Seasonal and diurnal water vapour distribution in the Sahelian area from microwave radiometric profiling observations
A dataset gathered over 369 days in various midlatitude sites with a 12-frequency microwave radiometric profiler is used to analyze the statistical distribution of tropospheric water vapor content (WVC) in clear and cloudy conditions. The WVC distribution inside intervals of temperature is analyzed. WVC is found to be well fitted by a Weibull distribution. The two Weibull parameters, the scale (l) and shape (k), are temperature (T) dependent; k is almost constant, around 2.6, for clear conditions. For cloudy conditions, at T , 2108C, k is close to 2.6. For T .2108C, k displays a maximum in such a way that skewness, which is positive in most conditions, reverses to negative in a temperature region approximately centered around 08C (i.e., at a level where the occurrence of cumulus clouds is high). Analytical l(T) and k(T) relations are proposed. The WVC spatial distribution can thus be described as a function of T. The mean WVC vertical profiles for clear and cloudy conditions are well described by a function of temperature of the same form as the Clausius– Clapeyron equation. The WVCcloudy/WVCclear ratio is shown to be a linear function of temperature. The vertically integrated WV (IWV) is found to follow a Weibull distribution. The IWV Weibull distribution parameters retrieved from the microwave radiometric profiler agree very well with the ones calculated from the 15-yr ECMWF reanalysis (ERA-15) meteorological database. The radiometric retrievals compare fairly well to the corresponding values calculated from an operational radiosonde sounding dataset.