Monsoon Trough Boundary Layer Experiment Research Articles

Simulation of Atmospheric Boundary Layer Characteristics during Indian Summer Monsoon using Observations from Monsoon Trough Boundary Layer Experiment at Jodhpur, India

The diurnal structure of the boundary layer during Indian summer monsoon period is studied using a one-dimensional meteorological boundary layer model and the observations collected from the Monsoon Trough Boundary Layer Experiment conducted in 1990 at Jodhpur, India. The model was initialized with the observed temperature profiles at 0530 LST on 17 July, 1990 at Jodhpur and was run for 26 hours. The study is carried out with a geostrophic wind speed of 9.5 m s−1 corresponding to the strong wind simulation. The mean thermodynamic and wind structure simulated by the model are in good agreement with those observed from 30 m tower. The computed surface layer characteristics such as the surface fluxes, TKE and standard deviations of velocity components are found to be reasonably in good agreement with those based on turbulence measurements. The shear and buoyancy budget computed from the model are also compared with the turbulence measurements. The integrated cooling budget in the nocturnal boundary layer is examined.

Interrelationship between cloud cover and sensible heat flux over land during MONTBLEX-1990

Micro-meteorological tower observations of MONTBLEX (Monsoon Trough Boundary Layer Experiment)-1990, combined with routine surface observations at Jodhpur in the dry convective sector of Indian summer monsoon trough are used to examine the interrelationship between total cloud cover (TCC) and surface sensible heat flux (SHF) during the summer monsoon of 1990. A significant inverse relationship between TCC and SHF is found during various Intensive Observation Periods of the experiment. This relationship holds for the various methods of estimation of SHF.

Identification of a surface layer structure and analysis of humidity data in two weather situations at Jodhpur (26°18′N, 73°04′E), India, during MONTBLEX 1990

The Monsoon Trough Boundary Layer Experiment held in 1990 was a multi-institutional effort to probe the atmospheric boundary layer over the monsoon trough over northern India. For this experiment, four micrometeorological towers were set up at four different locations along the normal position of the trough. One such tower of 30m height was located at Jodhpur (26‡18′N, 73‡04′E), Rajasthan. The fast and slow response data available during the experiment have been used in the present study to determine a suitable layer-structure of the surface layer for evaluation of sensible heat flux using the multilayer hypothesis of Kramm (1989).

Atmospheric surface layer spectra over a tropical desert station during Indian summer monsoon

Characteristics of turbulence have been studied using the observations of wind components (u, v and w) and temperature (T) collected by fast response sensor the sonic anemometer (8.33 Hz) during the Monsoon Trough Boundary Layer Experiment in 1990. The analysis showed that the spectra of wind and temperature obeyed the Monin-Obukhov scaling. The spectral curve showed a slope of –2/3 in high frequency portion which indicated the existence of inertial subrange. The diurnal variation in the eddy sizes were brought out.

Meso-scale convergence rainfall and surface fluxes during the Indian summer monsoon

Meso-scale convergence and vertical velocity were computed using Bellamy technique from rawindsonde and pibal data obtained from three stations. These stations formed the vertices of a meso-scale triangle around Kharagpur (22.3° N, 87.2° E) - a region of moist convection located near the eastern end of the monsoon trough, where the monsoon trough boundary layer experiment (MONTBLEX), was conducted during the Indian summer monsoon 1990. We find that the total rainfall over the meso-triangle depends on the proximity and intensity of the large-scale pressure system forming in the head Bay and moving W/NW towards the land during the onset and active phase of the monsoon. When the large-scale flow showed cyclic circulation extending to mid-troposphere, the rainfall over the triangle does not seem to be affected by the meso-scale divergence. However the rainfall at Kalaikonda (KKA) -an airforce station near Kharagpur (KGP) correlates with meso-scale divergence/convergence. Surface heat flux estimated using sonic anemometer at 8m AGL at KGP was large during meso-scale convergence. It increased with wind speed while the momentum flux showed large variation.

Prevailing weather conditions during the monsoon period of 1990 over Jodhpur region

The Monsoon Trough Boundary Layer Experiment (MONTBLEX) was an intense multi-institutional effort to probe the atmospheric boundary layer (ABL) over the monsoon trough region of the Gangetic Plains in 1990. For this experiment, four micrometeorological towers were set up at four different locations along the normal position of the trough axis over Northern India. One such tower of 30 m height was located at Jodhpur (26.3° N, 73° E), Rajasthan in the campus of Central Arid Zone Research Institute. This tower had six levels of instrumentation at 1m, 2m, 4m, 8m, 15m and 30m heights, which are termed here as Ist, 2nd, 3rd, 4th, 5th and 6th level respectively. Both fast as well as slow response sensors at various levels of the tower (Rudrakumar and Prabhu, 1991) were utilized. In the present study, we have presented the results evaluated both from fast and slow response data, 6 levels of cup anemometers and wind vanes, 4 levels of slow response temperature sensors and 2 levels of fast response temperature sensors are presented.
 
