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°S latitude between 156°E (west of the international date line) and 155°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 Väisälä sondes, Humicap‐H Väisälä 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.
Abstract. This is the first study that investigates the seasonal variability of nitrate (NO3) radicals in the marine boundary layer over the East Mediterranean Sea. An extensive data set of NO3 radical observations on the north coast of Crete for more than two years (June 2001–September 2003) is presented here. NO3 radicals follow a distinct seasonal dependency with the highest seasonally average mixing ratios in summer (5.6±1.2 pptv) and the lowest in winter (1.2±1.2 pptv). Episodes with high NO3 mixing ratios have been encountered mainly in polluted air masses originating from mainland Greece, Central and East Europe, and Turkey. Ancillary measurements of ozone, nitrogen dioxide (NO2) and meteorological parameters have been conducted and used to reveal possible relationship with the observed NO3 variability. The acquired NO2 nighttime observations provide the up-to-date most complete overview of NO2 temporal variability in the area. The data show correlations of the NO3 nighttime mixing ratios with temperature (positive), relative humidity (negative) and to a lesser extend with O3 (positive). As inferred from these observations, on average the major sink of NO3 radicals in the area is the heterogeneous reaction of dinitrogen pentoxide (N2O5) on aqueous particles whereas the homogeneous gas phase reactions of NO3 are most important during spring and summer. These observations support a significant contribution of NO3 nighttime chemistry to the oxidizing capacity of the troposphere.
