SIMULATION OF ATMOSPHERIC BOUNDARY LAYER

Abstract

It is important to study Atmospheric Boundary Layer (ABL) to understand the aeronautical meteorology, air pollution scenarios, and regional climate. Turbulence is the most critical activity in ABL, especially in the Convective Boundary Layer (CBL). Turbulence controls the wind pattern and temperature profile in the ABL. Turbulence may be generated mechanically and thermally, and it is determined that convection is one of the important factor, along with radiation, advection and diffusion, in the variation of turbulence in ABL. With increasing air pollution level and changing climate, better understanding on CBL and the improving reliability of prediction for pollutant concentration in terms of height of the CBL is necessary. Different dimensionalities of the model are reviewed with various surface conditions, atmospheric stability, atmospheric stratifications, and modeling approaches. On the basis of the above discussion, the following remarks are made. 1-D ABL modeling can be performed in the case when advection terms are found to be negligible. In 1-D model, the popular BRN based approach is found statistically comparable with physical process - turbulence closure based approach. 3-D ABL modeling requires a lot of computational means and a complex calculations that is why there is a compromise on 2-D modeling. However, physical processes can be better understood and problem can be accurately solved while going from 2-D to 3-D simulations. 2-D and 3-D RANS based modeling are used widely in atmospheric applications because RANS converts unsteady turbulent phenomenon to steady one and greatly saves the computational power. However, RANS based turbulence models are problem-specific and dependent on the flow boundary and inlet conditions, according to which turbulent model parameters have to be tuned best to simulate. Theoretically, 2-D turbulence modeling is not appropriate since eddies are the 3-D structures so to solve them 3-D simulation is the best solution. 2-D LES perhaps do not pretend for the accuracy of results for ABL. The 2-D simulations are quick in testing the parameters and features of employed models and also if the true behavior of the model can come up from 3-D simulations. It has been observed in 2-D LES that in absence of obstructions to the flow does not lead to formation of 2-D turbulence, e.g. bluff bodies. In flat terrain cases, 2-D studies suggest that turbulence did not develop even with superimposed perturbations with the initialization command. 3-D LES is a good option since it solves the larger eddies directly and the smaller eddies due to turbulence using SGS model. However, LES requires the great computational means since finer grid size is required at the region closed to the wall. For this, nowadays, DES is used whereby RANS simulations are performed nearby wall region and LES is used at the zones far from the wall. Scale-dependent dynamic SGS model of LES was used ix successfully for various conditions, viz., neutral atmospheric flow over heterogeneous surfaces and SBL, etc. Also, for turbulent stably-stratified flow over a steep 2-D hill immersed in ABL, sub-grid LES model was able to capture the flow having strong shear and thermal stratification. A numerical study has been carried out to assess changes in Atmospheric Boundary Layer (ABL) height and sub-surface conditions under changing hydro-climatic conditions. A copula-based statistical analysis is performed that shows ABL height determined by Garratt expression and Bulk Richardson Number approach are comparable. The sensitivity analysis of height of ABL against critical parameters reveals that surface condition (soil temperature) is more critical than atmospheric condition (net radiation) in forcing change on height of ABL in summer. Wavelet ARIMA is developed to forecast the ABL depth trends in 10 stations in India. Predicted ABL depth is comparable and in good agreement with the calculated ABL depth using the Bulk Richardson Number (BRN). To understand the spatial variability of ABL depth, a Variogram analysis is also performed taking 30 stations over Indian sub-continent. We found the ABL height is correlated within the lag distance of 150. Based on the variogram parameters, the spatial distribution of ABL depth is plotted using ordinary Kriging. A Coupled Atmospheric Turbulence and Dispersion Model is developed for Determining Vertical Profile of Gaseous Pollutants viz. NOx, SO2 across India. The spatial mapping of NO2 and SO2 concentration at various heights is also done and concentration hotspots are identified. The numerical study is further extended to a two-dimensional atmospheric boundary layer model to simulate numerically the observed temperature profiles. Wind rose diagram is plotted to determine the predominant wind direction in which simulation is performed.