 The purpose of the present work is to find out the suitable layer for evaluating sensible heat flux for the Jodhpur region with the help of multilayer hypothesis.

Some Atmospheric Surface Layer Characteristics over Varanasi from MONTBLEX-90

—An attempt has been made to study the atmospheric surface layer characteristics such as Richardson number (Ri). Monin-Obukhov length scale (L), friction velocity (U * ), friction temperature (θ * ), roughness length (Z 0 ), turbulent kinetic energy (TKE), ratio of eddy conductivity to eddy diffusivity (K m /K h ) over a semi-moist convective regime. Data which were collected at Varanasi (25°18′N, 83°E) as part of the experiment known as MONTBLEX-90 (Monsoon Trough Boundary Layer Experiment) during the summer monsoon season was used in the present study. The variation of the above parameters with stability has been discussed. The differences within the surface layer are also pointed out. Some broad features are found to coincide with that of Businger et al. (1971). The heterogeneity and the anisotropic turbulence typical of monsoon tropical atmosphere are shown to be responsible for the deviations noticed within the surface layer.

Wavelet Analysis of Intermittent Turbulent Transport in the Atmospheric Surface Layer over a Monsoon Trough Region

The structure of the turbulence in the atmospheric surface layer over a monsoon trough region has been studied using structural analysis based on wavelet transform. The observational site is located at the eastern (wet) end of the monsoon trough region, characterized by high moisture in the atmospheric surface layer. On the average relative humidity varied from 70% to 100% during the experiment. The wind and temperature data, collected at Kharagpur (22°25' N, 87°18' E) at six observational hours of a day in June 1990 during the Monsoon Trough Boundary Layer Experiment (MONTBLEX), have been utilized in the study. The wind and instantaneous momentum flux time series were decomposed into 12 scales using the Haar wavelet transform. The eddies exhibited a large temporal variability generating intermittency in the energy and flux distributions. A criterion based on the isotropy has been suggested for separating the large eddies from the small eddies. At the separation scale the isotropy coefficient drops sharply. It is shown that the intermittency in the small eddies resulted from the spatial variation of energy, and deviation of velocity statistics from the Gaussian distribution known as flatness. The deviation from the -5/3 power law has been attributed to the increased mean values of, (i) the coefficient of variation of energy, and (ii) the flatness factor, in the inertial subrange. The decomposition of the instantaneous momentum flux time series reveals that the major contribution to the total flux arises from the large eddies. The quadrant analysis of the momentum flux shows that ejections and sweeps account for a substantial part of the total flux, and quantifies the relative importance of the various spatial scales that contribute to the transport of momentum.

Estimation of drag coefficient at low wind speeds over the monsoon trough land region during MONTBLEX‐90

Using observational data acquired during the Monsoon Trough Boundary Layer Experiment (MONTBLEX‐90), the drag coefficient CD is estimated over two land stations, Jodhpur (arid region) and Kharagpur (moist region), by eddy correlation of velocity data from sonic anemometers. At low winds CD varies with wind speed according to a power law, which at Jodhpur takes the form CD=0.04 Ū10−1.06 (Ū10 being the mean wind speed at 10 m). The large values of the drag coefficient at low winds are associated with both near‐neutral and strongly unstable situations. On the other hand, the CD values at Kharagpur are less than at Jodhpur by nearly 50% for wind speeds > 3ms−1, and cannot be explained by the different roughness lengths at the two sites.

Estimation of surface heat flux and inversion height with a Doppler acoustic sounder

In this paper, acoustic sounder (sodar) derived vertical velocity variance (σw2) and inversion height (Zi) are used to compute the surface heat flux during the convective activity in the morning hours. The surface heat flux computed by these methods is found to be of the same order of magnitude as that obtained from tower measurements. Inversion heights derived from sodar reflectivity profiles averaged for an hour are compared with those obtained from the σw2/Z profile. Variation of σw2 in the mixed layer is discussed. The data were collected during the Monsoon Trough Boundary Layer Experiment 1990 at Kharagpur. The analysis is made for four days which represent the pre-monsoon, onset, active and relatively weak phases of the summer monsoon 1990. The interaction of the ABL with the monsoon activity is studied in terms of the variation of inversion height, vertical velocity variance and surface heat flux as monsoon progresses from June to August.

