Quantitative and qualitative assessment of diurnal variability in tropospheric humidity using SAPHIR on-board Megha-Tropiques

• Diurnal variability over the continents and the oceanic regions.
• Unique measurement of the vertical profile of humidity.
• Occurrence time of maximum and minimum diurnal variability differs in different layers of the troposphere.

The global diurnal variability of relative humidity (RH) from August 2012 to May 2014 is discussed for the first time using ‘Sounder for Atmospheric Profiling of Humidity in the Inter-tropical Regions (SAPHIR)’, a microwave humidity sounder onboard Megha-Tropiques (MT). It is superior to other microwave satellite humidity sounders in terms of its higher repetitive cycle in the tropics owing to its low-inclination orbit and the availability of six dedicated humidity sounding channels. The six layers obtained are 1000–850, 850–700, 700–550, 550–400, 400–250 and 250–100 hPa. Three hourly data over a month has been combined using equivalent day analysis to attain a composite profile of complete diurnal cycle in each grid (2.5°×2.5°). A distinct diurnal variation is obtained over the continental and the oceanic regions at all the layers. The magnitude in the lower tropospheric humidity (LTH), middle tropospheric humidity (MTH) and the upper tropospheric humidity (UTH) show a large variability over the continental regions compared to that over oceans. The monthly variability of the diurnal variation over the years has also been discussed by segregating into five different continental and four different oceanic regions. Afternoon peaks dominate in the LTH over the land and the desert regions. The MTH is found to vary between the evening and the early morning hours over different geographical regions and not as consistent as that of the LTH. The UTH maximum magnitude is generally observed during the early morning hours, over the continents. Interestingly, the Oceanic regions are found to have a dominant magnitude in the afternoon hours similar to that of the continents in the LTH, evening maximum in the MTH and the early morning maximum in the UTH. The underlying mechanisms involved in the variability of humidity over different regions are also discussed. The study reveals the complexity involved in the understanding the diurnal variability over the continents and open oceans.