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Pramod Kumar
and
Maithili Sharan

Abstract

An analysis in a plane of the stability parameters ζ = z/L (where z is a height above the ground surface and L is the Obukhov length) and bulk Richardson number Ri B is proposed to examine the applicability of Monin–Obukhov similarity (MOS) theory in stable conditions. In this analysis, the data available from two different experimental datasets [(i) Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) and (ii) Cardington] are divided into four quadrants. An unexpected behavior of decreasing ζ with increasing Ri B was observed with both datasets in quadrant II characterized by ζ < 1 and Ri B > 0.2 and in quadrant IV with ζ > 1 and Ri B < 0.2. This is in contrast to a commonly expected monotonically increasing behavior between ζ and Ri B . It is shown that the MOS theory is consistent for computing the surface fluxes corresponding to the data points lying in quadrants I (with ζ > 1 and Ri B > 0.2) and III (with ζ < 1 and Ri B < 0.2), whereas it may not be applicable for the points in quadrants II and IV. Thus, a breakdown of the relationship between observed ζ and Ri B with growing stability in these quadrants may limit the applicability of MOS theory in stable conditions. Since quadrant IV has very few data points, the applicability of MOS theory needs to be substantiated further with the availability of sufficient data points in this regime.

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Piyush Srivastava
and
Maithili Sharan

Abstract

The turbulent data over a tropical region are utilized to analyze the observational behavior of the drag coefficient with respect to wind speed U and the stability parameter in convective conditions. The drag coefficient is observed to follow the power-law profile with respect to U, with large values in low winds and relatively lower values with moderate-wind conditions. Depending on the stability regimes, regression curves for with U are proposed. The variation of with is bounded by a curve. This curve first shows increasing behavior with until it reaches a peak at and then decreases with increasing instability. A mathematical analysis based on Monin–Obukhov similarity (MOS) reveals that increases monotonically with increasing instability. This suggests that MOS theory is able to capture the increasing nature of in weakly to moderately unstable conditions. However, it is unable to explain the observed decreasing behavior of with in moderately to strongly unstable conditions in the tropics within the framework of commonly used similarity functions.

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Maithili Sharan
and
Piyush Srivastava

Abstract

The behavior of the heat flux H with respect to the stability parameter (=z/L, where z is the height above the ground, and L is the Obukhov length) in the unstable atmospheric surface layer is analyzed within the framework of Monin–Obukhov similarity (MOS) theory. Using MOS equations, H is expressed as a function of and vertical surface-layer potential temperature gradient . A mathematical analysis is carried out to analyze the theoretical nature of heat flux with the stability parameter by considering the vertical potential temperature gradient as (i) a constant and (ii) a power-law function of heat flux. For a given value of H, two values of associated with different stability regimes are found to occur in both the conditions, suggesting the nonuniqueness of MOS equations.

Turbulent data over three different sites—(i) Ranchi, India; (ii) the Met Office’s Cardington, United Kingdom, monitoring facility; and (iii) 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99; United States—are analyzed to compare the observed nature of H with that predicted by MOS. The analysis of observational data over these three sites reveals that the observed variation of H with is consistent with that obtained theoretically from MOS equations when considering the vertical temperature gradient as a power-law function of heat flux having the exponent larger than 2/3. The existence of two different values of the stability parameter for a given value of heat flux suggests that the application of heat flux as a boundary condition involves some intricacies, and it should be applied with caution in convective conditions.

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Piyush Srivastava
and
Maithili Sharan

Abstract

In this study, an attempt has been made to analyze the possible uncertainties in the parameterization of surface fluxes associated with the form of nondimensional wind and temperature profile functions used in weather and climate models under convective conditions within the framework of Monin–Obukhov similarity theory (MOST). For this purpose, these functions, which are commonly known as similarity functions, are classified into four categories based on the resemblance in their functional behavior. The bulk flux algorithm is used for the estimation of transfer coefficients of momentum and heat using four different classes of similarity functions. Uncertainty in the estimated values of fluxes is presented in the form of deviation in the predicted values of momentum and heat transfer coefficients and their variation with the Monin–Obukhov stability parameter. The analysis suggests that a large deviation in the values of estimated fluxes might occur if different forms of similarity functions are utilized for the estimation of surface fluxes. Recommendations are made for the form of similarity function for momentum based on the analysis of 1-yr-long turbulence observations over an Indian region. The study suggests that there is a distinct need to carry out a careful analysis of turbulence data in free-convective conditions for determining a consistent functional form of the similarity functions to be utilized in the atmospheric models universally.

