The radiosonde humidity profiles available during the Ganges Valley Experiment were compared to those simulated from the regional Weather Research and Forecasting (WRF) model coupled with a chemistry module (WRF-Chem) and the global reanalysis datasets. Large biases were revealed. On a monthly mean basis at Nainital, located in northern India, the WRFChem model simulates a large moist bias in the free troposphere (up to +20%) as well as a large dry bias in the boundary layer (up to -30%). While the overall pattern of the biases is similar, the magnitude of the biases varies from time to time and from one location to another. At Thiruvananthapuram, the magnitude of the dry bias is smaller, and in contrast to Nainital, the higher-resolution regional WRF-Chem model generates larger moist biases in the upper troposphere than the global reanalysis data. Furthermore, the humidity biases in the upper troposphere, while significant, have little impact on the model estimation of column aerosol optical depth (AOD). The frequent occurrences of the dry boundary-layer bias simulated by the large-scale models tend to lead to the underestimation of AOD. It is thus important to quantify the humidity vertical profiles for aerosol simulations over South Asia.Keywords: Aerosol optical depth and extinction, relative humidity, regional climate model. SOUTH ASIA, the Indian subcontinent in particular, has persistently high aerosol loadings during most of the year and is one of the regional hotspots for aerosol pollution 1-4 . Aerosols over this region originate mainly from industrial activities, residential cooking, agricultural waste and biomass burnings and transportation, and are mixed with dust and sea-spray particles transported from the adjacent desert and oceanic areas respectively. During their residence time in the atmosphere, aerosols can induce a significant radiative impact on the local energy balance by scattering and absorbing the incoming solar radiation 1,5-7 , and can cause severe air quality and human health issues 8 . Furthermore, these aerosols may be transported over long distances to affect the climatology and hydrological cycle at larger scales 9 , such as accelerating the retreat of the Himalayan glaciers 10,11 and causing a decline in the snow packs 12 . Therefore, it is important to quantify the aerosol loadings, chemical composition, radiative properties and seasonal cycles over the Indian subcontinent and adjacent areas.On regional to global scales, chemical transport models or aerosol and climate models have been used extensively to simulate the distribution and transport of aerosols over South Asia based on the compiled emission inventories [13][14][15][16][17] . While these model simulations yield insights on the regional-scale characteristics and radiative impact of aerosols, they are often subject to large uncertainties in emissions, aerosol parameterization, cloud representation, and meteorological fields that are used to drive aerosol transport and removal 15,18 . As a result, aerosol concentration ...

The intraseasonal and interannual characteristics and the vertical distribution of atmospheric water vapor from the tropical coastal station Thiruvananthapuram (TVM) located in the southwestern region of the Indian Peninsula are examined from continuous multiyear, multifrequency microwave radiometer profiler (MRP) measurements. The accuracy of MRP for precipitable water vapor (PWV) estimation, particularly during a prolonged monsoon period, has been demonstrated by comparing with the PWV derived from collocated GPS measurements based on regression model between PWV and GPS wet delay component which has been developed for TVM station. Large diurnal and intraseasonal variations of PWV are observed during winter and premonsoon seasons. There is large interannual PWV variability during premonsoon, owing to frequent local convection and summer thunderstorms. During monsoon period, low interannual PWV variability is attributed to the persistent wind from the ocean which brings moisture to this coastal station. However, significant interannual humidity variability is seen at 2 to 6 km altitude, which is linked to the monsoon strength over the station. Prior to monsoon onset over the station, the specific humidity increases up to 5-10 g/kg in the altitude region above 5 km and remains consistently so throughout the active spells.

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