Through acquisition of well-calibrated near-nadir and oblique-angle imagery (0° - 70° zenith angles) at moderately high spatial resolution (275 m - 1.1 km), the Multi-angle Imaging SpectroRadiometer (MISR) experiment aboard NASA's Terra satellite has taken atmospheric remote sensing in new directions. Retrieval algorithms that were largely conceptual prior to Terra launch in 1999 have led to publicly available aerosol and cloud products with direct application to global climate and particulate air quality research. Automated algorithms making use of stereoscopic parallax, time lapse among the nine angular views, and the variation in radiance with view angle, scattering angle, and wavelength (446-866 nm) make possible unique data sets including geometric cloud and aerosol plume heights derived independently of emissivity or temperature assumptions; height-resolved cloud-tracked winds; and aerosol optical depth and particle type over a wide variety of surfaces including bright desert source regions. To illustrate these capabilities, examples of regional and global MISR data products, quantitative evaluations of product accuracies based on comparisons with independent data sources, and time series showing seasonal and interannual variations are presented here. Future sensor improvements aimed at building upon MISR heritage, including expanding the spectral coverage to ultraviolet and shortwave infrared wavelengths, adding polarization channels, and widening the sensor swath, are also discussed.

ROSA (Radio Occultation Sounder of Atmosphere) is a space instrument which uses the Radio Occultation Technique to provide highly accurate measurements of the atmospheric refractive indexes from which it is possible to derive, atmospheric vertical profiles of temperature, pressure and humidity, as well as profiles of electron content in the ionosphere. The measurements, generally, are made by using meteorological balloons, able to give a very detailed atmospheric profiles but only on a local scale. From the space, ROSA, flying on a LEO orbit, is able to perform more than 500 atmospheric profiles per day on a global scale, The ROSA instrument is the core of a scientific program, of the same name, promoted by the Italian Space Agency (ASI) to contribute to a better understanding of Climate Change. This program foresees the installation of the ROSA instruments on several space missions and the developments of an ad hoc Ground Segment for the acquisition and processing of the data. The ISRO (Indian Space Research Organization) mission OCEANSAT-2 will be the first mission that will embark ROSA. OCEANSAT-2 is an operative mission for the study of the Ocean that will be launched next July 2007.

Emission based radiative transfer simulations have been carried out to study the impact of atmospheric humidity on clear-sky microwave emissions at various channels of Megha-Tropiques SAPHIR and NOAA AMSU-B sounders. Detailed investigations reveal that under cool and drier conditions, water vapour channels in the far wing region like 183.31±7 GHz of AMSU-B and others behave like microwave imagers in contradiction to these being sounding channels. This feature affects their utility for sounding the lower atmosphere. Simulation study confirms that the layer average relative humidity is retrieved better as compared to its other forms requiring temperature information and has logarithmic dependency on radiation. Present study deals with development of retrieval algorithms using multi-channel sounder data for deriving average relative humidity for different layers of the atmosphere. AMSU-B data during June and October 2002 over Indian region have been used for testing the algorithms to derive relative humidity in three layers between 300 to 1000 hpa. The satellite derived humidity fields have been compared and found to be in good agreement with those from NCEP data.

Clouds and the Earth's Radiant Energy System (CERES) instruments were designed to measure the reflected shortwave and emitted longwave radiances of the Earth's radiation budget and to investigate the cloud interactions with global radiances for the long-term monitoring of Earth's climate. CERES instrument has three scanning thermistor bolometers that measure broadband radiances in the shortwave (0.3 to 5.0 micrometer), total (0.3 to >100 micrometer) and 8 - 12 micrometer water vapor window regions. Four CERES instruments (Flight Models1 through 4) are flying aboard EOS Terra and Aqua platforms with two instruments aboard each spacecraft. The pre-launch accuracy requirements for CERES were 1.0% in the shortwave and 0.5% in longwave regions. The in-flight calibration of CERES sensors are carried out using the internal calibration module (ICM) comprising of blackbody sources and tungsten lamp, and a solar diffuser plate known as the Mirror Attenuator Mosaic (MAM). The ICM and MAM calibration results are instrumental in understanding the ground to flight shift and in-flight drifts in CERES sensors' gains. Inter and intra instrument validation studies are conducted on the CERES measurements to monitor the behavior of the sensors in various spectral regions. Targets such as deep convective clouds and tropical ocean are used to evaluate the sensors' stability within an instrument. With two CERES instruments on same platform, inter comparison of similar sensor measurements viewing the same geolocation are also conducted. The results from these individual studies have collectively given an understanding of each CERES sensor's behavior in different spectral regions. This paper discusses the results from each of these studies which facilitated the correction of CERES data products with a calibration stability better than 0.2%. Keywords: CERES, EOS Instrument, Radiometry, Calibration, Validationt

Present study describes a methodology to establish an empirical expression to estimate the upper tropospheric humidity (UTH) from brightness temperature observations in water vapour channel of Very High Resolution Radiometer (VHRR) onboard Indian geostationary satellites INSAT-3A and Kalpana. Radiative transfer simulations for VHRR water vapour channel were made using SBDART model for tropical atmosphere with different upper tropospheric relative humidity values and varying zenith angles. INSAT-3A and Kalpana VHRR sensor response functions (SRF) for water vapour channel were used to simulate the convolved radiances. Estimated UTH values have been compared with corresponding Meteosat-5 observations. Comparison of retrieved UTH is also made with radiosonde observations of relative humidity weighted by water vapour channel weighting function.