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Maithili Sharan
and
S. G. Gopalakrishnan

Abstract

Five local K-closure formulations and a TKE closure were incorporated in a one-dimensional version of the Pielke’s model, and a comparative evaluation of the closure schemes was made for strong and weak wind stable boundary layer (SBL). The Cabauw (Netherlands) and EPRI-Kincaid site (United States) observations were used for this purpose. The results indicate that for the strong wind case study, the profiles of turbulent diffusivities in terms of shape, depth of significant mixing, and the height above the surface where diffusion reaches a maximum are more or less the same for the various closure schemes. Only the magnitudes of mixing produced by various closure schemes are different. This difference produced by various closure formulations causes minor but noticeable changes in the mean wind field and thermodynamic structure of the model SBL. However, although the profiles of turbulent diffusivities become weak, variable, and poorly defined under weak wind conditions, the mean profiles become insensitive to the differences in the diffusion that arise due to various parameterization schemes. Apart from the TKE closure scheme, Estournel and Guedalia simple local closure scheme is able to produce the essential features of the SBL quite well.

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Jagabandhu Panda
,
Maithili Sharan
, and
S. G. Gopalakrishnan

Abstract

Extensive contrasts of land surface heterogeneities have a pivotal role in modulating boundary layer processes and consequently, the regional-scale dispersion of air pollutants. The Weather Research and Forecasting (WRF) modeling system has been used to analyze the regional-scale boundary layer features over northern India. Two cases, 9–11 December 2004 and 20–22 May 2005, representing the winter and summer season, respectively, are chosen for the simulations. The model results have been compared with the observations from the India Meteorological Department (IMD) and Wyoming Weather Web data archive over three cities: Delhi, Ahmedabad, and Jodhpur. The simulations show that the thermal stratifications and the associated wind pattern are very well supported by land surface characteristics over the region. The results signify that the underlying land surface along with the prevailing hemispheric-scale meteorological processes (synoptic conditions) is the driver of the simulated patterns. The study implies that thermally driven regional circulations play a major role in the transport of particulate matter from the Thar Desert to Delhi and its neighboring regions during summer.

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Maithili Sharan
,
S. G. Gopalakrishnan
, and
R. T. McNider

Abstract

Turbulence in stable conditions is local, that is, it is locally defined by small eddies. A local formulation for σ w based on a level 2 approximation of is proposed. The proposed formulation is able to describe the nondimensional profile of (σ w /U∗)2 against Z/H consistently when compared with the Minnesota observations, where H is the height of the turbulent stable boundary layer.

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Maithili Sharan
,
Anil Kumar Yadav
, and
M. P. Singh

Abstract

Short-range diffusion experiments conducted at Idaho National Engineering Laboratory in light-wind stable conditions have been simulated using a mathematical model that combines plume segment and Gaussian puff approaches. The model is shown to describe reasonably well the situations such as in run 12 wherein the plume resulted in nonzero concentrations only on a 100-m arc and not on 200- and 400-m arcs in a particular sector. The results obtained have been found to be encouraging in terms of accuracy of agreement with the observed values.

Results obtained from the model have been evaluated using some statistical measures and compared with two steady-state models based on constant and variable diffusivities.

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S. G. Gopalakrishnan
,
Maithili Sharan
,
R. T. McNider
, and
M. P. Singh

Abstract

The role of radiation and turbulence was studied in a weak wind nocturnal inversion layer using a one-dimensional model. In contrast to a strong wind stable boundary layer where cooling within the surface inversion layer is dominated by turbulence, radiative cooling becomes larger than turbulent cooling under weak wind conditions. Further, the surface inversion layer was found to grow all through the night under weak wind conditions, whereas it attained a near equilibrium in the case of strong wind.

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Maithili Sharan
,
S. G. Gopalakrishnan
,
R. T. McNider
, and
M. P. Singh

Abstract

A three-dimensional mesoscale model was used to understand the meteorological conditions and the influence of the terrain on the local flow pattern during the Bhopal methyl isocyanate (MIC) gas leak. The study reveals that under the prevailing conditions of weak wind and strong stability the lakes in Bhopal influenced the local circulation significantly and caused northwesterly flow near the surface. The modified flow pattern resulted in the transport of MIC into the city area of Bhopal. However, with the increase in the ambient synoptic wind, the role of the lakes was found to diminish. Further, the other topographical features such as the hillocks in and around the city and the gently rolling terrain toward the southeastern sector of the city seem to have played a secondary role in influencing the meteorological conditions on the episodic night.

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