Investigation of Dynamical Aspects of the Atmosphere Over Central Himalayas Using ST Radar and Other Observations

Abstract

The swath of the Himalayan ranges spans the vast diversity of climatic patterns varying from subtropical to boreal. This interaction between the tropical and extratropical weather systems combined with substantial modifications induced by highly elevated and complex topography frequently leads to weather extremes like cloud bursts, hailstorms, flash floods in the region. However, the paucity of an observational network over the Himalayan region has restricted the analysis of these weather extremes up to synoptic scale only. It is essential to acquire systematic observations at the finer scales for better analysis of these weather extremes and improvement in their forecast skill. In the past three decades, VHF radars have contributed significantly to the study of the lower and middle atmosphere and proved to be an indispensable tool to study the mesoscale convective systems over tropical and middle latitude regions. In this context, a Stratosphere Troposphere (ST) radar has been established at the high altitude subtropical site of Nainital (29.4oN, 79.5oE; 1793m amsl) and operates at a high VHF frequency of 206.5 MHz. It has the capability to provide three-dimensional profile of winds up to 20 km at a fine vertical resolution of 37.5 m/75 m/150 m and a typical temporal resolution of one minute. The quality of the wind profiles from ST radar has been validated against the co–located balloon–borne radiosonde wind profiles resulting in a good correlation of 0.91 and 0.81 for zonal and meridional wind velocities respectively. The probing of dynamics of the Himalayan atmosphere using ST radar began with the first ever estimation of turbulence parameters over this region. The early experiments using simultaneous observations from radar and radiosonde have estimated turbulent kinetic energy dissipation rates, eddy diffusivity coefficients due to thermal and momentum fluctuations as well as inner and outer scales of turbulence regime. It was inferred that the strength of turbulence over this region was greater than those estimated at the tropical site of Gadanki by the Indian MST radar. Further, the seasonal variation of these turbulence parameters were analysed. The kinetic energy dissipation rate and eddy diffusivity coefficient over the site varies from 10-2.4 m2s-3 to 10-5 m2s-3 and 100.2 to 102.8 m2s-1 respectively. In the lower troposphere persistent layers of enhanced turbulence were observed and found to be correlated with the trapping of humidity within stable layers and Kelvin Helmholtz instability. The capability of the VHF radar to estimate refractivity fluctuations has been utilised to retrieve optical turbulence for astronomical observations. Seeing and astroclimatic parameters are estimated integrated in the 2 – 20 km region. The best median seeing of 0.4″ was observed for the winter season followed by 0.6″ in the post – monsoon season. Pre monsoon and monsoon seasons were the most turbulent with median seeing of 0.8″ and 0.7″ respectively. The largest contribution of seeing are observed from the 2 – 5 km region followed by the 12 – 16 km region. Among the integrated astroclimatic parameters the isoplanatic angle ranges from 0.8″ to 2.5″ and wavefront coherence time varies from ~ 3 – 5 ms. The mean scintillation rate over the site varies from ~ 10 – 45 % of the time. Attributes of the pre – monsoon and summer monsoon deep convective systems were delineated by observing the hailstorm and heavy rainfall events during these seasons. Pre – monsoon convective hailstorm was characterised by the intense updrafts and downdrafts of the order of 15 ms-1 in the mid – upper troposphere and lower troposphere respectively. Stable layer structures associated with tropopause seems to weaken during the intense convective activity. This penetrative convection generated upward propagating gravity waves in the upper troposphere and lower stratosphere region. Associated with gravity wave, a ten times enhancement in the exchange of momentum flux has been estimated both in upward and downward directions. A deep convective system in the monsoon season was observed with moderate vertical velocities (~ 4 ms-1) and a trailing stratiform region identified by the double bright band structure in radar reflectivity. Eventually, it is planned to incorporate the polarimetric observations of nearby IMD X band Doppler weather radar at Mukteshwar to examine the microphysical characteristics of convective clouds and the assimilation of radar observations for the simulation of thunderstorms in mesoscale models.