The AIRS instrument was launched in May 2002 into a polar sun-synchronous orbit onboard the EOS Aqua Spacecraft. Since then we have released three versions of the AIRS data product to the scientific community. AIRS, in conjunction with the Advanced Microwave Sounding Unit (AMSU), produces temperature profiles with 1K/km accuracy on a global scale, as well as water vapor profiles and trace gas amounts. The first version of software, Version 2.0 was available to scientists shortly after launch with Version 3.0 released to the public in June 2003. Like all AIRS product releases, all products are accessible to the public in order to have the best user feedback on issues that appear in the data. Fortunately the products have had exceptional accuracy and stability. This paper presents the improvement between AIRS Version 4.0 and Version 5.0 products and shows examples of the new products available in Version 5.0.

A climatological study of cirrus occurrence has been carried out using the ground-based lidar observations over Gadanki (13.5°N, 79.2° E) during 1998-2004. The Moderate Resolution Imaging Spectroradiometer (MODIS) measurements on the Terra spacecraft are also used for remote sensing of high clouds cirrus from space during 2001-2004. The interannual study using LIDAR and MODIS shows an enhancement in the cirrus occurrence during 2001 and fewer amount during 2002. The interseasonal variation of cirrus occurrence frequencies shows much of the occurrences during the monsoon season. Further lidar observations shows that the cirrus cloud tops typically extended to near the 16.53 km, the average tropical tropopause height. The distribution of maximum cloud base height frequency is confined to 10-12km. Frequency of occurrence of cloud physical thickness with respect to cloud base height (Zb) gives a higher occurrence between 11- 15 km and typically the thickness of 2-4 km. At the cloud base height Zb>15km, which is in the vicinity of tropopause, the cirrus is found to have lesser thickness. A significant observation from this statistical study over this latitude shows appearance of cirrus at two different altitudes because of different formation mechanisms. We will also discuss the formation mechanisms for the occurrence of tropical cirrus at this latitude.

Quantitative estimates of the spatio-temporal variations in deep convective events over the Indian subcontinent, Arabian Sea, Bay of Bengal, and tropical Indian Ocean are carried out using the data obtained from Advanced Very High Resolution Radiometer (AVHRR) onboard NOAA-14 and NOAA-16 during the period 1996-2003. Pixels having thermal IR brightness temperature (BT) less than 245K are considered as high altitude clouds and those having BT<220 K are considered as very high altitude clouds. Very deep convective clouds are observed over north Bay of Bengal during the Asian summer monsoon season when the mean cloud top temperature reaches as low as 190K. Over the Head Bay of Bengal (HBoB) from June to September, more than 50% of the observed clouds are deep convective type and more than half of these deep convective clouds are very deep convective clouds. Histogram analysis of the cloud top temperatures during this period shows that over HBoB the most prominent cloud top temperature of the deep convective clouds is ~205K over the HBoB while that over southeast Arabian Sea (SEAS) is ~220K. This indicates that most probably the cloud top altitude over HBoB is ~2 km larger than that over SEAS during the Asian summer monsoon period. Another remarkable feature observed during the Asian summer monsoon period is the significantly low values of deep convective clouds observed over the south Bay of Bengal close to Srilanka, which appears as a large pool of reduced cloud amount surrounded by regions of large-scale deep convection. Over both SEAS and HBoB, the total, deep convective and very deep convective cloud amounts as well as their corresponding cloud top temperatures (or the altitude of the cloud top) undergo large seasonal variations, while such variations are less prominent over the eastern equatorial Indian Ocean.

Satellite images are useful for creating updated land cover maps. But the major problem in these images is that the region below the clouds are not covered by the sensor. Hence cloud detection and removal is very vital in the processing of satellite imagery. The objective of this study is to propose an approach for automatic detection and removal of cloud and its shadow contamination from Satellite Images. After detection and removal of the contamination the method will selectively replace the data from different images of the same area to minimize the cloud contamination effect. Detection is achieved by performing two cloud segmentation algorithms namely Average brightness thresholding (ABT) algorithm and Region growing algorithm. Finally the performance of the two algorithms are compared to detect the exact cloud region. This is followed by the detection of the corresponding shadow pair. Finally the detected cloud contamination is removed and replaced with the data from different images of the same area. The algorithms were tested using multispectral ASTER(Advanced Space Borne Thermal Emission and Reflection Radiometer) and LISS III data. The procedure is computationally efficient and hence could be very useful in providing improved weather forecast, land cover and analysis products.

