Hiren Jethva

Seasonal variability of aerosols over the Indo-Gangetic basin

We examine the spatio-temporal characteristics of aerosols in the recent years (2000-2003) over the Indian region with special emphasis on the Indo-Gangetic basin (northern India) using data from Moderate Resolution Imaging Spectroradiometer (MODIS), Aerosol Robotic Network (AERONET) and Total Ozone Mapping Spectroradiometer (TOMS). First, we have compared the MODIS-derived aerosol optical depth (AOD) and fine-mode aerosol fraction (FMAF ratio of the fine-mode AOD to the total mode AOD) with those of AERONET at Kanpur (26.45 degrees N, 80.346 degrees E). It has been found that the MODIS captures the major part of the seasonal variation of aerosols in terms of abundance as well as aerosol type. The absolute errors in AOD were within the predicted uncertainty of Delta \tau = +/-0.05 +/- 0.2 \tau. The monthly mean regional maps of MODIS show high aerosol optical depth (AOD) over the Indo-Gangetic basin in the range 0.6-1.2 at 550 nm wavelength with significant spatial and temporal variation during the summer (April to June). The associated FMAF was found to be low (<0.4). This indicates that the coarse-mode particles are dominant in the summer. The spatial distribution of absorbing aerosol index (AAI) derived from TOMS, Angstrom exponent (alpha) and aerosol volume size distribution measured at Kanpur also indicated the presence of absorbing coarse-mode aerosols during summer. On the other hand, the entire Indo-Gangetic basin was dominated by the fine-mode particles during the winter (November to January) with AOD in the range 0.4-0.6. Their spatial and temporal variations, however, were quite low compared to the summer. Results reported in this paper indicate that the Indo-Gangetic basin has the largest aerosol optical depth in India during both the seasons. The region is dominated by the large absorbing coarse-mode particles (possibly transported dust from the northwest of India) in the summer and by the probable widespread emission sources of fine-mode aerosols (primarily of anthropogenic origin) in the winter. The unique topography and weather condition of the region have impact on the observed spatial and temporal distribution of aerosols.

A Color Ratio Method for Simultaneous Retrieval of Aerosol and Cloud Optical Thickness of Above-Cloud Absorbing Aerosols From Passive Sensors: Application to MODIS Measurements

The presence of absorbing aerosols above cloud decks reduces the amount of upwelling ultraviolet (UV), visible (VIS), and shortwave infrared radiation reaching the top of atmosphere. This effect is often referred to as “cloud darkening,” which can be seen by eye in images and quantitatively in the spectral reflectance measurements made by passive sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS) in the regions where light-absorbing carbonaceous and dust aerosols overlay low-level clouds. Radiative transfer simulations support the observational evidence and further reveal that the reduction in the spectral reflectance and color ratio between a pair of wavelengths is a function of both aerosol and cloud optical thickness (AOT and COT). For a prescribed set of aerosol and cloud properties and their vertical profiles, thus, the measured reflectance can be associated with a pair of AOT and COT. Based on these results, a retrieval technique has been developed, which is named as the “color ratio method,” which utilizes the measurements at a shorter (470 nm) and a longer (860 nm) wavelength for the simultaneous derivation of AOT and COT. The retrieval technique has been applied to the MODIS 1-km reflectance measurements for the two distinct above-cloud smoke and dust aerosols events. This study is an extension of the previously developed near-UV method to the VIS spectral region. However, it constitutes the first attempt to use non-UV wavelengths to retrieve above-cloud AOT by a passive nonpolarized sensor. An uncertainty analysis has been presented, which estimates the expected error associated with these retrievals.

Assessment of second‐generation MODIS aerosol retrieval (Collection 005) at Kanpur, India

The second-generation MODIS aerosol retrieval (Collection 005) from EOS- Aqua (2002–2005) was evaluated using ground-based AERONET measurements at Kanpur

How Good is the Assumption About Visible Surface Reflectance in MODIS Aerosol Retrieval Over Land? A Comparison With Aircraft Measurements Over an Urban Site in India

(26.45 ^\circ N, 80.35^ \circ E)

Role of variation in vertical profiles of relative humidity on retrieval of humidity from AMSU‐B data

northern India. We found that the aerosol optical depth (AOD) retrievals are more accurate compared to that of previous retrieval (Collection 004). About 70\% of the total retrievals at 0.47 \mu m and 0.55 \mu m and 60\% of the retrievals at 0.66 \mu m wavelength in the new version fall within the pre-launch uncertainty (\Delta \tau = \pm 0.05 \pm 0.15 \tau, where \tau is AOD) with better correlation

