Publications

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2012
Auffhammer, M, Ramanathan V, Vincent JR.  2012.  Climate change, the monsoon, and rice yield in India. Climatic Change. 111:411-424.   10.1007/s10584-011-0208-4   AbstractWebsite

Recent research indicates that monsoon rainfall became less frequent but more intense in India during the latter half of the Twentieth Century, thus increasing the risk of drought and flood damage to the country's wet-season (kharif) rice crop. Our statistical analysis of state-level Indian data confirms that drought and extreme rainfall negatively affected rice yield (harvest per hectare) in predominantly rainfed areas during 1966-2002, with drought having a much greater impact than extreme rainfall. Using Monte Carlo simulation, we find that yield would have been 1.7% higher on average if monsoon characteristics, especially drought frequency, had not changed since 1960. Yield would have received an additional boost of nearly 4% if two other meteorological changes (warmer nights and lower rainfall at the end of the growing season) had not occurred. In combination, these changes would have increased cumulative harvest during 1966-2002 by an amount equivalent to about a fifth of the increase caused by improvements in farming technology. Climate change has evidently already negatively affected India's hundreds of millions of rice producers and consumers.

2006
Auffhammer, M, Ramanathan V, Vincent JR.  2006.  Integrated model shows that atmospheric brown clouds and greenhouse gases have reduced rice harvests in India. Proceedings of the National Academy of Sciences of the United States of America. 103:19668-19672.   10.1073/pnas.0609584104   AbstractWebsite

Previous studies have found that atmospheric brown clouds partially offset the warming effects of greenhouse gases. This finding suggests a tradeoff between the impacts of reducing emissions of aerosols and greenhouse gases. Results from a statistical model of historical rice harvests in India, coupled with regional climate scenarios from a parallel climate model, indicate that joint reductions in brown clouds and greenhouse gases would in fact have complementary, positive impacts on harvests. The results also imply that adverse climate changes due to brown clouds and greenhouse gases contributed to the slowdown in harvest growth that occurred during the past two decades.

2005
Ramanathan, V, Ramana MV.  2005.  Persistent, widespread, and strongly absorbing haze over the Himalayan foothills and the Indo-Gangetic Plains. Pure and Applied Geophysics. 162:1609-1626.   10.1007/s00024-005-2685-8   AbstractWebsite

We examine the impact of the Atmospheric Brown Clouds on the direct radiative forcing of the Himalayan foothills and the Indo-Gangetic Plains (IGP) regions, home for over 500 million S. Asians. The NASA-Terra MODIS satellite data reveal an extensive layer of aerosols covering the entire IGP and Himalayan foothills region with seasonal mean AODs of about 0.4 to 0.5 in the visible wavelengths (0.55 micron), which fall among the largest seasonal mean dry season AODs for the tropics. We show new surface data which reveal the presence of strongly absorbing aerosols that lead to a large reduction in solar radiation fluxes at the surface during the October to May period. The three-year mean (2001 to 2003) October to May seasonal and diurnal average reduction in surface solar radiation for the IGP region is about 32 (+/- 5) W m(-2) (about 10% of TOA insolation or 20% of surface insolation). The forcing efficiency (forcing per unit optical depth) is as large as -27% (note that the forcing is negative) of top-of-atmosphere (TOA) solar insolation, and exceeds the forcing efficiency that has been observed for other polluted regions in America, Africa, East Asia, and Europe. General circulation model sensitivity studies suggest that both the local and remote influence of the aerosol induced radiative forcing is to strengthen the lower atmosphere inversion, stabilize the boundary layer, amplify the climatological tendency for a drier troposphere, and decrease evaporation. These aerosol-induced changes could potentially increase the life times of aerosols, make them more persistent, and decrease their single scattering albedos, thus potentially leading to a detrimental positive feedback between aerosol concentrations, aerosol forcing, and aerosol persistence. In addition, both the model studies and observations of pan evaporation suggest that the reduction in surface solar radiation may have led to a reduction in surface evaporation of moisture. These results suggest the vulnerability of this vital region to air pollution related direct and indirect (through climate changes) impacts on agricultural productivity of the region.

