Observation and Modelling Studies on Meteorology Over Central Himalaya

Abstract

The sensitive ecosystem of the Himalayas, particularly the central Himalayan (CH) region, is experiencing enhanced stress from anthropogenic forcing that requires adequate atmospheric observations and an improved representation of the Himalayas in the models. However, the accuracy of atmospheric models remains limited in this region due to highly complex mountainous topography. The convective-permitting scale simulations by the limited area models, e.g., the Weather Research and Forecasting (WRF) model, are broadly used for a wide range of applications, including climate projections, weather predictions, and air quality forecasts. Such state-of-the-art models fill the gap in data scarcity over high mountain areas like the Himalayas. Their evaluation with observations and fine-tuning increases the certitude of the generated regional scale information. This thesis work first delineates the effects of spatial resolution on the modeled meteorology and dynamics over the CH by utilizing the Weather Research and Forecasting (WRF) model and extensively evaluated against the ground-based, remote-sensing, and re-analysis products. This analysis highlights that the WRF model set up at finer spatial resolution can significantly reduce the biases in simulated meteorology, and such an improved representation of CH can be adopted through domain feedback into regional-scale simulations. Model simulation implementing a high-resolution (3s) topography input (SRTM) improved the model performance immensely. Apart from the model resolution and unresolved topographic features, model physics, which is generally known as parameterization schemes, plays an important role in stimulating accurate simulation. After the successful evaluation of the model at various grid resolutions, the impact of the model physics schemes related to the sub-grid scale turbulence (six boundary layer schemes) and microphysical processes (seven microphysics schemes) are also evaluated over the central Himalaya region. The evaluation of the boundary layer schemes within the Weather Research and Forecasting (WRF) model is nearly non-existent over complex terrains of the Himalayan region. To achieve this, six different planetary boundary layer (PBL) schemes Yonsei University (YSU), Mellor-Yamada-Nakanishi-Niino level 3 (MYNN3), Shin-Hong Scale-aware (SHSS), Mellor-Yamada-Janjić (MYJ), Asymmetric Convective Model version 2 (ACM2), Quasi Normal Scale Elimination (QNSE) in the WRF model are evaluated against observational datasets. The evaluation is carried out for spring during clear-sky conditions using surface-based, balloon-borne, and radar wind profiler (RWP) observations. YSU and SHSS schemes are found to be the best schemes in reproducing the meteorology over the region. Model performance varies significantly with the employed PBL schemes in simulating the localized boundary layer height (LBL), and all PBL schemes can capture the daytime maxima of LBL heights with some overestimation, while YSU and SHSS schemes are closer to the observations followed by MYJ. The competing effect between the synoptic and local circulation was found to be a vital controlling factor in the boundary layer evolution over the mountain peak and affected the model performance. Further, cloud microphysics schemes are used to simulate the spatio-temporal distribution of the precipitation, especially at the convection-permitting scales, and are generally one of the major sources of uncertainty in the model. To test it, seven different microphysics schemes (Lin, Eta, WSM6, WSM5, WDM, Thompson, and Morrison) are evaluated during a heavy precipitation event (HPE) over the CH. The model successfully reproduced structural and dynamical characteristics associated with the extreme event, including water vapor transport and meteorological fields. Most of the microphysics schemes are found to simulate the temperature and specific humidity reasonably, except the Eta schemes. The precipitation pattern is found to be very sensitive to both the grid resolution and microphysics schemes. Further, model results suggested that the interaction between the moisture convergence system and orography caused the HPE. Additionally, the impact of the enhanced aerosol loading on the surface layer characteristics has been investigated using micrometeorological measurements during a severe dust storm that resulted in a five-fold enhancement in the fine particulate matter (PM2.5) over the central Himalaya. Interestingly, dust storms also had significant impacts on turbulent kinetic energy (2.9–9.6 m2 s−2), vertical momentum flux (0.9–3.3 Nm−2), and sensible heat flux (34.8 to −33.9 Wm−2), suggesting turbulent mixing of aerosols and cooling near the surface over the Himalayas. This analysis highlights that large-scale dust storms can profoundly impact air quality and surface layer characteristics by perturbing micrometeorology over the CH. Overall, this thesis work is based on meteorological studies, which have been carried out using the WRF model and observational datasets over the central Himalayas.