Satellite-based evidence of wavelength-dependent aerosol absorption in biomass burning smoke inferred from Ozone Monitoring Instrument

(R^2 \sim 0.83)

OMI and MODIS observations of the anomalous 2008–2009 Southern Hemisphere biomass burning seasons

at all three wavelengths. However, MODIS still tends to over-estimate AOD for a few retrievals in the presence of dust aerosols. The error in the fine-dominated AOD was large for most retrievals in the C005. However, the fine-dominated AOD of Collection 004 was better correlated with the equivalent AERONET data. This suggests that the fine-dominated AOD retrievals need to be re-examined further.

Evaluation of Moderate‐Resolution Imaging Spectroradiometer (MODIS) Collection 004 (C004) aerosol retrievals at Kanpur, Indo‐Gangetic Basin

In this paper, we discuss the measurements of spectral surface reflectance (rholambda s) in the wavelength range 350-2500 nm measured using a spectroradiometer onboard a low-flying aircraft over Bangalore (12.95degN, 77.65deg E), an urban site in southern India. The large discrepancies in the retrieval of aerosol properties over land by the Moderate-Resolution Imaging Spectroradiometer (MODIS), which could be attributed to the inaccurate estimation of surface reflectance at many sites in India and elsewhere, provided motivation for this paper. The aim of this paper was to verify the surface reflectance relationships assumed by the MODIS aerosol algorithm for the estimation of surface reflectance in the visible channels (470 and 660 nm) from the surface reflectance at 2100 nm for aerosol retrieval over land. The variety of surfaces observed in this paper includes green and dry vegetations, bare land, and urban surfaces. The measured reflectance data were first corrected for the radiative effects of atmosphere lying between the ground and aircraft using the Second Simulation of Satellite Signal in the Solar Spectrum (6S) radiative transfer code. The corrected surface reflectance in the MODISs blue (rho470 s), red (rho660 s), and shortwave-infrared (SWIR) channel (rho2100 s) was linearly correlated. We found that the slope of reflectance relationship between 660 and 2100 nm derived from the forward scattering data was 0.53 with an intercept of 0.07, whereas the slope for the relationship between the reflectance at 470 and 660 nm was 0.85. These values are much higher than the slope ( ~0.49) for either wavelengths assumed by the MODIS aerosol algorithm over this region. The reflectance relationship for the backward scattering data has a slope of 0.39, with an intercept of 0.08 for 660 nm, and 0.65, with an intercept of 0.08 for 470 nm. The large values of the intercept (which is very small in the MODIS reflectance relationships) result in larger values of absolute surface reflectance in the visible channels. The discrepancy between the measured and assumed surface reflectances could lead to error in the aerosol retrieval. The reflectance ratio (rho660 s/rho2100 s) showed a clear dependence on the NDVI SWIR where the ratio increased from 0.5 to 1 with an increase in NDVI SWIR from 0 to 0.5. The high correlation between the reflectance at SWIR wavelengths (2100, 1640, and 1240 nm) indicated an opportunity to derive the surface reflectance and, possibly, aerosol properties at these wavelengths. We need more experiments to characterize the surface reflectance and associated inhomogeneity of land surfaces, which play a critical role in the remote sensing of aerosols over land.

Improved retrieval of aerosol size-resolved properties from moderate resolution imaging spectroradiometer over India: Role of aerosol model and surface reflectance

In this paper, we have compared the relative humidity (RH) retrieved from 183 GHz channel brightness temperature (BT) of AMSU-B (Advanced Microwave Sounding Unit-B) with those obtained from radiosondes in the Bay of Bengal during BOBMEX (Bay of Bengal Monsoon Experiment) and in the Arabian Sea during ARMEX (Arabian Sea Monsoon Experiment). The NCAR (National Center for Atmospheric Research) radiative transfer (RT) model has been used to obtain the relationship between layer-averaged RH and BT. With the present RT model, it has been found that the linear dependence of natural logarithm of layer-averaged RH on BT is valid, if the profile of humidity is invariant. However, the relative humidity profiles obtained during BOBMEX and ARMEX experiments showed a large variability in the vertical profiles of humidity in middle and upper troposphere. In such situation, the relation between layer-averaged RH and BT was found to be linear.