Turbulent heat flux variation over the Monsoon-Trough region during MONTBLEX-90

Tower data collected during the Monsoon-Trough Boundary Layer Experiment (MONTBLEX-90) have been analysed to understand the observed structure of the surface layer over an arid region (Jodhpur) and a moist region (Kharagpur) during active and weak phases of the 1990 southwest monsoon. Turbulent heat and momentum fluxes are estimated by the eddy correlation method using sonic data. The turbulent momentum flux at both Jodhpur and Kharagpur was larger when the winds were stronger, reaching a maximum of the order of 0.5 N m −2 on 5 and 6 August when a low pressure system was located over the region. The heat flux at Jodhpur is high during weak monsoon days, the maximum being 450 W m −2, whereas during active days the flux never exceeds 200 W m −2. At Kharagpur, the flux does not vary significantly between active and weak monsoon days, the maximum in either phase being 160 W m −2. At Jodhpur, there is significant contrast in the near-surface air temperature, being higher during weak monsoon days as compared to active days. Cloud cover did not vary significantly in both the regions. The turbulent heat flux variation at both the sites appears to be correlated mainly with soil mixture, and less sensitive to cloud cover.

Study of horizontal scales of motion observed over Kharagpur during MONTBLEX-1990

Characteristic wavelengths for theu andv components of wind are studied using the Monsoon Trough Boundary Layer Experiment (MONTBLEX) data obtained from a Doppler Sonic Detection and Ranging System (sodar) over the land station Kharagpur (near sea-coast). The principal stability parameter (Z i/Lo) is used to infer the behaviour of the non-dimensional form of the characteristic wavelength (L H) within the entire stability range occurring during the sounding periods. This is compared with GATE - 1974 results (over the sea surface) published by Fitzjarrald (1978).

MONTBLEX data archival centre

A co-ordinated project Monsoon Trough Boundary Layer Experiment (MONTBLEX) to study the atmospheric boundary layer in the monsoon trough region was taken up during 1990. 30-m high instrumented towers were erected at Kharagpur, Banaras, Delhi and Jodhpur. Sophisticated equipment like Doppler sodar and Kytoon were used at Kharagpur. Sodars were exposed at Calcutta, Delhi and Jodhpur. ORVSagarkanya cruises were arranged in the Bay of Bengal. The India Meteorological Department set up new surface and radiation observatories and released special radio-sonde, pilot balloons. Using the above mentioned platforms, data were collected during April - September 1990 and after proper editing the entire data were archived at the Indian Institute of Tropical Meteorology, Pune. The DST-MONTBLEX data bank was started at IITM on 25th November 1991. The paper contains the details of this data.

Estimation of drag coefficient over the western desert sector of the Indian summer monsoon trough

In the estimation of momentum fluxes over land surfaces by the bulk aerodynamic method, no unique value of the drag coefficient (C D) is found in the literature. The drag coefficient is generally estimated from special observations at different parts of the world. In this study an attempt is made to estimate drag coefficient over the western desert sector of India using data sets of Monsoon Trough Boundary Layer Experiment (MONTBLEX) during the summer monsoon season of 1990. For this purpose, the fast and slow response data sets obtained simultaneously from a 30 m high micro-meteorological tower at Jodhpur are used. All the observations used in this study are confined to a wind speed regime of 2.5–9.0 ms−1. A comparison of momentum fluxes computed by eddy correlation (direct estimation) with profile and bulk aerodynamic (C D = 3.9 × 10−3, Garratt, 1977) methods revealed that though the nature of variation of the fluxes by all these methods is almost similar, both the indirect methods give an under-estimated value of the fluxes. The drag coefficient is estimated as a function of wind speed and surface stability by a multiple regression approach. An average value of the estimated drag coefficient is found to be of the order of 5.43 × 10−3. The estimated value ofC D is validated with a set of independent observations and found to be quite satisfactory. The recomputed momentum fluxes by bulk aerodynamic method using the estimated drag coefficient are in close agreement with the directly estimated fluxes.