The cloud microphysical properties retrieved from MODIS sub-sampling radiance dataset (MOD02SSH) over the extended GAME region are shown in this paper. The spatial distributions and temporal variations of the cloud optical thickness (τ_c) and effective particle radius (r_e), are important observation targets to understand the role of clouds in the radiation budget estimations, especially in the cloud-aerosol interaction studies, because they will have information of cloud growth process. Thus, the wide-area and relatively high-temporal observations of the cloud properties are necessary. The most passed researches that retrieved near-global distributions of the cloud properties, used the reduced data volume satellite radiance subset, such as the globally map-projected radiances dataset at the discretized degrees in latitude and longitude (e.g. 0.5 degrees) or narrow-swath (around nadir) radiance subset. In fact, these subsets reduced the computing time but unexpectedly reduced the observation samplings. In this study, we used MODIS 5-km subsampling radiance subsets (MOD02SSH) for the retrievals. The MOD02SSH conserves scene texture with moderately reduced data volume of 1/25 from original MODIS scene, so that they will be suitable for the more precise estimations of τ_c and r_e over synoptic to global scale. We retrieved τ_c and r_e from one-month MOD02SSH over the extended GAME region (60E-180E longitude and 20S to 60N latitude) in July 2004. In the obtained τ_c versus r_e scatter plots, we found interesting features that will be explained as the cloud properties under the pristine and polluted environments in the region. The non-hydrostatic spectral microphysics cloud model simulates these phenomena well. In the presentation, we will discuss about the MODIS radiance dataset for the cloud analysis, retrieval algorithm, then show the typical and interesting features appeared in observed τ_c - r_e scatter plots over the extended GAME regions.

We studied a method to retrieve the optical thickness and effective particle radius of water clouds using the split-window channels and the 8.7-μm channel of Meteosat-8. Valid ranges are approximately from 1 to 9 for optical thickness and smaller than 18 micron for effective particle radius. The retrieval used the brightness temperature (TBB) and brightness temperature difference (BTD) between the split windows, as computed with the radiation code RSTAR5b for various properties of water clouds and vertical profiles of temperature and water vapor. The retrieved cloud parameters were then compared to those retrieved by the solar reflection method, which employs the 0.6-, 3.9-, and 11- μm channels of Meteosat-8. Comparison between the two methods revealed that the split-window technique could capture spatial features for both optical thickness and effective particle radius. The BTD is a good indicator for optical thickness. The diurnal variation of BTD shows the minimum value (thickest) before sunrise. Further precipitation and optical depth estimated from TMI/TRMM are compared with optical properties.

The importance of measuring the complete global range of cloud optical depth distribution is reviewed. While current techniques do a fairly good job of measuring small and moderate thicknesses, the difficulty of measuring large optical depths is noted. The use of multiangle views of cloud reflectivity offers a possible solution to this difficulty, at least for those clouds that have unobscured sides. Preliminary work with Multiangle Imaging SpectroRadiometer (MISR) data shows the ability to constrain the retrieved optical depths using a combination of geometrical reconstruction and three-dimensional radiative transfer modeling.

The south-west monsoon seasonal variations of the rainfall and water vapor over the Indian subcontinent and oceans are studied using microwave (MW) and near-Infrared (NIR) satellite measurements on monthly scales. The total precipitable water (TPW) derived from multi channel imaging data acquired with the Moderate Resolution Imaging Spectrometer (MODIS) on the Terra Spacecraft and rainfall data from merged infrared estimates calibrated against Tropical Rainfall Measuring Mission (TRMM) microwave data respectively are used. Since TPW is an important link connecting the various components of the hydrological cycle, its variability with rainfall on monthly scales have been used to meet this objective in the present study during the three successive contrasting good (normal), bad (drought) and good (normal) south-west monsoon years of 2001 to 2003 respectively.

The Micro Rain Radar (MRR) a highly resolution radar operates at a frequency of 24 GHz installed at Thumba (8.5°N, 76.9°E) under Ka band propagation experiment is used extensively to characterize the tropical rain. This radar measurements of rain were obtained with fine spatial and temporal resolutions like One minute time resolution and 200 m height resolution. With this radar for the first time classification of precipitating systems are studied. With the presence or absence of bright band a radar signature of melting layer one can classify particular rain type as convective or stratiform. For present study MRR data from September 2005 onwards are collected. The main objective is to classify precipitation system into Stratiform and Convective with the presence or absence of Bright band. Another potential of this radar is ability to give information of vertical structure of fall velocity of hydrometeors. This also gives profiles of number concentration of various ranges of Drop sizes, liquid water content and rain rate for different heights. These results are compared with the collocated ground based Disdrometer. Attenuation at Microwave frequencies during the presence of rain is a serious concern to the communication. Once temporal and spatial information of DSD is known microwave attenuation can be studied. These results will be presented in this paper.

Characteristics of atmospheric aerosols and their radiative impacts show large variations in space and time, leading to large uncertainties in the climate impact assessment. Despite the concerted efforts in the last decade through several field campaigns, the uncertainty still persists. This essentially arises due to lack of a comprehensive data on several aerosol parameters with adequate spatial resolution and for long duration. This is particularly true for the Indian region, with large geographical diversities, large density of population, diverse living habits and rapid industrialization. With a view to addressing to this problem and to evolve a regional scale database for assessing the radiative impacts of aerosols, an integrated, multi-platform field campaign, ICARB, was carried out during March-May 2006, under the ISRO's Geosphere Biosphere Programme. The campaign involved participation by more than 100 scientists from about 35 different institutions across the country and collocated observation over the mainland, over oceans and same time measurements of altitude profiles using aircrafts. A campaign of this magnitude is carried out for the first time in this region, and perhaps for the first time globally. The mainland observations included a network of aerosol observatories over Indian landmass and from islands in the Arabian Sea and Bay of Bengal, while a dedicated scientific cruise of 64- days duration covered the vast oceanic regions around India. Altitude profiles were obtained using research aircraft of the National Remote Sensing Agency, from 5 different bases. The details of the campaign and the preliminary findings will be presented.