2002
Meywerk, J, Ramanathan V.  2002.  Influence of anthropogenic aerosols on the total and spectral irradiance at the sea surface during the Indian Ocean Experiment (INDOEX) 1999. Journal of Geophysical Research-Atmospheres. 107   10.1029/2000jd000022   AbstractWebsite

[1] Unique measurements of the spectral signature of the aerosol forcing at the surface between 350 and 1050 nm wavelength during the 1999 haze event over the tropical Indian Ocean are presented and discussed. The aerosol visible optical depths reached values as high as 0.7 in the northern Indian Ocean and decreased to about 0.1 south of the Intertropical Convergence Zone (ITCZ) in the Southern Hemisphere (SH) tropical oceans. Measurements of aerosol optical depth and global (direct plus diffuse) irradiance have been taken with a grating spectroradiometer with a resolution of 3 nm onboard R/V Ronald H. Brown. The radiometer was calibrated onboard in real time. We inverted the direct solar spectra to obtain aerosol optical depth and to compare these with three independent sunphotometer optical depth at selected spectral intervals to assess the validity of the retrieved optical depth. We took the difference between aerosol optical depth between polluted and pristine air mass spectra to obtain the spectral signature. The difference in optical depth between the polluted regime north of the ITCZ and the pristine air masses south of the ITCZ were between 0.5 at 350 nm wavelength, 0.35 at 500 nm wavelength, and 0.1 at 1050 nm wavelength. Next, we obtained the aerosol forcing by correlating optical depth variations (for the duration of the cruise) with corresponding global irradiance variations. We found the Southern Asian aerosol reduces the solar irradiance (for a unit increase in optical depth at 500 nm wavelength) by about 25% at 350 nm decreasing to about 10% in the near-infrared. The reduction in the direct solar was a factor of 2 to 3 larger. The spectral data shown here will provide a critical test for aerosol-radiation models used for aerosol-forcing estimates.

2001
Wilcox, EM, Ramanathan V.  2001.  Scale dependence of the thermodynamic forcing of tropical monsoon clouds: Results from TRMM observations. Journal of Climate. 14:1511-1524.   10.1175/1520-0442(2001)014<1511:sdottf>2.0.co;2   AbstractWebsite

Clouds exert a thermodynamic forcing on the ocean-atmosphere column through latent heating, owing to the production of rain, and through cloud radiative forcing, owing to the absorption of terrestrial infrared energy and the reflection of solar energy. The Tropical Rainfall Measuring Mission (TRMM) satellite provides, for the first time, simultaneous measurements of each of these processes on the spatial scales of individual clouds. Data from TRMM are used to examine the scale dependence of the cloud thermodynamic forcing and to understand the dominant spatial scales of forcing in monsoonal cloud systems. The tropical Indian Ocean is chosen, because the major monsoonal cloud systems are located over this region. Using threshold criteria, the satellite data are segmented into rain cells (consisting of only precipitating pixels) and clouds (consisting of precipitating as well as nonprecipitating pixels), ranging in scales from 10(3) km(2) to 10(6) km(2). For each rain cell and cloud, latent heating is estimated from the microwave imager and radiative forcing is estimated from the Cloud and the Earth's Radiant Energy System radiation budget instrument. The sizes of clouds and rain cells over the tropical Indian Ocean are distributed lognormally. Thermodynamic forcing of clouds increases with rain cell and cloud area. For example, latent heating increases from about 100 W m(-2) for a rain cell of 10(3) km(2) to as high as 1500 W m(-2) for a rain cell of 10(6) km(2). Correspondingly, the liquid water path increases tenfold from 0.3 to nearly 3 kg m(-2), the longwave cloud forcing from 30 to 100 W m(-2), and the diurnal mean shortwave cloud forcing from -50 to -150 W m(-2). Previous studies have shown that in regions of deep convection, large clouds and rain cells express greater organization into structures composed of convective core regions attached to stratiform anvil cloud and precipitation. Entrainment of moist, cloudy air from the stratiform anvil into the convective core helps to sustain convection against the entrainment of unsaturated air. Thus large clouds produce more rain, trap more terrestrial radiation, and reflect more solar energy than do smaller clouds. The combined effect of increased forcing and increased spatial coverage means that larger clouds contribute most of the total forcing. Rain cells larger than 10(5) km(2) make up less than 2% of the rain cell population, yet contribute greater than 70% of the latent heating. Similarly, the clouds larger than 10(5) km(2), in which the largest rain cells are embedded, make up less than 3% of clouds, yet are the source of greater than 90% of the total thermodynamic forcing. Significant differences are apparent between the scales of latent heating and radiative forcing, as only about 25% of cloud area is observed to precipitate. The fraction of clouds that contain some rain increases dramatically from about 5% for the smaller scale (10(3) km(2)) to as high as 90% for the largest scale considered here (10(6) km(2)). The fractional area of the precipitating cloud ranges from 0.2 to 0.4 with a hybrid-scale dependence. Greater than one-half of radiative forcing is provided by nonprecipitating anvil portions of deep convective cloud systems. The results presented here have significant implications for the parameterization of clouds and rain in GCMs and washout of solute trace gases and aerosols in chemistry and transport models.

