G. Pandithurai

Microphysics of Premonsoon and Monsoon Clouds as Seen from In Situ Measurements during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX)

AbstractAnalysis of the microphysical structure of deep convective clouds using in situ measurements during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the Indian peninsular region is presented. It is shown that droplet size distributions (DSDs) in highly polluted premonsoon clouds are substantially narrower than DSDs in less polluted monsoon clouds. High values of DSD dispersion (0.3–0.6) and its vertical variation in the transient and monsoon clouds are related largely to the existence of small cloud droplets with diameters less than 10 μm, which were found at nearly all levels. This finding indicates the existence of a continuous generation of the smallest droplets at different heights. In some cases this generation of small droplets leads to the formation of bimodal and even multimodal DSDs. The formation of bimodal DSDs is especially pronounced in monsoon clouds. Observational evidence is presented to suggest that in-cloud nucleation at elevated layers is a f...

A dust outbreak episode in sub-Sahel West Africa

Wind blown dust is a major contributor to the tropospheric aerosol mass loading and has a significant effect on the local radiative forcing. Information on aerosol optical properties and their temporal and spatial distribution is very limited. Attempts to derive such information from space, in particular, over land, are in a preliminary stage. Recently, information on aerosol optical properties is becoming available from ground networks within one day from the time the observations are made. In this study, use was made of such “real-time” aerosol observations, to characterize their optical properties, during a heavy dust event in the sub-Sahel in January 2000. Aerosol optical depths at all wavelengths showed a sharp increase when compared to the average for the season, reaching values up to 3.5 at 500 nm. The Angstrom exponent was reduced from 1.2 to 0.3, and a sharp increase in the single-scattering albedo was found. The aerosol optical properties differed significantly from climatologically available information on dust aerosols, and therefore their timely assimilation into transport models or weather prediction models could be of great relevance. Developments in the ways aerosol are presently observed could influence future treatment of aerosols in climate research.

Investigations of aerosol optical depth variations using spectroradiometer at an urban station, Pune, India

Abstract The high spectral resolution radiometer at the Indian Institute of Tropical Meteorology (IITM), Pune [18°, 32′N, 73° 51′ E, 559 m Above Mean Sea Level (AMSL)], India has been operated on 79 clear-sky days during April 1993–April 1995 in order to study the temporal-spectral variations in aerosol optical depth (AOD) at the station. From the records of solar irradiance at different zenith angles of the sun, AODs were estimated at a spectral interval of 5 nm in the wavelength region 400–700 nm. The temporal variations in AOD in different spectral regions showed a relationship with those observed in the concurrent surface-level meteorological parameters at the experimental site. The seasonal variations in AOD indicated maximum during summer months and minimum during winter months. In addition, the afternoon (AN) AODs are found to be higher as compared to the forenoon (FN) depths in summer months of the year, whereas the FN AODs dominate in winter months at almost all wavelengths over the observing station.

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.

Spectral width of premonsoon and monsoon clouds over Indo‐Gangetic valley

[1] The combined effect of humidity and aerosol on cloud droplet spectral width (s) in continental monsoon clouds is a topic of significant relevance for precipitation and radiation budgets over monsoon regions. The droplet spectral width in polluted, dry premonsoon conditions and moist monsoon conditions observed near the Himalayan Foothills region during Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) is the focus of this study. Here s is small in premonsoon clouds developing from dry boundary layers. This is attributed to numerous aerosol particles and the absence/suppression of collision-coalescence during premonsoon. For polluted and dry premonsoon clouds, s is constant with height. In contrast to premonsoon clouds, s in monsoon clouds increases with height irrespective of whether they are polluted or clean. The mean radius of polluted monsoon clouds is half that of clean monsoon clouds. In monsoon clouds, both mean radius and s decreased with total cloud droplet number concentration (CDNC). The spectral widths of premonsoon clouds were independent of total droplet number concentrations, but both s and mean radius decreased with small droplet (diameter < 20 mm) number concentrations in the diluted part of the cloud. Observational evidence is provided for the formation of large droplets in the adiabatic regions of monsoon clouds. The number concentration of small droplets is found to decrease in the diluted cloud volumes that may be characterized by various spectral widths or mean droplet radii.

