Roughness length and drag coefficient at two MONTBLEX-90 tower stations

Using MONTBLEX-90 mean velocity data, roughness lengths and drag coefficients are estimated at Jodhpur and Kharagpur. At Jodhpur, since the surface is not uniform the roughness length is estimated separately in three different subsectors within the range of prevailing wind directions and averages to 1.23 cm in the sector between 200° and 230° which is relatively flat with no obstacles on the ground. At Kharagpur, where the terrain is more nearly homogeneous, the average value (for all prevailing wind directions) is 1.94 cm. The drag coefficient C_D at Jodhpur shows variation both with the roughness subsector and with wind speed, the average over all directions increasing rapidly as themean wind speed Ū₁₀ at 10m height drops according to the power lawC _D = 0.05 Ū ₁₀ ^t-1.09 in trie range 0.5 < Ū₁₀ < 7 m s⁻¹. At Kharagpur, the drag coefficient is smaller than at Jodhpur by nearly 50% for the same range of wind speeds (> 3 ms⁻¹).     

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.     

Thermal and wind structure of the monsoon trough boundary layer

Radiosonde data from Jodhpur, taken at 0530, 1730 and around 1100 hr IST during MONTBLEX 1990, reveal that the distribution of virtual potential temperature0 _v below about 500 hPa has a structure characterized by up to three layers each of approximately constant gradient. We are thus led to introduce a characterization of the observed thermal structure through a sequence of the symbolsN, S andU, standing respectively for neutral, stable or unstable conditions in the different layers, beginning with the one closest to the ground. It is found that, of the 29 combinations possible, only the seven classes,S, SS′, SNS′, NS, NSS′, USS′ andUNS are observed, whereS′ stands for a stable layer with a different gradient of0 _r. than in the layerS. It is also found that, in 90% of the launches at 0530 hr, 48% of the launches at 1730 hr and 69% of the launches around 1100 hr, the first radiosonde layer near the ground is stable; the classical mixed layer was found in only 11 % of the data set analysed, and, if present on other occasions, must have been less than 250 m in height, the first level at which radiosonde data are available. Supplementing the above data, sodar echograms, available during 82% of the time between June and August 1990, suggest a stable layer up to a few tens of metres 48% of the time. A comparative study of the radiosonde data at Ranchi shows that the frequent prevalence of stability near the surface at Jodhpur cannot be attributed entirely to the large scale subsidence known to be characteristic of the Rajasthan area. Further, data at Jodhpur reveal a weak low level jet at heights generally ranging from 400 to 900 m with wind speeds of 6 to 15 m/s. Based on these results, it is conjectured that the lowest layers in the atmosphere during the monsoons, especially with heavy clouding or rain, may frequently be closer to the classical nocturnal boundary layer than to the standard convective mixed layer, although often with shallow plumes that penetrate such a stable layer during daytime.     

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 boundary layer studies at Jodhpur during MONTBLEX using sodar and tower

A monostatic sodar was set up at Jodhpur, near the western end of the monsoon trough, to investigate the atmospheric boundary layer dynamics. A 30 m instrumented tower was also located close to the sodar antenna. Data were collected from June to August during the monsoon period of 1990, as also from July 1992 to September 1993. Thermal plumes, surface-based stable layers (both flat or short spiky top and tall spiky top), elevated/multi-layers with or without undulations and dot echo structures were seen; however, erosion of the morning inversion layer in the form of a rising layer with growing thermal plumes under it was rarely seen, and that too only during the winter period. The observed structure of the stable layer with tall spikes and its depth have been found to be correlated with the intensity of the monsoon spell; the dot echoes have been found to be correlated with the approach of a monsoon depression near Jodhpur; and the elevated/multilayers have been attributed to the formation of a subsidence (shear instability).     

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

In this paper, acoustic sounder (sodar) derived vertical velocity variance (σ _w ² ) and inversion height (Z _i) 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 σ _w ² /Z profile. Variation of σ _w ² 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.     

