convective transport convergence is estimated as a residual from large-scale budgets and directly from sounding time series by an independent method, which shows a narrow maximum at 500 mb. Half of the paper addresses the question of data accuracy, including sounding and analyzed data, as it pertains to the question at hand. It is concluded that the moisture budgets from the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are of useful accuracy despite some significant mean descrepancies between the analyses and sounding observations in convective areas. The budget is found to be similar to that of a general circulation model based on the ECMWF forecasting model. Humidity measurements from operational soundings appear responsive below 300 mb, but then abruptly become unresponsive.