The AERONET inversion products provide powerful information for understanding column integrated aerosol properties particularly given the wide global distribution of sites and the 13 year record for some sites. Significant evolution of the instrument, data quality, ancillary input data and inversion algorithm has necessitated release of Version 2.0 and establishment of criteria for quality assured products. This paper documents version 1.0 quality assurance criteria and the analysis of the entire retrieval record available for the Version 2.0 to revise the quality assured criteria. The result is an improvement in the number and quality of aerosol inversion parameters for most sites through the entire AERONET data record.

Optical observation around near UV spectral region potentially enables us to retrieve light absorbing features of aerosol, such as type as well as optical thickness. We analyzed near UV observation data to identify haze properties around Japan in the autumn of 2003, using Global Imager onboard Advanced Earth Observing Satellite-II (ADEOS-II/GLI), which has 380nm and 400nm window channels. At the same time, we had optical observation, such as a ground-based LIDAR measurement and a shipborne skyradiometer measurement, so as to retrieve vertical profile, particle sphericity, particle size distribution, and optical thickness of the haze. Based upon the three kinds of analyses with remotely sensed data, such as satellite, LIDAR, and skyradiometer, we have the following characteristics of the haze: little UV absorbing, of optical thickness 0.5 (around 500nm), within lower boundary layer (less than around 1km a.s.l.), and of spherical and fine particles (0.2 μm in radius). We also have some direct sampling measurements onboard Research Vessel Shirase, such as integrated nephelometer, particle soot / absorption photometer, and optical particle counter, so as to identify optical and microphysical properties of the haze as well as chemical composition analyses. The results of the surface direct sampling showed the dense haze dominantly consisted of smaller (0.2 μm in radius) and sulfate particulates, which is consistent to the remotely sensed results. Backward trajectory simulations also indicate that the hazy air mass had arrived from / through some mega cities over East Asia. Further, we are going to investigate the consistency between optical, microphysical, chemical, and dynamical aspects using a chemical transport model.

Continuous and near-real-time measurements of BC were made for a period of two years from the I-LARC (ISRO Laboratory for Aerosol Radiation and Chemistry) station in Port Blair as a part of the ISRO Geosphere Biosphere Programme (I-GBP). These are used to characterize BC, for the first time over the Bay of Bengal (BoB), which is surrounded by distinct landmasses having highly varying anthropogenic activities. Significantly high concentrations (~2.4 μg m^-3) occur during the period September to April. During this period, BC contributes ~ 6.5 % to the composite aerosol mass concentration. The concentration and its share to the composite aerosols decrease rapidly (by a factor of >3) and remain so during the period June to August when the station is under the influence of monsoon winds coming from the Indian Ocean. Back-trajectory analyses reveal five potential advection pathways, which are seasonal in nature and have a strong influence on the BC concentrations over the island. The results and their implications will be discussed.

Enhanced aerosol loading over the Indo-Gangetic Plain (IGP) is a regular feature during winter months. In addition to the environmental degradation and reduced visibility, these aerosols can cause significant radiative impact also. In view of this, a campaign mode observation under ISRO-GBP was conducted in December 2004 to characterize the aerosol properties over the IGP. As part of this, extensive measurements of aerosol BC were made from Kharagpur, an inland rural location lying at the eastern end of the Indo Gangetic Plain. It also lies close to several industrialized regions and area having lot of mining activities Results showed, extremely high BC concentration, often exceeding ~20 mg m^-2, prevailed during December. During this period, BC concentration also showed large diurnal variation. Simultaneous measurements of the local atmospheric boundary layer height and wind fields revealed a very close association between the BC concentration and the ventilation coefficient (defined as the product of the boundary layer height and the transport wind). Back trajectory analyses using HYSPLIT revealed that in addition to the local boundary layer dynamics, the changes in the advection pathways also influence the concentration of BC.

We made the operational sky measurements for two and half years from April 2003 to September 2005, using the sky radiometer and presented the analized results of the aerosol optical properties over Kanazawa area, Japan, namely, the optical thickness τ_a (500) at the wavelength of 0.5μm and Ångstroem exponent α. The reflectance ratios between the visible and short wave infrared bands were computed for typical ground covers, such as the vegetation, urban, and others (soil and sand), using several image data sets of Terra/MODIS and these sky observation data sets.

