R. S. Maheskumar

Aerosol effect on droplet spectral dispersion in warm continental cumuli

In situ aircraft measurements of cloud microphysical properties and aerosol during the 1st phase of the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX-I) over the Indian sub-continent provided initial opportunities to investigate the dispersion effect and its implications for estimating aerosol indirect effects in continental cumuli. In contrast to earlier studies on continental shallow cumuli, it is found that not only the cloud droplet number concentration but also the relative dispersion increases with the aerosol number concentration in continental cumuli. The first aerosol indirect effect estimated from the relative changes in droplet concentration and effective radius with aerosol number concentration are 0.13 and 0.07, respectively. In-depth analysis reveals that the dispersion effect could offset the cooling by enhanced droplet concentration by 39 in these continental cumuli. Adiabaticity analysis revealed aerosol indirect effect is lesser in subadiabatic clouds possibly due to inhomogeneous mixing processes. This study shows that adequate representation of the dispersion effect would help in accurately estimating the cloud albedo effect for continental cumuli and can reduce uncertainty in aerosol indirect effect estimates.

Lidar measurements of aerosol column content in an urban nocturnal boundary layer

Abstract Lidar aerosol measurements made at Pune (lat. 18°32′N, long. 73°51′E, 559 m AMSL), India, a tropical urban station, during the nine-year period from October 1986 to September 1995 have been used to study the temporal variations in the aerosol-loading in the nocturnal boundary layer. There is a long-term increasing trend in the aerosol column content in the 50–1100 m layer that has been attributed to be due to the increasing anthropogenic activity around the lidar site. The seasonal variations in aerosol content show a maximum in the pre-monsoon month of May and a minimum in the SW monsoon month of July. The percentage contribution of the 50–200 m layer to the total loading in the 50–1100 m layer is about 41% which points to the predominant surface source of particulate matter. On a seasonal scale, there is a decrease of about 36% in the aerosol content from premonsoon (March-May) to monsoon (June-September) season and this decrease on year-to-year basis is directly related to the amount of rainfall received at the ground at Pune during the monsoon season. This effect has not changed appreciably over the years whatever be the increase in the aerosol loading. The temporal evolution of aerosol content in the nighttime showed a rapid decrease soon after sunset and a slower rate of decrease in the midnight hours. The study also showed that surface relative humidity and winds influence the temporal variations in the aerosol column content.

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 pre-monsoon and monsoon months prevailing over the experimental station. Copyright

Study of total column atmospheric aerosol optical depth, ozone and precipitable water content over Bay of Bengal during BOBMEX-99

The spatial and temporal variations in aerosols and precursor gases over oceanic regions have special importance in the estimation of radiative forcing parameters and thereby in the refinement of general circulation models. Extensive observations of the columnar aerosol optical depth (AOD), total column ozone (TCO) and precipitable water content (PWC) have been carried out using the on-line, multi-band solar radiometers onboard ORV Sagar Kanya (Cruise # SK 147B) over Bay of Bengal during 11th–28th August 1999. Aerosol optical and physical properties (optical depth and angstrom parameter) have been estimated at six wavelengths covering from UV to NIR (380–1020 nm) while TCO and PWC have been determined using the UV band around 300 nm and NIR band around 940 nm, respectively. Added, concurrent meteorological and satellite observations during this field phase of BOBMEX-99 have been utilized to investigate spectral-temporal variations of AOD, TCO and PWC in marine environment.The results indicate lower AODs (around 0.4 at characteristic wavelength of 500 nm) and size distributions with abundance of coarse-mode particles as compared to those aerosols of typical land origin. An interesting result that is found in the present study is the significant reduction in AOD at all wavelengths from initial to later part of observation period due to cloud-scavenging and rain-washout effects as well as signature of coastal aerosol loading. The clear-sky daytime diurnal variation of TCO shows gradual increase during post-sunrise hours, broad maximum during afternoon hours and gradual decrease during pre-sunset hours, which is considered to be due to photochemical reactions. The diurnal variation curve of PWC showed maximum (~ 4 cm) during morning hours and gradual decrease (~ 3.5 cm) towards evening hours, which are found to be greater as compared to typical values over land. Another interesting feature observed is that although the PWC values are very high, there was no proportionate or appreciable enhancement in AOD—a feature that can be utilized to infer composition of aerosols over the study region.

