Towards Understanding Planetary Boundary Layer Regimes in Relation to Indian Summer Monsoon

Abstract:

Atmospheric boundary layer processes play crucial role in modulating weather and climate of the earth. Information on the planetary boundary layer characteristics are important in various aspects. Analyses presented in the study are mainly carried out using Modern Era Retrospective analysis for Research and Applications (MERRA) reanalysis data products. Hourly values of PBL height, soil moisture, fluxes, cloud cover, and atmospheric stability in the region 5-38° N, 60 – 100^o E are used. The MERRA PBL heights are validated with PBL heights calculated using GPS RO atmospheric profiles during 2007-09 and radiosonde observations in order to assess the suitability of MERRA data for the PBL analysis. The radiosonde data used are from two sources: (i) routine radiosonde observations conducted by India Meteorological Department over the Indian subcontinent and (ii) additional radiosonde observations conducted by the Indian Institute of Tropical Meteorology as a part of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment during theSW monsoon, 2009. Spatio-temporal variations of PBL height in relation to different phases of monsoon and intra-seasonal variations are investigated in detail. Seasonal variations show a deeper premonsoon boundary layer and a shallower monsoon boundary layer, with large spatial variations. The PBLH variations over inland locations are found to be in good agreement with onset and progress of monsoon rainfall and associated soil moisture variations. The active and break spell monsoon PBL heights analyzed using 20-year PBL data showed deeper PBLHs during break periods compared to active period. Based on the maximum PBLH and growth characteristics, different regimes are identified which are mainly controlled by soil moisture/ evaporative fraction, but further influenced by stability of the surface, cloudiness, wind shear, etc. resulting in complex PBL regimes in relation to monsoon. The maximum PBLH, growth rate, time of occurrence of maximum PBLH, duration of large PBLHs, and their relation to surface fluxes are found to be significantly different for wet and dry periods, and for inland and coastal locations. Temporal scales associated with PBLH variation and their relationships with different controlling factors are investigated with the help of wavelet analysis.