Estimation of atmospheric aerosols by remote sensing is very important because aerosols cool/warm the Earth atmosphere system through scattering/absorption of solar radiation. The information on aerosol optical thickness (AOT) is also essential in atmospheric correction of satellite imagery. Due to non-uniformity and extreme reflectivity of land targets, it is challenging to monitor aerosols over land-surfaces. This paper reports a new approach of estimating AOT over land using dual viewing angle observations (FORE: 26 degree and AFT: -5 degree) in panchromatic channel (0.50- 0.85μm) of Cartosat-1 satellite. Differential responses of targets observed in two atmospheric path lengths through Fore and Aft observations of Cartosat-1 were analyzed using atmospheric radiative transfer model (6S-code). A number of forward simulations over dark to bright targets (having 1-70% reflectivity) were carried out to derive top-of-atmosphere (TOA) reflectance for various AOT conditions to arrive at a particular reflectance condition called- cross over reflectance (ρ_co: a reflectance value for which the difference between TOA_Aft and TOA_Fore equals zero). The shift in position of ρ_co for dual look angle was modeled as a function of AOT. Using this method, AOT was estimated for two sites representing clear and turbid atmosphere conditions. A cross over reflectance of 11 percent and 13 percent was observed for clear and turbid atmospheres, respectively. Corresponding modeled AOT estimates were 0.32 and 0.78, respectively. Validation of these estimates with the MODIS AOT showed good agreement with 0.26 in clear and 0.68 in turbid atmosphere case. The present approach enables to retrieve single AOT value for a scene and for known aerosol meteorology. Initial results are encouraging, however further analyses are in progress to verify it in different atmospheric conditions.

The Ozone Monitoring Instrument (OMI) is first-of-its-kind hyperspectral instrument that employs two dimensional UV-enhanced CCD's to measure radiation backscattered by the Earth's atmosphere from 270-500 nm at high spectral resolution (0.45-0.63 nm), and with higher spatial resolution compared to the predecessor instruments (TOMS, SBUV, GOME, and SCIAMACHY). OMI is a Dutch-Finnish contribution to the NASA EOS Aura satellite, which was launched on 15 July 2004. The hyperspectral capability of OMI allows one to measure several trace gases in the boundary layer (NO₂, SO₂, HCHO, BrO) at urban scale resolution. In addition, OMI continues the 28-year record of data collected by the TOMS-series of instruments since Nov 1978, and SBUV-series of instruments since April 1970. These products include ozone profile, total column ozone, tropospheric column ozone, volcanic SO₂, and daily global maps of UV-absorbing aerosols. In this paper we discuss recent results from OMI, focusing on OMI products related to air quality over the Asian-Pacific region. OMI has the unique capability of seeing transport of dust and smoke above clouds. Recent refinements to the algorithm are providing estimate of aerosol absorption, important for estimating the solar radiation reaching the ground.

The LIDAR equation contains four unknown variables in a two-component atmosphere where the effects caused by both molecules and aerosols have to be considered. The inversion of LIDAR returns to retrieve aerosol extinction profiles, thus, calls for some functional relationship to be assumed between these two. The Klett's method, assumes a functional relationship between the extinction and backscatter. In this paper, we apply a different technique, called the optical depth solution, where we made use of the total optical depth or transmittance of the atmosphere along the LIDAR-measurement range. This method provides a stable solution to the LIDAR equation. In this study, we apply this technique to the data obtained using a micro pulse LIDAR (MPL, model 1000, Science and Engineering Services Inc) to retrieve the vertical distribution of aerosol extinction coefficient. The LIDAR is equipped with Nd-YLF laser at an operating wavelength of 523.5 nm and the data were collected over Bangalore. The LIDAR data are analyzed to get to weighted extinction coefficient profiles or the weighted sum of aerosol and molecular extinction coefficient profiles. Simultaneous measurements of aerosol column optical depth (at 500 nm) using a Microtops sun photometer were used in the retrievals. The molecular extinction coefficient is determined assuming standard atmospheric conditions. The aerosol extinction coefficient profiles are determined by subtracting the molecular part from the weighted extinction coefficient profiles. The details of the method and the results obtained are presented.

Atmospheric aerosols are among the most variable components of the Earth's atmospheric environment important in general circulation models related to climate change. The presence of aerosols in the lower atmosphere affects primarily the incoming solar radiation by scattering and absorbing the solar radiation. Aerosols have significant impact on climate through their influence on cloud formation and on minor species concentrations. Studies on Aerosols with respect to temporal and spatial variations in different environments gains importance. Synchronous measurement of Aerosol Optical Depth (AOD), solar irradiance in different wavelength bands, aerosol particle size distribution measurements and Black Carbon (BC) aerosol mass concentration were made at urban area of Hyderabad, India as a part of ISRO-GBP initiative. The Julian day variation of AOD, Particulate Matter (PM) and BC showed higher values on certain days suggesting additional sources of aerosols over urban area of Hyderabad. In order to understand the additional sources of aerosol, daily satellite data sets of MODIS/DMSP-OLS were processed for forest fires over the Indian region. The higher values in black carbon aerosol mass concentration and aerosol optical depth correlated well with forest fires over the region. Radiative forcing estimated from synchronous measurements of AOD and ground reaching broadband solar irradiance. Ground AOD measurements correlated well with MODIS derived AOD at different wavelengths. Results are discussed in the paper.

