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Free keywords:
SEA-SURFACE TEMPERATURE; CONVECTION; STRATOSPHERE; VARIABILITY;
DEHYDRATION; ATMOSPHEREMeteorology & Atmospheric Sciences; balloon-borne sensors; humidity; tracer transport; tropical convection;
Abstract:
Tropospheric water-vapour and ozone measurements, using calibrated balloon-borne sensors, are reported from the Central Equatorial Pacific Experiment (CEPEX). The sensors were launched from the Research Vessel Vickers along 2 degrees S latitude between 156 degrees E (west of the international date line) and 155 degrees W (east of the date line). These measurements are combined with those from water-vapour sondes launched over the western Pacific warm pool, during the Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Taking the two experiments CEPEX and TOGA-COARE together, the sensors included frost-point hygrometers, Humicap-A Vaisala sondes, Humicap-H Vaisala sondes and electrochemical ozone-sondes. Taken together, the CEPEX and TOGA-COARE data provide over 150 vertical profiles of water vapour within the troposphere in varied conditions of convective activity ranging from disturbed to suppressed. The primary motivation behind the present analyses is to understand the role of tropical deep convection in the vertical distribution of water-vapour. With this in mind, the profiles have been analysed in relation to occasions of recent deep convection and occasions when convection was suppressed.
We employ three different criteria to identify the profiles influenced by deep convection brightness temperature in the infrared-window channel of the Japanese Geostationary Meteorological Satellite (GMS); ozone as a quasi-conservative tracer for deep convection; and using water vapour itself, that is the wettest versus the driest soundings. Irrespective of the criteria used, we report here that the atmosphere, while under the influence of active deep convection, was found to have relative humidities in excess of 75% over most of the troposphere between the surface and about 14 km. The sondes were launched routinely over a period of 45 days (between CEPEX and TOGA-COARE), without biasing the sample towards convectively disturbed conditions.
A feature of the convectively disturbed profile is a distinct minimum in relative humidity at about 700 hPa, where it was as low as 65%. The low relative humidity was accompanied by relatively high ozone mixing ratios, which raises the possibility of long-range transport of dry sub-tropical air into the warm, convectively disturbed, regions of the equatorial Pacific Ocean. Inspection of the analysed fields, and the wind fields from the sondes, supports this assertion.
It then follows that the omnipresent minimum of moist static energy and minimum relative humidity at 700 hPa in the inner tropics may be the result of long-range, inclined, transport of dry air from non-convective regions. This detection suggests a linkage between the large-scale circulation, deep convection and the thermodynamic structure within the equatorial troposphere.
The results presented here demonstrate the applicability of ozone as a quasi-conservative tracer of transport in the context of deep convection.
The ozone-based criterion is used to diagnose recent deep convection, independent of the GMS satellite observations, and allows one to follow the evolution of relative humidity and of water-vapour mixing ratio after the dissipation of the cloud anvil to optically thin conditions. We show that the troposphere dries to low humidity soon after anvil dissipation. This observation leads to the hypothesis that moistening of the atmosphere, away from the core of Cb convection, occurs by evaporation of precipitation falling out of the anvils. After anvil dissipation, the ensuing subsidence in clear air causes the relative humidity and the water mixing ratio to decrease.