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.     

Some observations from the data taken in and around Kharagpur during the onset of the monsoon, 1990

During MONTBLEX 1990, various observational platforms were operated at Kharagpur and the nearby Kalaikunda Air Base. Using the data from all the platforms, one can draw the following conclusions. The temperature and wind data obtained from various sensors have overall compatibility. Sodar wind data indicate the presence of a low level jet at around 300 m above ground. The inversion height may be evaluated from the vertical profile of the sodar back-scatter echo intensity. The sub-synoptic or synoptic scale convergence modulates the inversion height and the presence of cloud-base within the inversion height in turn modulates the sensible heat and momentum fluxes.     

An analysis of MONTBLEX data on heat and momentum flux at Jodhpur

Parameterization of sensible heat and momentum fluxes as inferred from an analysis of tower observations archived during MONTBLEX-90 at Jodhpur is proposed, both in terms of standard exchange coefficientsC _H andC _D respectively and also according to free convection scaling. Both coefficients increase rapidly at low winds (the latter more strongly) and with increasing instability. All the sensible heat flux data at Jodhpur (wind speed at 10 m Ū₁₀ < 8 ms⁻¹) also obey free convection scaling, with the flux proportional to the ‘4/3’ power of an appropriate temperature difference such as that between 1 and 30 m. Furthermore, for Ū₁₀ < 4 ms⁻¹ the momentum flux displays a linear dependence on wind speed.     

Characterization of C‐isotope variability in the Delhi Supergroup,northwestern India and the Mesoproterozoic ocean

Marine carbonate rocks of the Delhi Supergroup of northwestern India show little deviation in whole‐rock δ ¹³Ccarb and δ ¹⁸Ocarb values, which generally are around 0 and –10‰ respectively. These narrow ranges and almost constant δ ¹³Ccarb values persist despite close sampling through long sections. The data suggest that the global rate of organic carbon burial was probably constant during deposition of the Delhi Supergroup. The nearly invariant C isotopic profile of the Delhi Supergroup is similar to C isotopic profiles of Mesoproterozoic carbonates older than 1.3 Ga, as reported from different parts of world. Carbonate units on the western margin of the Delhi Supergroup however, have on average moderately positive δ ¹³C values (from 2 to +4.96‰). These high δ ¹³C carbonates may represent the Mesoproterozoic–Neoproterozoic transition (from ～1.25 to ～0.85 Ga), a period characterized by high positive δ ¹³C values globally.     

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.     

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/L_o) 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).     

A case study of atmospheric boundary layer features during winter over a tropical inland station — Kharagpur (22.32°N, 87.32°E)

The local weather and air quality over a region are greatly influenced by the atmospheric boundary layer (ABL) structure and dynamics. ABL characteristics were measured using a tethered balloon-sonde system over Kharagpur (22.32°N, 87.32°E, 40m above MSL), India, for the period 7 December 2004 to 30 December 2004, as a part of the Indian Space Research Organization-Geosphere Biosphere Program (ISRO-GBP) Aerosol Land Campaign II. High-resolution data of pressure, temperature, humidity, wind speed and wind direction were archived along with surface layer measurements using an automatic weather station. This paper presents the features of ABL, like ABL depth and nocturnal boundary layer (NBL) depth. The sea surface winds from Quikscat over the oceanic regions near the experiment site were analyzed along with the NCEP/NCAR reanalysis winds over Kharagpur to estimate the convergence of wind, moisture and vorticity to understand the observed variations in wind speed and relative humidity, and also the increased aerosol concentrations. The variation of ventilation coefficient (V C), a factor determining the air pollution potential over a region, is also discussed in detail.     