Impact of atmospheric aerosols on the radiation budget of the Earth-atmosphere system and climate is well recognized. The spatial distribution and the properties of atmospheric aerosols over the oceanic areas around the Indian subcontinent during the Asian dry season (November-April period) are significantly governed by the transport of continental air from the adjoining landmass. In the present study, we report the estimated spatial distribution of monthly mean aerosol direct radiative forcing (ADRF) at the top-of-atmosphere (TOA), within the atmosphere, and at the earth's surface over the Arabian Sea, Bay of Bengal and Equatorial Indian Ocean during the Asian dry season, based on the regional distribution of aerosol optical depth (AOD) derived from NOAA14/16-AVHRR data during 1996-2003, and the model estimates of the diurnal mean normalized aerosol radiative forcing (NADRF) which also compares fairly well with the earlier observations reported over this region. The diurnal mean NADRF varies with latitude and Julian day. Its value at TOA (for AOD at 550nm) is in the range -26.5 Wm^-2AOD ^-1 to -29.5 Wm^-2 AOD^-1 while the corresponding value at surface is in the range -77 to -95 Wm^-2 AOD^-1. In the Northern Hemisphere, ADRF is in the range -4 to -14 Wm^-2 at TOA, -2 to - 42 Wm^-2 at the surface and 8 to 28Wm^-2 in the atmosphere. During the Asian dry season highest DMADRF is observed in the northwest Bay of Bengal followed by the southeast Arabian Sea during the March-April period.

In this paper, we report the results of extensive, and all-season, collocated, measurements of several aerosol parameters [such as spectral aerosol optical depth (AOD) at 10 bands spanning from UV to IR; mass size distribution and mass concentration of composite aerosols; as well as mass concentration and mass mixing ratio of aerosol black carbon (BC)] for over a 4-year period (January 2000 to December 2003), from an unindustrialized coastal location, Trivandrum (8.55°N, 76.9°E), close to the southern tip of Indian peninsula and use these properties to estimate the aerosol short wave radiative forcing. The results show that the top of the atmosphere (TOA) forcing is significantly positive during winter while it changes to negative during monsoon and post monsoon seasons. The surface forcing decreases from winter to summer. Consequently, the net atmospheric absorption decreases from a high value in winter to low values during monsoon.

The role of local production and transport in governing the spatial distribution of aerosol loading over the Indian subcontinent and adjoining oceanic regions are examined using satellite derived aerosol optical depth (AOD) and wind speed obtained from National Centre for Environmental Prediction (NCEP) reanalysis for the year 2004 (study domain: 20S°-40°N, 40°-120°N). The association of AOD with wind speed, wind convergence, vorticity and vertical velocity are examined for the Arabian Sea, Bay of Bengal, equatorial Indian Ocean as well as over the land surface in central India for different seasons. This analysis shows that the AOD dependence on these dynamical variables are significantly different for different seasons. Introducing MODIS derived AOD and the NCEP winds into an aerosol flux continuity equation, the locations of aerosol generation are identified and their strengths estimated for different months.

We have compared the spectral aerosol optical depth (AOD) and aerosol fine mode fraction (AFMF) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) with those of Aerosol Robotic Network (AERONET) at Kanpur (26.45N, 80.35E), northern India for the pre-monsoon season (March to June, 2001-2005). We found that MODIS systematically overestimates AOD during pre-monsoon season (known to be influenced by dust transport from north-west of India). The errors in AOD were correlated with the MODIS top-of-atmosphere apparent surface reflectance in 2.1 μm channel (ρ^*_2.1). MODIS aerosol algorithm uses (ρ^*_2.1) to derive the surface reflectance in visible channels (_{ρ0.47, ρ0.66}) using an empirical mid IR-visible correlation (_ρ0.47= _ρ2.1/4, _{ρ0.66 ρ2.1}/2). The large uncertainty in estimating surface reflectance in visible channels (Δ_ρ0.66±0.04, Δ_ρ0.47±0.02) at higher values of ρ^*_2.1 (ρ^*_2.1>0.18) leads to higher aerosol contribution in the total reflected radiance at top-of-atmosphere to compensate for the reduced surface reflectance in visible channels and thus leads to overestimation of AOD. This was also reflected in the very low values of AFMF during pre-monsoon whose accuracy depends on the aerosol path radiance in 0.47 and 0.66 μm channels and aerosol models. The errors in AOD were also high in the scattering angle range 110°-140°, where the effect of dust non-spherity on its optical properties is significant. The direct measurements of spectral surface reflectance are required over the Indo-Gangetic basin in order to validate the mid IR-visible relationship. MODIS aerosol models should also be modified to incorporate the effect of non-spherity of dust aerosols.

