Abstract
Abstract. Thermodynamic soundings for pre-monsoon and monsoon seasons from the Indian subcontinent are analysed to document differences between convective environments. The pre-monsoon environment features more variability for both near-surface moisture and free-tropospheric temperature and moisture profiles. As a result, the level of neutral buoyancy (LNB) and pseudo-adiabatic convective available potential energy (CAPE) vary more for the pre-monsoon environment. Pre-monsoon soundings also feature higher lifting condensation levels (LCLs). LCL heights are shown to depend on the availability of surface moisture, with low LCLs corresponding to high surface humidity, arguably because of the availability of soil moisture. A simple theoretical argument is developed and showed to mimic the observed relationship between LCLs and surface moisture. We argue that the key element is the partitioning of surface energy flux into its sensible and latent components, that is, the surface Bowen ratio, and the way the Bowen ratio affects surface buoyancy flux. We support our argument with observations of changes in the Bowen ratio and LCL height around the monsoon onset, and with idealized simulations of cloud fields driven by surface heat fluxes with different Bowen ratios.
Highlights
The convective environment over the Indian subcontinent changes significantly from hot and dry pre-monsoon conditions to cooler and wetter monsoon conditions
Since no surface flux information is available for the region where long period soundings were obtained, we use data collected at another location in the rain shadow area to document changes in the Bowen ratio and lifting condensation levels (LCLs) height between pre-monsoon and monsoon conditions
The pre-monsoon boundary layers (BLs) is typically topped by a strong inversion that is accompanied by a decrease of relative humidity (RH) within a few hundred metres
Summary
The convective environment over the Indian subcontinent changes significantly from hot and dry pre-monsoon conditions to cooler and wetter monsoon conditions. The active/break monsoon conditions are characterized by lower/higher boundary layer heights Transition from pre-monsoon to monsoon conditions is associated with an increase in soil moisture (Sathyanadh et al, 2016) and with the change of the partitioning of the surface energy flux into its sensible and latent fluxes. One might expect different boundary layer characteristics in surface-forced pre-monsoon and monsoon conditions due to different Bowen ratios for the two environments. The free-tropospheric conditions, impacted by larger scale atmospheric dynamics, may affect the strength of convection as measured by parameters such as CAPE, LCL height, or maximum pseudo-adiabatic parcel buoyancy. Since no surface flux information is available for the region where long period soundings were obtained, we use data collected at another location in the rain shadow area to document changes in the Bowen ratio and LCL height between pre-monsoon and monsoon conditions.
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