Real-time monitoring of atmospheric aerosols using a computer-controlled lidar

Regular monitoring of aerosol parameters during nighttime in the tropical urban lower atmosphere using a tunable, continuous wave, bistatic argon ion laser radar (lidar) system was initiated at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°N51′E), India in 1986. So far, the measurements have been made from manual operation of the lidar which consumes a lot of time and inhibits interpretation of results immediately after completion of the experiment. In order to improve and expedite the data acquisition-processing and to avoid extra manual work to a considerable extent, the lidar has recently been made fully automatic. This paper discusses the on-line control and digital data system that provides real-time acquisition, analysis and display of lidar data. The improvements in the data quality and application of the system to the real-time observation of various features connected with atmospheric aerosol variations in both space and time are presented. The previously used lidar data acquisition and handling techniques and those of the present auto-mode operation are compared. Such real-time aerosol observations on a long-term basis are expected to lead to a better understanding of the optical properties of aerosols and their interaction with clouds and climate and will hopefully add to the global monitoring efforts and attempts at incorporating aerosols into climate models.

Satellite estimates of surface radiative fluxes for the extended San Pedro Basin : sensitivity to aerosols

Surface downwelling and upwelling radiative fluxes are important inputs into hydrologic models that evaluate water budgets, and into land surface data assimilation schemes which are driven with radiative fluxes. For large-scale needs, only remote sensing methods can provide such information. The accuracy of the derived fluxes depends on the inference schemes and on the quality of auxiliary input parameters. At present, information on surface short-wave (SW) radiative fluxes over the United States is produced in real time by the National Oceanic and Atmospheric Administration (NOAA)/National Environmental Satellite Data and Information Service (NESDIS) at 0.5 resolution, at hourly time intervals, using independently derived auxiliary inputs. Information on aerosol properties and their temporal variability is not available, and at best, is only estimated. During 1997 information on aerosol optical properties was collected at the USDA-Agricultural Research Service Walnut Gulch Experimental Watershed, Arizona, in preparation for future validation efforts in support of new satellite observations (e.g., ADEOS-II). This data set was used to test the sensitivity of a radiation inference scheme to aerosols, in particular, on the determination of clear sky fluxes and the surface albedo. Data from the Arizona meteorological network (AZMET) have been utilized to evaluate the satellite estimates for 1997. It was found that the current satellite estimates are within 70 W m 2 of the ground observations on an hourly time scale and within 24 W m 2 on a daily time scale. In the latter case this is less than 10% of the mean. Use of actual observations of aerosols, as compared to climatological values, reduces the bias substantially, while less significant changes in the r.m.s. were found.

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.

On the response of Indian summer monsoon to aerosol forcing in CMIP5 model simulations

The Indo-Gangetic plains (IGP), which hosts 1/7th of the world population, has undergone significant anomalous changes in hydrological cycle in recent decades. In present study, the role of aerosols in the precipitation changes over IGP region is investigated using Coupled Model Inter-comparison Project-5 (CMIP5) experiments with adequate representation of aerosols in state-of-the art climate models. The climatological sea surface temperature experiments are used to explore the relative impact of the aerosols. The diagnostic analysis on representation of aerosols and precipitation over Indian region was investigated in CMIP5 models. After the evaluation, multi-model ensemble was used for further analysis. It is revealed from the analysis that aerosol-forcing plays an important role in observed weakening of the monsoon circulation and decreased precipitation over the IGP region. The significant cooling of the continental Indian region (mainly IGP) caused by the aerosols leads to reduction in land sea temperature contrast, which further leads to weakening of monsoon overturning circulation and reduction in precipitation.

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