Correct assessment of aerosol properties is a pre-requisite for climate change study. On account of large heterogeneity in their properties both on spatial and temporal scales, satellite remote sensing is an ideal tool to study them. But the advantage offered by satellites is inhibited by contamination from surface reflectance and cloud interference. In the past, satellite remote sensing of aerosols was limited to over oceans, which offered a dark background. With the launch of MODIS instrument onboard Terra and Aqua satellites observations have been extended to over land. MODIS derives aerosol properties by making an assessment of surface reflectance at visible wavelengths based on mid-IR reflectance. This process has an empirical basis but the validity can only be verified by comparing the results against ground truth data. In this study MODIS derived AOD is validated against the ground based sunphotometer observations made at Ahmedabad (23.03° N, 72.53°E), an urban location in Western India, from 2002 to 2005. The local meteorology is summer from March till July, monsoon during July to September and winter from October to February. MODIS AOD data at 470 nm and 660 nm from both Terra and Aqua averaged over a 0.5×0.5 degree box centered at Ahmedabad are compared with the ground truth data. An overestimation up to 150% by MODIS during April- June and an underestimation up to 50% during October to March is found. An attempt to explain these differences in terms of seasonal variation in surface reflectivity and cloud contamination is presented and discussed.

Clouds are the obstruction of visual and infrared remote sensing and their shadows may also lead up to an intolerable bias of the true reflectance of the underlying terrain elements. Thus a reliable cloud and shadow mask is essential before the further processing. Clouds cast shadows on the earth's surface. On the high resolution remote sensing images, clouds' profiles and their shadows' are resemblant. Based on this truth, we employed a robust image matching algorithm called Modified Partial Hausforff Distance(MPHD) to find the match with every cloud and its shadow and finally calculated the pixel distance between them. Before the match task we took into account topologic relationships such as coverage and fragmentation to improve the match result. Not only were the match pairs detected but also the pixel distances from each cloud to its shadow were obtained. Then we can use a pixel distance to predict a shadow of a cloud by translating the cloud. Given sunbeam's direction and viewing angles we may get cloud height with simple geometry calculation.

A study of detecting the sea fog from NOAA-16, and -17 Advanced Very High Resolution Radiometers (AVHRR3) is presented. Sea fog occurs frequently over China Sea at different season, especially on spring, and forms an obstacle for sea traffic. In this paper, several sea fog events are selected, analyzed and compared; their radiance and texture characterization with other bodies from satellite observations is given. Firstly, It is found that the satellite measured reflectance of channel 3a (1.58-1.64μm) is between the channel 1(0.58-0.68μm) and channel 2(0.725-1.0μm), and the one of channel 2 is lowest, which is different from the ocean, the land and some cloud body. Those results are explained with Mie scatter theory. Secondly, the sea fog, which is the cloud clung the earth surface, is generally characterized by higher reflectance than the surrounding water in visible, while it is similar to the surrounding in the infrared radiance brightness temperature. According those different feathers, a parameter, which is the standard deviation of the reflectance (or brightness temperature) differences between the adjacent eight pixels and the one, is defined as the texture to detect the marine fog. Thirdly, the sea fog is often coherent and continuous in spatial. It could be used to modify and restore some misjudge point or segment. Based on those analyses, finally, we design an algorithm for detect the sea fog at day. The results show that this method is valid to extract the fog region from NOAA/AVHRR3.

On the basis of the numerical results by using finite-difference time-domain (FDTD) methods, we discuss the light scattering and its dependence on the particles' physical properties, such as particle shape, surface roughness, and porosity. The FDTD calculations were conducted for single hexagonal columns and aggregates with size parameter up to 50 taking into account the particles' random orientation. The discussion includes the depolarization ratio as well as basic radiative transfer parameters: phase function, extinction efficiency and asymmetry factor. Results of our light-scattering calculations show that the simple deformation of the original shape alters the optical properties. Comparing with the shape effects, surface micro-roughness and porosity cause minor changes in some scattering parameters However, the micro-roughness makes the depolarization ratios increase as well as the irregular shaped aggregate particles.

The Atmospheric Infrared Sounder (AIRS) onboard Aqua satellite is providing a wealth of highly accurate atmospheric and surface information using 2378 high-spectral-resolution infrared (3.7 - 15.4 μ) channels. Cooperative Institute for Meteorological Satellite Studies (CIMSS) has developed International MODIS/AIRS Processing Package (IMAPP) to retrieve atmospheric and surface parameters from AIRS-L1B radiance measurements. CIMSS retrieval algorithm is based on principal component regression technique. In order to account for retrieval dependency on zenith angle and regional/seasonal variations a classification scheme is employed based on scan angle classification and window-channel brightness temperature classification. To improve atmospheric sounding retrieval for a specific region, which is useful for AIRS direct broadcast users, regional regression coefficients have been generated for Indian region. Training dataset of radiosonde observations over India and surrounding region have been used to generate regional regression coefficients for IMAPP-AIRS processing. Retrieval error statistics was generated using simulated radiances from independent dataset of radiosonde observations over Indian region. This study shows that the Root Mean Square (RMS) error in humidity profile is reduced by ~25% when compared to the global regression coefficients, whereas RMS error for temperature profile is reduced by ~0.2 K. This study is also useful for sounding retrieval from geostationary sounder measurements, for example, for Geostationary Operational Environmental Satellite (GOES) Sounder and INSAT-3D Sounder that have observations over a limited region with high spatial and temporal resolution.

