Herman G. J. Smit
h.smit@fz-juelich.de
12.12.2003, 13.00 c.t.
Raum N3380
Tropospheric water vapor plays a crucial role in our Earth’s climate and its interaction with atmospheric chemistry. While it is clear that evaporation from the surface is the ultimate source of water vapor in the troposphere and precipitation is the sink, it is less clear what processes control the upper tropospheric humidity (UTH). Whether UTH is under thermodynamic or dynamic control is still one of the unresolved questions regarding climate feedback, important for a better understanding and prediction of the global temperature changes. A large deficiency is that very little is known about the spatial and temporal distribution of UTH content. Since 1994 an important gap is filled by MOZAIC (Measurement of Ozone and Water Vapor by Airbus In Service Aircraft) UTH-observations made at 9 to 12 km cruise altitude aboard five A340 aircraft during normal in-service flights.
After a short introduction of the MOZAIC- water vapor sampling program a comprehensive climatology of the UTH distribution over the Atlantic obtained from MOZAIC observations will be presented and discussed. The UTH measurements reveal a strong spatial and temporal variability on different scales. Very pronounced are the seasonal cycle and inter-annual variability of specific humidity which are often positively correlated with temperature. However, in summer a negative correlation of relative humidity and temperature is observed. The relative humidity frequency distribution in the tropics and sub-tropics show thereby to be clearly bi-modal with a dry and a wet branch.
The observed seasonal and inter-annual variations of the MOZAIC-UTH climatology will be compared with results obtained from ECMWF-analysis. Thereby, special attention will be paid to the fact that a substantial fraction of the MOZAIC-UTH-measurements show supersaturation with respect to ice which is not represented in the ECMWF-analysis. Finally, it will be shown that MOZAIC-observations can be used to validate UTH-measurements made from satellites and to integrate with other non-satellite observations (e.g. radiosoundings) into a global data set of UTH-observations aiming to increase the diagnostic and prognostic strength of numerical model simulations of chemistry, weather and climate of the atmosphere.