Spectral characteristics of urban aerosols and their association with relative humidity

Abstract Multi-spectral extinction measurements made with co-located high-spectral resolution radiometer (spectroradiometer) and sunphotometer at the Indian Institute of Tropical Meteorology (IITM), Pune (18°32′N, 73°5′E, 559 m AMSL) on 188 cloud-free days between March 1993 and May 1995 are presented. The observed aerosol optical size spectrum was approximatd by a composite power-law distribution function. The seasonal mean size distributions, inferred from both the radiometers, exhibit a power-law type distribution with different exponents changing at an intermediate size. The mean Junge size exponents ( ν 1 and ν 2 ) and switching radius ( r 0 ) obtained with spectroradiometer are found to be in good agreement with those obtained with sunphotometer. The relationships among aerosol optical depths and derived size distributions, and meteorological parameters (height-integrated) are discussed.

Atmospheric aerosol–cloud-stability relationship as observed with optical and radio remote sensing techniques

Abstract Atmospheric aerosol characteristics are being monitored, on a routine basis, with lidars and spectroradiometer since 1985 and 1992, respectively, at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°51′E, 559 m AMSL), a tropical Indian urban station. The lidar-derived nocturnal atmospheric structures, transport of surface-generated aerosols (pollutants) and their participation in the formation of clouds and their time evolution in the planetary boundary layer (PBL) are studied in relation to the local diabatic conditions and height-integrated relative humidity (RH) inferred from radiometersonde and pilot balloon (pibal) observations. This paper mainly addresses (i) how the integrated lidar, spectroradiometer and coincident aerometric data can be used to delineate the atmospheric structure and transport mechanisms, and (ii) the extent to which the lidar observations during different atmospheric conditions can be useful to explain the phenomenon of cloud scavenging of aerosols over the experimental station.

Macrophysical and microphysical properties of monsoon clouds over a rain shadow region in India from ground‐based radiometric measurements

The important radiative properties of clouds such as cloud optical depth (COD) and droplet effective radii (Re) are retrieved from the simultaneous measurements by ground-based multifilter rotating shadowband radiometer (MFRSR) and microwave radiometric profiler (MWRP), colocated at Mahabubnagar, a rain shadow region in southern Indian peninsula. Min and Harissons (1996) retrieval algorithm is used for the first time to derive monsoon cloud properties in India. COD and liquid water path (LWP) retrieved from two independent instruments of MFRSR and MWRP showed reasonably good correlation. During monsoon (July to September) and postmonsoon (October) months, the maximum probability of occurrence of COD for overcast sky is 20. The maximum probability of occurrence of LWP is 100 gm−2 for water clouds during monsoon months, while October showed maximum occurrence at a lower value of 50 gm−2, where most of the times the cloud bases are above freezing level indicating mixed phase clouds. Maximum Re varied from 14–16 µm (10–12%) to 12 µm (9%) during monsoon to postmonsoon transition with very less probability of occurrence indicating the characteristic feature of this region. A case study showed that the mean Re from ground-based and aircraft measurements are 12.0 ± 3.7 µm and 8.14 ± 1.4 µm, respectively, indicating a fairly good agreement within the experimental constraints. Intercomparison of ground-based and Moderate Resolution Imaging Spectroradiometer (MODIS)-Terra and MODIS-Aqua-derived COD, LWP and Re over the observational site for overcast and warm clouds indicates that on an average, MODIS-retrieved mean COD and LWP are underestimated, while mean Re is overestimated as compared to ground retrievals.

Tropical urban aerosol distributions during pre-sunrise and post-sunset as observed with lidar and solar radiometer at Pune, India

Abstract The pre-sunrise and post-sunset differences in the tropical urban aerosol distributions have been studied by conducting coordinated experiments using a continuous wave, bistatic Argon ion lidar and a spectroradiometer at the Indian Institute of Tropical Meteorology, Pune, India during 1997–2000. The results of the study indicate higher aerosol concentration in the air layers close to the ground, and lower concentration aloft on all the days of observations. Further, the concentrations are found to be greater in the early-morning (pre-sunrise) hours and lower in the late-evening (post-sunset) hours during the winter season and vice versa during the pre-monsoon season. These deviations are considered to be due to the convective activity and associated turbulent mixing during the pre-monsoon and close-to-ground and elevated haze layer formations during the winter months over the experimental station.

Results of Sun Photometer¿Derived Precipitable Water Content over a Tropical Indian Station

Abstract 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 as...

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.