Simultaneous occurrence of two bands of inter tropical convergence zone (ITCZ) on either side of the equator, generally known as double-ITCZ (DITCZ), over the Equatorial Indian Ocean (EIO) is investigated using the cloud characteristics derived from NOAA-14/16-AVHRR data (1996-2003), the monthly mean cloud characteristics obtained from the International Satellite Cloud Climatology Project (ISCCP-D2) (1984-2004) and the monthly mean outgoing long wave radiation (OLR: 1974-2004) data obtained from NOAA. A well discernible signature of DITCZ could be observed over the EIO in terms of total as well as high cloud amount and OLR. The doubling of ITCZ occurs mainly in the western part of EIO between 50°E and 80°E. The frequency of occurrence of DITCZ over the Indian Ocean is largest in November (percentage of occurrence ~85%) and December (~62%), which is significantly larger than that reported from earlier studies. The most preferred latitude for the northern and southern bands of DITCZ is ~5°N and ~7.5°S respectively in November and ~5°N and ~10°S in December. The amplitude of the DITCZ, defined as the difference between the total cloud fraction in the equatorial region of minimum cloudiness and that in the respective bands of the DITCZ, is 0.05 to 0.25 for the southern branch and 0.05 to 0.15 for the northern branch. The corresponding amplitude in terms of OLR is 10Wm^-2 to 15Wm^-2 in the southern band and 5Wm^-2 to 10Wm^-2 in the northern band.

The paper deals with the vectorial radiative transfer equation (VRTE) problem for a homogeneous strongly anisotropic scattering slab illuminated by a plain unidirectional source of light with an arbitrary angle of irradiance and polarization state. The problem is a theoretical base for the polarized satellite remote sensing (POLDER, PARASOL and others). The VRTE boundary problem decomposition allows reducing to the nonreflecting bottom with subsequent including its polarization properties. We give the complete analysis for the solution smooth non-small angle part for the vectorial small angle modification of the spherical harmonics method (VMSH) built upon the smoothness of the spatial spectrum of the light field distribution vector-function caused by mathematical singularities of the top-boundary condition for the VRTE boundary problem and the anisotropy of many natural scattering media (clouds, ocean). The VMSH itself is described as well.

In the present study Lower Atmospheric Wind Profiler (LAWP) operating at a frequency of 1.3 GHz is installed at Gadanki (13.50°N, 79.20°E) is used to study the characteristics of radar Bright Band. Radar observations of the melting layer of precipitation have been made from few decades. It has been known since then that the melting of precipitation is often associated with an increase of the reflectivity of weather targets. The primary cause for this is a rapid increase in the dielectric constant of hydrometeors at the top of the melting layer followed by an increase of the fall velocities of melting snowflakes towards the end of the melting process. VHF/UHF radars can also be used to study radar bright band, a strong enhancement in reflectivity at around 0°C isotherm level. Using VHF/UHF radars bright band structure are studied particularly in the tropics. Though lot of work has been done on the radar signature of the melting layer, still microphysics behind its formation possesses a challenging task to the atmospheric scientists. The ability of the profilers to clearly resolve and identify the melting layer when it is present gives very good information regarding the classification of various precipitating systems within the Tropical Mesoscale Convective Systems (TMCS). VHF/UHF radars are the wonderful tools in classifying the systems. LAWP data during the period of March 1999 to September 2000 used for the present study. Dependency of bright band height and thickness on various parameters is investigated. An automated bright band detection algorithm is made to detect the height and thickness of bright band. It is compared well with the radiosonde observations. A statistical study has been done in correlating bright band thickness with that of the precipitation intensity. Further in the present paper altitude variation of bright band with the seasons are calculated.

Tropical regions can be characterized as large fields of convective clouds of all sizes. Latent heat released is different for different precipitating systems like convective and stratiform. So we need to classify various precipitating systems. In the present study, ground based observations of Joss-Waldvogel Disdrometer (JWD) which was installed at Thumba (8.5°N, 76.9°E) under Ka band propagation experiment is used extensively to characterize the tropical rain. It can be noticed that the JWD is placed at calm and noise-free places, in order to make it sensitive to smaller drops. The JWD is a standard tool for precipitation measurements such as Drop Size Distribution (DSD), rainfall intensity, R, rain accumulation and liquid water content, W, reflectivity factor, Z. The range of drop diameters that can be measured spans from 0.3 to 5 mm with an accuracy of 5%. For present study Disdrometer data from June 2005 onwards are collected. The main objective of the present study is to classify precipitation system into Convective, Transition (an intermediate region) and stratiform. Since DSD integral parameters like rain rate (R), liquid water content (LWC), Reflectivity (Z) are different for different precipitating systems, so we need to classify these systems. There is a dearth of raindrop Size data and distribution models for the tropics, especially over Indian continent. Models for drop size distribution are required for the evaluation of microwave and millimeter wave propagation effects due to rainfall. In the present paper various DSD models namely gamma model and lognormal model with different combination of moments for observing the characteristic features of tropical rain are studied.

At NASA Langley Research Center (LaRC), radiances from multiple satellites are analyzed in near real-time to produce cloud products over many regions on the globe. These data are valuable for many applications such as diagnosing aircraft icing conditions and model validation and assimilation. This paper presents an overview of the multiple products available, summarizes the content of the online database, and details web-based satellite browsers and tools to access satellite imagery and products.