B. S. Murthy

An Overview of the Land Surface Processes Experiment (Laspex) over a Semi-Arid Region of India

To understand and quantify the land-surface-vegetation interactionwith the atmospheric boundary layer, and validate or improve upon the existing surfaceflux parameterization schemes in various weather forecast models, a LAnd SurfaceProcesses EXperiment (LASPEX), was designed and executed in the semi-arid regionof Gujarat, India during January 1997–December 1998. Micrometeorological tower observations,soil and vegetation parameters, radiation, turbulence and upper airobservations were taken continuously for two years at five sites, separated by about60–100 km from each other. Towers of 9 m height with instruments at four levels wereinstalled at sites that are agricultural fields and characterized with a variety of soilproperties, vegetation and diverse crops. An overview of the experiment is presented.Some results, such as the seasonal variation of surface energy balance and turbulence statistics,are discussed.

Sodar observations of the nocturnal boundary layer at Kharagpur, India

Sodar has been installed at Kharagpur (22.2 ° N, 87.3 ° E) as a part of the MONTBLEX-90 experiment and data were collected during the monsoon period. The variation of the nocturnal boundary layer (NBL) during the monsoon period is discussed. The height corresponding to the low-level wind maximum in the sodar wind profile during night time is identified as the NBL height. Mean monthly winds for July and August, plotted as time-height cross sections, reveal the height of the ground-based stable layer. The average NBL heights in the months of July and August are found to be 324 m and 296 m respectively. It is observed that the NBL height is relatively high in the month of July (active phase of monsoon) compared to that during August (weak phase). The months of July (total rainfall = 901 mm) and August (total rainfall = 134 mm) are associated with cloudy and relatively clear sky conditions. This indicates that clouds (through their effect on longwave cooling to space) play an important role in determining the NBL height during the monsoon.

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.

Temporal variation of “solar dimming” induced by composite and carbonaceous aerosols: Observations from mineral‐rich eastern Indian region

Composite and carbonaceous aerosol radiative forcing (RF) over Ranchi (23.5°N, 85.3°E) in eastern India at monthly and seasonal scales during February 2011 to January 2012 is derived from mean optical properties obtained from Sun-sky radiometer and a radiative transfer model. Ranchi is located on the Chotanagpur plateau at 650 m above mean sea level; the region is unique with dense open active mines, a source of mineral aerosols with opposing optical properties such as coal and limestone. Diurnal mean composite aerosol RF at the surface, in the spectral band 0.3–3.0 µm increases from winter (December, January, February) to premonsoon (March, April, May) with maximum (−65 Wm−2) aerosol RF in March that is associated with highest black carbon (BC) aerosol optical depth (AOD), 0.05. Minimum surface aerosol RF occurs in July with minimum values of AOD and AODBC. Aerosol RF at top of the atmosphere is maximum (−17 Wm−2) in April, and is positive (+1.2 Wm−2) in March. On an average, carbonaceous aerosols (BC) contribute ~8.8% to total AOD and about 60% to total atmospheric absorption. A rapid increase of BC is seen in the postmonsoon with 1.0 µg m−3 in September to 3.0 µg m−3 in October. Aerosol-induced solar dimming is about 9.4% (premonsoon), 7.0% (monsoon), 10.6% (postmonsoon), and 10.2% (winter) of the surface radiation. Seasonal mean aerosol heating rate is observed to be maximum in premonsoon (~1.15°K/d−1), followed by postmonsoon, while winter and monsoon experience minimum (~0.45°K/d−1) heating, assuming typical aerosol profiles of the model.

Air-quality monitoring by optical and acoustic radars at Pune, India

The co-located optical (argon-ion lidar) and acoustic (Doppler sodar) radar systems at the Indian Institute of Tropical Meteorology (IITM), Pune (, , 559 m AMSL), India, have been employed to study the nocturnal aerosol pollution dynamics or air quality. Both the systems have been operated simultaneously in order to sample the common atmospheric volume. The results of the experiments thus conducted on some typical nights are presented in this paper. The time - height cross section of the mean, horizontal and vertical components of the wind field derived from the sodar observations indicate that the wind is either easterly or northerly with its vertical component negative (downdraft) for most of the observational period. The interesting feature is that the winds are either calm or relatively weaker in the night-time boundary layer compared with those in the region aloft. This is more evident in the north - south direction in which the lidar and sodar equipment are located. Furthermore, the regions where stable/elevated layer formation is active are found to be associated with the smallest wind gradients, leading to accumulation of aerosol particles or larger temperature structure parameters. The time evolution of the mixed-layer heights and the associated ventilation coefficients, determined from the combined lidar - aerosol and sodar - wind observations, are found to be useful to explain the transport and diffusion of pollutants across the elevated temperature inversions over the experimental station. The details of the experimental techniques are presented and their value for application in the study of regional air quality and pollution potential is discussed.

Wind profiles in the boundary layer over Kharagpur associated with synoptic scale systems

Doppler sodar wind data for the boundary layer over Kharagpur obtained during MONTBLEX-1990 at a height interval of 30 m from surface up to 1500 m have been analysed for the periods when intense synoptic scale disturbances from north Bay of Bengal moved along the eastern end of the monsoon trough. The variation in the vertical wind profile in the lower boundary layer over Kharagpur during the passage of synoptic scale disturbances has been discussed in the paper. The analysis indicates that the mean winds over Kharagpur veered with height in the lower boundary layer near the surface suggesting divergence over Kharagpur when the system lay south/southwest of the station. No such veering has been noticed when the centre of the system lay very close to the station.

A study of turbulent characteristics of atmospheric boundary layer over monsoon trough region using Kytoon and Doppler sodar

As a part of the MONTBLEX-90 observational programme, Kytoon and Doppler sodar observations were taken at Kharagpur. These data are analysed to study the turbulent characteristics of the atmospheric boundary layer in terms of stability, temperature structure function (CT2) and velocity structure function (Cv2).CT2follows aZ−4/3 law on most of the days, whereas the variation ofCV2is not systematic.CV2andCT2values are found to vary between 10−5−10−1 m4/3s−2 and 10−5−10−2°C2 m−2/3 respectively.