Atmospheric surface layer characteristics with MONTBLEX data

Data obtained from the 30m high MONTBLEX tower installed at the Indian Institute of Technology, Kharagpur are described. Data on wind speed and direction, temperature and humidity were recorded during the first week of July 1989 at six levels on the tower. They indicate some of the atmospheric surface layer characteristics. Using two levels of tower data involving wind speed and temperature a profile method was used for computing the surface fluxes of heat and momentum. A method for calculating the surface roughness length using the data was also used to obtain the vertical windspeed profile. The computed wind profile was compared with observations, while the surface roughness length was compared with values obtained by a least square fit. The computed surface fluxes were compared with theoretical values obtained by an energy budget method which uses only wind data at one level. The agreement was reasonably good.     

Critical evaluation and geological correlation of teleseismic phase data from Indian seismological stations

We evaluated the quality of seismic phase data from Indian seismological stations through the analysis of teleseismic travel times reported during 1976–83 and infer that only WWSSN stations (NDI, SHL, POO, KOD) apart from GBA and HYB can be rated satisfactory while the majority of stations (more than 40) produce very poor quality data sets. Detailed analysis of teleseismic P-wave travel time residuals shows that while the average structure of the upper mantle beneath India has high velocity (negative residuals) there are marked lateral variations. In particular, three zones of anomalous positive residuals (low velocity) are observed: one beneath the north western part of the Deccan trap, the second covering the southernmost peninsula (granulite terrain) and a third rather localized one, to the north of Delhi coinciding with Delhi-Haridwar ridge. New Delhi exhibits strong negative residuals in the E-SE quadrant along with negative station anomaly, implying that it is underlain by an anomalous high velocity crust/upper mantle. The negative residuals observed over India, continue beneath the Himalaya till the south of Lhasa but change sign further northward, suggesting the northern limit of the Indian upper mantle structure.     

Study of near surface boundary layer characteristics during pre-monsoon seasons using micrometeorological tower observations

Studying the boundary layer is imperative because severe weather in this portion of the atmosphere impacts on environment and various facets of national activities and affects the socioeconomic scenario of a region. Near surface boundary layer characteristics are investigated through the vertical variation of fluxes of heat, moisture, momentum, kinetic energy and Richardson number during the pre-monsoon season (April-May) at Kharagpur (22° 30’ N, 87° 20’ E) and Ranchi (23° 32’ N, 85° 32’ E) with 50 and 32 m tower data, respectively, on thunderstorm and non-thunderstorm days. The temporal variation of fluxes within the boundary layer and the kinetic energy at different logarithmic heights are observed to vary significantly between thunderstorm and non-thunderstorm days. The heat and momentum fluxes show a maximum peak while the moisture flux shows a sudden attenuation just before the occurrence of thunderstorms. The wind field depicts to play a crucial role at the inland station Kharagpur, which is in the proximity of the Bay of Bengal, compared to the station Ranchi, situated over hilly terrain on Chotanagpur. The micrometeorological study of the boundary layer reveals a significant finding pertaining to observe the passage of thunderstorms. It is observed that the ratio of the potential temperature (θ) and equivalent potential temperature (θe) remains confined within a critical range between 0.85 and 0.90 during the passage of thunderstorms.     

Stable Isotopic Compositions of Carbonates from the Mesoproterozoic Delhi Supergroup,Northwestern India

Marine carbonate rocks from the Delhi Supergroup show little deviation in whole-rock δ¹³Ccarb values, which are generally around 0% PDB. This narrow range and almost constant δ¹³Ccarb values persist despite close sampling through long, geographically widespread sections. The data suggest that, in contrast to the Neoproterozoic, the global rate of organic carbon burial was probably constant during deposition of the Delhi Supergroup, and perhaps generally during the Mesoproterozoic, as was the redox state of the atmosphere and hydrosphere.     

