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Moisture flux is the driver of the Indian summer monsoon on all space-time scales and its accurate representation in monsoon simulation models is crucial. For the first time, the moisture flux measurements are made over India during the PRWONAM (Prediction of Regional Weather using Observational meso-Network and Atmospheric Modelling) program. These measurements best represented low wind free convection regime (U-10 < 2 m/s). Here we show moisture flux scaled linearly with appropriate near-surface humidity and temperature differentials. The drag scaled linearly with wind speed unlike the moisture and sensible heat fluxes. The sensible heat flux scaled as "4/3" power of a near-surface temperature differential consistent with Rao et al. (1996a, https://doi.org/10.1029/96GL02368) and Rao (2004, https://doi.org/10.1023/B:BOUN.0000016495.85528.d7). A striking result is that the moisture flux is a linear function of the sensible heat flux at all wind speeds; while drag does not show any dependence on heat flux.

Plain Language Summary The Indian summer monsoon largely impacts agricultural production and hence the Indian economy. Thus, it is important to unravel vagaries of monsoon to improve the skill of monsoon rainfall prediction. Rainfall is largely accounted by moisture transports from the Earth's surface into atmosphere. In this study, long-term moisture flux measurements are made for the first time over India and scaling laws are proposed for its accurate representation. These scaling laws may replace the widely used Monin-Obukhov similarity theory in models to estimate surface fluxes at low wind speeds prevailing all-through monsoon season. The question is how best the moisture flux can be parameterized at low wind speeds. Major advancement here is the new finding that the moisture flux is a linear function of the sensible heat flux at all wind speeds. The sensible heat flux scaled as 4/3 power of an appropriate temperature differential. Momentum flux scaled linearly with wind speed. Accurate representation of moisture flux is the need of the hour to improve monsoon prediction, in view of its teleconnections with El Nino-Southern Oscillation, North-Atlantic Oscillation, and so forth, with implications for global climate change. Cloud parameterization, air-pollution, dispersion-modeling communities are directly impacted as surface fluxes serve as sources/sinks.