Lower Atmospheric Studies Using ARIES StratosphericTropospheric (ST) Radar in Conjunction with In-situ and Auxiliary Observations

Abstract

The lower atmospheric processes impose a number of challenges, such as fundamental understanding of small-scale processes and extreme weather events in the area of atmospheric physics. Atmospheric turbulence is one of the important microscale processes due to its characteristics of efficient mixing of air having different properties within a time scale of less than 1 sec to typically 1 hour and corresponding length scales from 1 mm to 1 km. Consequently, it influences the dynamics and thermodynamics of precipitating systems, transport of water vapor and dust, heat transfer, as well as impacts telecommunications, remote sensing, and aviation, etc. To understand such processes, the study of variation in the vertical structure of turbulence characteristics and their scale sizes over different regions of the globe is a need of the hour. Further, over the mountainous terrain, the intensive interaction of atmospheric circulation on different spatial and temporal scales results in a combination of multiple transport processes such as turbulent motions, thermally driven circulations, terrain-forced winds, and moist convection. Therefore, mountainous regions serve as a natural laboratory to explore the role of orography in the various aspects of turbulence characteristics, diurnal variation of the atmospheric boundary layer, and different precipitating systems. Weather balloon experiments using radiosonde are one of the oldest in-situ measurement techniques to retrieve the vertical profiles of the turbulent characteristics in the lower atmosphere. Also, wind profiler radars operating at various frequencies have been extensively utilized throughout the globe for continuous monitoring of microscale and mesoscale weather phenomena and estimating their corresponding physical mechanisms and processes. The central objective of the present thesis is focused on the study of lower atmospheric dynamics and associated meteorological processes, considering the importance of the Himalayan mountainous site in terms of meteorological and climatic conditions. The dynamical features and variability of the turbulence characteristics and scale-size calculations at different temporal scales are carried out using radiosonde observations in the lower atmosphere up to 20 km. Turbulence characteristics are specifically focused on the variation of energy dissipation rate (ε), eddy diffusivity (K), refractive index structure parameter (Cn2), inner scale (lo), and outer scale (LB) over a mountainous site, Manora Peak Nainital. Results showed the lowest dissipation rate (− 4.5 m2 s-3) during the post-monsoon season while constant dissipation rate (~− 4 m2 s-3) below 5 km in winter. A constant variation of log K (~ − 0.01 m2 s-1) is observed above 8 km during all the seasons except during the monsoon, where larger buoyancy scales (>100 m) are found in the altitude range of 8–14 km. This study also reveals that orographic-induced local circulations can significantly control the large turbulence intensity even up to the deeper layer of 2 km from the surface during a weak wind regime as compared to a strong wind regime where large turbulence intensity is confined within 0.5 km. An attempt is also made in this thesis to investigate the lower troposphere dynamics within 2.5 km altitude by estimating the diurnal variation of turbulence characteristics along with the atmospheric boundary layer height (BLH) during two contrasting seasons, winter and spring, using 1290 MHz radar wind profiler measurements. Analysis showed deep convective (CBL ~ 750 m) and stable boundary layer (SBL ~ 500 m) with average values of log Cn2 and log ɛ ≈ -12 m-2/3 and -2 m2 s-3, respectively, in spring. During winter, BLH varies from shallow SBL of about 270 m to the maximum 630 m daytime CBL with mean values of about 405 ± 125 m. One major cause of deep SBL in spring is found under the weak stratification and weak flow regime case, where shear-induced turbulences are responsible for robust vertical mixing. Further, from the variation of BLH with aerosol loading and background atmospheric conditions (stability and wind shear), different pollution transport mechanisms are suggested over the region during the seasons. Additionally, the preliminary analysis of the data from only the wind profiler ARIES ST radar (ASTRad) operating at 206.5 MHz frequency and installed over the central Himalayan site is performed by validating the estimated wind profiles with concurrent radiosonde profiles. Additionally, dynamical and microphysical aspects of two cases of precipitation events during monsoon (Case-I) and western disturbances (Case-II) are investigated over the Manora Peak Nainital using ASTRad data and the Weather Research and Forecasting (WRF) model simulations. The vertical structure of precipitation systems is analyzed in this study using three Doppler moments (equivalent Reflectivity dBZe, Doppler velocity, and Spectral width) estimated from the ASTRad observations. The investigation of the dynamical structure of these events suggested the role of orography in the formation of a precipitation system for Case-II. Microphysical analysis using WRF simulations revealed Case-I as a mixed-phase and Case-II as a cold-phase microphysical process resulting in liquid precipitation during the summer monsoon while both solid and liquid precipitation during the western disturbance. Overall, all objectives of this thesis work are based on lower atmospheric dynamical processes over the central Himalayas, which have been investigated here using the wind profiler radar, radiosonde, WRF simulations, and other auxiliary datasets over the central Himalayas.