UHF wind profiler observations of monsoon low-level jet (MLLJ) and its association with rainfall over a tropical Indian station

High resolution data of horizontal winds profiles (zonal and meridional) in the lower troposphere, derived from a UHF wind profiler at a tropical Indian station, Pune (18º 32' N, 73º 51' E, 559 masl) during a 3-yr period (June 2003-May 2006) has been utilized to study seasonal and intra-seasonal variability of winds. Winds display a systematic seasonal evolution with behavior opposite in phase in the two altitude regimes below and above a height of 4-5 km. In the lower region, during the southwest monsoon months (June to September) winds are predominantly westerly with a peak in the 1.5-3.0 km range indicating the occurrence of the monsoon low-level jet (MLLJ). Soon after September, winds in this height region change from westerly to easterly and these easterlies continue in winter months (December to February). Above a height of 4 km, westerlies are observed during post-monsoon (October to November) and winter periods. The MLLJ is observed to be strong during normal/good monsoon years. On a day-to-day scale during southwest monsoon months, winds exhibit considerable intra-seasonal variability and periods of strong MLLJ seem to be associated with occurrence of spells of rainfall over the region

Atmospheric stability effects on aerosol structure and stratification

This paper essentially addresses the structure and stratification of the nocturnal boundary layer (NBL) derived from the vertical profile measurements of aerosol concentration made with a computer-controlled Argon ion lidar system at the Indian Institue of Tropical Meteorology (IITM), Pune, India. Companions are made between the lidar observations during clear night-sky conditions and concurrent aerometric observations carried out on some selected experimental days. The results show multiple stratified aerosol layer structures in the nocturnal lower atmosphere, which drift either upward or downward depending on atmospheric stability conditions prevailing at different altitudes. The normalized aerosol concentration gradient (NCG) profiles indicate the variations in the nocturnal mixing depth from 200 to 426 m and that in the stable layer height from 325 to 725 m during the period of observations. The importance of such observations in the context of monitoring and/or assesment of airborne particulate pollutants over the urban environments associated with non-uniform terrain is discussed

Relationship between lidar-based observations of aerosol content and monsoon precipitation over a tropical station, Pune, India

This paper reports the results of the aerosol lidar experiments that have been performed at the Indian Institute of Tropical Meteorology (IITM), Pune (18.54°N, 73.85°E, 559 m amsl), a tropical station in India. The lidar-observed cloud macro-physical parameters (cloud-base and cloud-ceiling heights, vertical thickness, etc.) and polarisation characteristics and their association with surface-generated aerosols at the experimental site are presented and discussed. The correspondence among the lidar-derived aerosol distributions, meteorological parameters and south-west (SW) monsoon (June-September) activity over Pune during 12 successive SW monsoon seasons (1987-98) including two pairs of contrasting seasons of 1987-8 and 1993-4 is also examined. The results indicate an association between variations in aerosol loading in the boundary layer during the pre-monsoon season (March-May) and precipitation intensity during the ensuing monsoon season. Moreover, the decrease in aerosol content from pre-monsoon to monsoon season is found to follow the SW monsoon season total precipitation. Thus the results suggest that (i) the IITM lidar can also be a useful remote sensor for aerosol characterisation studies from polarisation measurements, and some important physical properties of clouds in the lower atmosphere over the station, and (ii) there exists a correspondence between boundary-layer aerosol content and SW monsoon precipitation over Pune, which is explained in terms of the type of aerosols and the environmental and meteorological processes, particularly during premonsoon and monsoon months prevailing over the experimental station

Ground-based lidar study of aerosol and boundary layer characteristics during INDOEX first field phase

This paper presents the extensive observations of atmospheric aerosol vertical distributions that have been carried out during the first field phase (FFP) of the Indian Ocean experiment (INDOEX-FFP 98) using the computer-controlled Argon-ion lidar system at the Indian Institute of Tropical Meteorology (IITM), Pune, a tropical urban station in India. These observations primarily will help explaining the teleconnections between the inland aerosol characteristics, and the marine aerosol properties observed from the ORV Sagar Kanya cruise during the same period. Vertical profiles of night-time aerosol concentration up to about 7 km above ground level were obtained on 25 days during the INDOEX-FFP 98 (17 February-31 March 1998). These profiles have also been utilized to investigate the nocturnal structure and stratification of the planetary boundary layer and associated air quality. The aerosol vertical distributions and the associated aerosol columnar content show significant day-to-day variations, particularly in the nocturnal boundary layer (NBL) and exhibit increasing trend during the study period. The derived ventilation coefficients are found to associate more closely with wind speed as compared to mixing depth

Recent trends in aerosol climatology and air pollution as inferred from multi-year lidar observations over a tropical urban station