Understanding the turbulent structure of the atmospheric boundary layer: A diagnostic approach

In this paper, we have attempted a diagnostic study of the turbulence characteristics of the ABL by means of two one-dimensional models. The first model uses a first order non-local closure, based on the Transilient Turbulence Theory, for parameterizing turbulent fluxes. while the second model uses second order local closure for parameterizing these. The models have been applied to conduct case studies using the Kytoon data taken at Kharagpur, during 17th–21st June, 1990, as part of the MONTBLEX programme. Our findings bring out various interesting features regarding the non-local and local turbulent statistics such as kinematic fluxes, turbulence kinetic energy, vertical velocity variance, the contribution of the eddies of various sizes to the fluxes at different level and the mixing lengths. The one-dimensional anisotropy of the turbulent eddies has been revealed by the findings from the transilient model. The vertical variation of the turbulence kinetic energy, as computed directly by the second order model, is found to be strongly correlated with the vertical velocity variance. In particular, for stably stratified boundary layers, identification of two distinct zones of the turbulence kinetic energy and corresponding vertical velocity maxima is possible, which has been interpreted as positive evidence of patchy turbulence in the boundary layer.     

Urban growth and quality of life: inter-district and intra-district analysis of housing in NCT-Delhi, 2001–2011–2020

One out of three people in India is urban. In 2011, there were about 53 urban agglomerations larger than 1 million population as against only 35 in 2001. Much of this urban expansion has been occurring in the country’s largest metropolises including the National Capital Territory of Delhi which has expanded horizontally and vertically both. This has also added to overall decline in its already dilapidated housing stock and quality of life. Delhi, a historical hub for regional, national, and international commerce, and a place for the socio-political elites, has failed to provide basic life amenities to its average citizens. This research critiques the (un)sustainable elements of Delhi’s urbanization and concomitant decline in basic amenities pertaining to quality-of-life by examining the growth and expansion of its urban-built-up areas during 2001–2011–2020 and provides nuanced insights into its ‘livability’ by examining select quality-of-life attributes. The LANDSAT imageries for 2010 and 2020 are used to measure NDB-Index that assesses its built-up area and change, which are later corroborated with Census household data to examine change in its ‘livable’ and ‘dilapidated’ housing structures. Significant sub-regional disparity exists in the availability of good and livable homes, with almost 20–30% of several districts still without drinking water source inside premises. However, significant progress is also noted for basic amenities like lighting, latrine and bathing facilities, and majority of Delhi’s built-up area has expanded along newer developments and transportation corridors. This calls for goal-oriented strategic interventions by policymakers to help achieve the SDG-11 on Sustainable Cities.

     

Long-term variations in outgoing long-wave radiation (OLR), convective available potential energy (CAPE) and temperature in the tropopause region over India

Relationship of outgoing long-wave radiation (OLR) with convective available potential energy (CAPE) and temperature at the 100-hPa pressure level is examined using daily radiosonde data for a period 1980–2006 over Delhi (28.3°N, 77.1°E) and Kolkata (22.3°N, 88.2°E), and during 1989–2005 over Cochin (10°N, 77°E) and Trivandrum (8.5°N, 77.0°E), India. Correlation coefficient (R _xy) between monthly OLR and CAPE shows a significant (~???0.45) anti-correlation at Delhi and Kolkata suggesting low OLR associated with high convective activity during summer (seasonal variation). Though, no significant correlation was found between OLR and CAPE at Cochin and Trivandrum (low latitude region); analysis of OLR and temperature (at 100-hPa) association suggests that low OLR peaks appear corresponding to low temperature at Delhi (R _xy~ 0.30) and Kolkata (R _xy ~ 0.25) during summer. However, R _xy between OLR and temperature becomes opposite as we move towards low latitudes (~8°–10°N) due to strong solar cycle influence. Large scale components mainly ENSO and quasi-biennial oscillaton (QBO) that contributed to the 100-hPa temperature variability were also analyzed, which showed that ENSO variance is larger by a factor of two in comparison to QBO over Indian region. ENSO warm conditions cause warming at 100-hPa over Delhi and Darwin. However, due to strong QBO and solar signals in the equatorial region, ENSO signal seems less effective. QBO, ENSO, and solar cycle contribution in temperature are found location-dependent (latitudinal variability) responding in consonance with shifting in convective activity regime during El Niño, seasonal variability in the tropical easterly jet, and the solar irradiance.