Genesis of meteorological disturbances and thermohaline variability of the upper layers in the head of the Bay of Bengal during MONsoon Trough Boundary Layer EXperiment (MONTBLEX-90)

Time-series measurements of surface meteorological elements and near-surface thermohaline profiles made at a station at the head of the Bay of Bengal during the MONsoon Trough Boundary Layer EXperiment (MONTBLEX-90; August–September 1990) were utilized to describe and explain the observed short-term variability in the thermohaline structure during active and inactive regimes of the summer monsoon. The observed near-surface thermal structure showed the occurrence of distinct isolated mini-warm pockets with temperatures in excess of 29°C extending from the surface to about 30 m depth, with a temporal correspondence to the formation of meteorological disturbances over the Bay's head. Accumulation of heat in the upper layers in excess of 0.4 × 108 J m−2 (with respect to the 28.7°C isotherm) appeared to have triggered the formation of meteorological disturbances that extracted this surplus energy to lower the surface temperature to around 28.7°C. The depth-time sections of thermohaline fields suggest the signature of a transient eddy or propagating wave. The small-scale structure in the thermohaline profiles was more prominent during September 1990 compared to August 1990. Two simple one-dimensional numerical models following Kraus and Turner (1967, Tellus, 19, 98–106), Denman (1973, Journal of Physical Oceanography, 3, 173–189), Miller (1976, Journal of Physical Oceanography, 6, 29–35) and Niiler (1975, Journal of Marine Research, 33, 405–422), Niiler and Kraus (1977, Modelling and prediction of the upper layers of the ocean, Pergamon, pp. 143–172) were evaluated to assess the relative importance of the local atmospheric forcing to account the observed variability in the near-surface mixed layer characteristics.

A convectively-driven boundary layer in the monsoon trough

Characteristic features of the convectively driven monsoon-trough boundary layer have been explored using the conserved-variable method of analysis. Aerological observations during the Monsoon Trough Boundary Layer Experiment 1990 (MONTBLEX-90) during 18–20 August have been used to investigate the thermodynamic features of the Convective Boundary Layer (CBL). Thermodynamic parameters such asθe,θes have been used to study the dynamical aspects of the CBL. Also, mixed-layer heights at an inland station, in the monsoon trough region, obtained from SODAR, are used to document the saturation of the mixed layer after the onset of the monsoon.

Thermodynamic structure of the monsoon boundary layer under the influence of a large-scale depression

Using data from the MONsoon Trough Boundary Layer EXperiment (MONTBLEX), the thermodynamic structure of the atmospheric boundary layer (ABL) under the influence of a monsoon depression has been studied. When the depression was in the vicinity of the observing station, the soundings showed an increase in potential temperature, the sub-cloud layer was well mixed, the wind speed increased to 35 m/s, and the monsoon boundary layer was convectively more unstable at night than in the daytime. Cloud-top processes, which lead to an apparent breakdown of the boundary layer, seem to explain this.

Sea breeze activity at an inland station Kharagpur (India) ? A case study

Surfaces fluxes, turbulent kinetic energy and Flux Richardson number are calculated for three typical sea breeze days characterizing three types of sea breeze onset at an inland station Kharagpur (22°21′ N, 87°19′ E), 80 km inland, one of the sites for MONTBLEX (MONsoon Trough Boundary Layer EXperiment), in India. The sea breeze onset is associated with a decrease in momentum and heat fluxes and an increase in moisture flux. The turbulent kinetic energy shows quite a significant value even in the late afternoon. The surface layer becomes nearly stable even before sunset, due to the passage of the sea breeze.

Characteristics of turbulent fluxes of sensible heat and momentum in the surface boundary layer during the Indian summer monsoon

Turbulent fluctuations of wind and temperature were measured using a three-component sonic anemometer at 8 m on a 30 m micro-meteorological tower erected at the Indian Institute of Technology (IIT) Kharagpur (22.3° N, 87.2° E), India, as part of the Monsoon Trough Boundary Layer Experiment (MONTBLEX). Diurnal and nocturnal variations of fluxes of sensible heat and momentum were estimated by the eddy correlation technique from 42 observations, each of 10 min duration during 6–8 July in the monsoon season of 1989. The estimated heat flux shows a diurnal trend while the momentum flux shows variability but no particular trend. The nocturnal heat flux changes sign intermittently.

Convective boundary layer in the region of the Monsoon Trough—A case study

A case study of the convectively driven monsoon boundary layer has been carried out using the acrological observations at four stations in the region of monsoon trough during Monsoon Trough Boundary Layer Experiment (MONTBLEX) 1988. The Convective Boundary Layer (CBL) in the region of monsoon trough did not show double mixing line structure. A single mixing line representing the CBL with different stabilities with respect to the convective activities was observed.