Regular nighttime monitoring of aerosol and other atmospheric parameters was initiated in 1985 at the Indian Institute of Tropical Meteorology, Pune. This is a tropical urban station (18°32'N, 73°51'E, 559 m AMSL), situated approximately 100 km inland from the west coast of India. The multi-year aerosol vertical profile database, utilized in the present study, consisted of more than 1200 vertical aerosol concentration profiles. These data were collected with a computer-controlled, bistatic, argon-ion lidar system over a 12 year period from October 1986 to September 1998 and have been utilized to study the morphology of the nighttime atmospheric boundary layer and associated air quality. The recent climatological trend in the aerosol loading at the experimental station has also been studied. The study reveals higher pollution potential during late evenings in the winter and a total increase of about 3 in the aerosol loading over the 12 year observational period. This increase can be attributed partly due to the urban heat island effect and due to growing urbanization and industrialization, as well as to the land-usage patterns in proximity to the experimental station. Further, it has been found that the long-term trend in aerosol loading was not uniform, but it changed from year to year depending on meteorological parameters (precipitation, in particular) and local anthropogenic activities. The short-term variations in aerosol loading and their relationship with concurrent meteorological parameters over the observational site are discussed also

Variability in Sun photometer-derived total ozone over a tropical urban station

A portable, handheld, multichannel Sun photometer (ozonometer) has been used to measure total column ozone over Pune (18320N, 73510E, 559 m above mean sea level), India, a tropical urban station. Data collected on about 575 mostly clear-sky days during the 5 year period from May 1998 to May 2003 have been used in this study to examine diurnal and seasonal variations. The overall daily mean total ozone at this station is 254 Dobson units (DU), with a variability of about 7%, while the most frequently occurring value is in the range 241–250 DU. The diurnal (daytime) patterns are observed to be different in the winter (December–February), premonsoon (March–May), and postmonsoon (October–November) seasons

Results of sun photometer-derived precipitable water content over a tropical Indian station

A compact, hand-held multiband sun photometer (ozone monitor) has been used to measure total precipitable water content (PWC) at the low-latitude tropical station in Pune, India (18°32′N, 73°51′E). Data collected in the daytime (0730–1800 LT) during the period from May 1998 to September 2001 have been used here. The daytime average PWC value at this station is 1.13 cm, and the average for only the clear-sky days is 0.75 cm. PWC values between 0.75 and 1.0 cm have the maximum frequency of occurrence. There is a large day-to-day variability due to varied sky and meteorological conditions. Mainly two types of diurnal variations in PWC are observed. The one occurs in the premonsoon summer months of April and May and shows that forenoon values are smaller than afternoon values. The other type occurs in November and December and shows a minimum around noontime. There is a diurnal asymmetry in PWC in which, on the majority of the days, the mean afternoon value is greater than the forenoon value. This asymmetry is more pronounced in the summer and southwest monsoon months (i.e., March–June). Monthly mean PWC is highest in September and lowest in December. The increase in PWC from the winter (December–February) to summer (March–May) seasons is about 50% and from the summer to southwest monsoon seasons (June–September) is almost 98%. Sun photometer–derived PWC shows a fairly good relationship with surface relative humidity and radiosonde-derived PWC, with a correlation coefficient as high as 0.8

Correlative measurements of aerosol optical depth and size distribution around INDOEX-FFP 98 from multi-spectral solar radiometry

Spectral aerosol optical depth measurements by means of ground-based solar radiometry have been made at the Indian Institute of Tropical Meteorology, Pune, a continental urban station in India, as part of the INDOEX-FFP 98 during 17 February through 31 March, 1998. The aerosol optical depths and corresponding size distributions observed with both a multi-channel radiometer (sun-photometer) and a spectroradiometer agree well and show marked variations during the experimental period. The day-to-day variation in the aerosol optical depth at 0.50 μm indicates an increasing trend. The results of a comparative study made between the continental urban and coastal aerosol characteristics using the sun-photometric observations carried out around the INDOEX-FFP 98 are also presented

Remote sensing of spectral signatures of tropospheric aerosols

With the launch of the German Aerospace Agency's (DLR) Modular Opto-electronic Scanner (MOS) sensor on board the Indian Remote Sensing satellite (IRS-P3) launched by the Indian Space Research Organization (ISRO) in March 1996, 13 channel multi-spectral data in the range of 408 to 1010nm at high radiometric resolution, precision, and with narrow spectral bands have been available for a variety of land, atmospheric and oceanic studies. We found that these data are best for validation of radiative transfer model and the corresponding code developed by one of the authors at Space Applications Centre, and called ATMRAD (abbreviated for ATMospheric RADiation). Once this model/code is validated, it can be used for retrieving information on tropospheric aerosols over ocean or land. This paper deals with two clear objectives, viz., (1) Validation of ATMRAD model/code using MOS data and synchronously measured atmospheric data, and if found performing well, then to (2) derive relationship between MOS radiances and Aerosol Optical Thickness (AOT). The data validation procedure essentially involves near-synchronous measurements of columnar aerosol optical thickness and altitude profiles of aerosol concentration using ground-based multi-filter solar radiometers and Argon-ion Lidar, respectively and computation of the top-of-the-atmosphere (TOA) radiances from a low reflecting target (near clear water reservoir in the present study) using the ATMRAD model. The results show that the model performance is satisfactory and a relationship between the spectral parameters of MOS radiances and aerosol optical thickness can be established. In this communication, we present the details of the experiments conducted, database, validation of the ATMRAD model and development of the relationship between AOT and MOS radiance