DIAGNOSING THE GLOBAL HYDROLOGICAL CYCLE FROM ROUTINE ATMOSPHERIC ANALYSES

This paper discusses the extent to which atmospheric analyses produced routinely by forecasting centres can be used to diagnose the global h ydrological cycle. These analyses directly represent the storage and l arge-scale advection of water vapour through a combination of observat ions and a short model forecast. In this work, the analyses produced b y the European Centre for Medium-Range Weather Forecasts are used. Due to changes in the model formulation, which have a profound effect on the analyses, only results from three years of these initialized analy ses are presented. The equations representing the conservation of wate r and water vapour in the atmosphere are derived and discussed, includ ing consideration of the approximations commonly made. Diagnostics of the storage and transport of water vapour have been calculated. The re sults are qualitatively similar to diagnostics from other sources. In addition, with regard to attempts to derive a global relationship betw een the surface specific humidity and the total column water vapour, s pecial consideration is given to the variations in the vertical distri bution of water vapour. The profile of water vapour is found to be ver y sensitive to the competing processes responsible for the water balan ce in the atmosphere. Although surface fluxes of water cannot be direc tly derived from these analyses, a technique is introduced which puts a lower bound on the evaporation and precipitation that would be consi stent with the analyses. The results of this technique are rather enco uraging, and comparisons with other climatologies of surface water flu xes could provide another way of validating these atmospheric analyses . A diagnostic characterizing the time scale of the the small-scale pr ocesses responsible for surface evaporation and precipitation is also introduced. This time-scale is found to vary widely but is of the orde r of one day where variability in the hydrological cycle is important.