2000
Lubin, D, Vogelmann A, Lehr PJ, Kressin A, Ehramjian J, Ramanathan V.  2000.  Validation of visible/near-IR atmospheric absorption and solar emission spectroscopic models at 1 cm(-1) resolution. Journal of Geophysical Research-Atmospheres. 105:22445-22454.   10.1029/2000jd900317   AbstractWebsite

A Fourier transform infrared (FTIR) spectrometer, operating at 1 cm(-1) resolution between 9000 and 24,669 cm(-1) (0.405-1.111 mu m) has been used to check the spectral composition of databases that form the basis for most atmospheric absorption parameterizations used in climate models, remote sensing, and other radiative transfer simulations. The spectrometer, operating near sea level under clear skies, obtained relative atmospheric transmission measurements of the direct solar beam by means of a heliostat. The spectroscopic data were compared with a line-by-line radiative transfer model (LBLRTM) calculation of direct solar beam flux, which used a input data a monochromatic model extraterrestrial solar flux spectrum currently in common use. This intercomparison revealed that the extraterrestrial solar flux spectrum contains 266 solar absorption features that do not appear in the data, resulting in an excess of approximately 1.92 W m(-2) in the model's solar constant. The intercomparison also revealed 97 absorption features in the data that do not appear in the HITRAN-96 database as used by LBLRTM, resulting in a model underestimate of shortwave absorption of similar to 0.23 W m(-2) for a solar zenith angle of 42 degrees. These small discrepancies revealed by the intercomparison indicate that current extraterrestrial solar irradiance models and spectroscopic databases used by shortwave atmospheric radiative transfer models are nearly entirely complete for purposes of atmospheric energy budget calculation. Thus clear or cloudy sky "excess absorption" is unlikely to be related to an incomplete identification of atmospheric absorbing gases and their spectroscopic features, at 1 cm(-1) resolution, for a clean troposphere of normal composition.

1996
Lubin, D, Chen JP, Pilewskie P, Ramanathan V, Valero FPJ.  1996.  Microphysical examination of excess cloud absorption in the tropical atmosphere. Journal of Geophysical Research-Atmospheres. 101:16961-16972.   10.1029/96jd01154   AbstractWebsite

To investigate the excess shortwave absorption by clouds, a numerical cloud generation model has been coupled to a plane-parallel discrete ordinates radiative transfer model. The former was used in a time-dependent fashion to generate a cumulonimbus turret and three types of cirrus anvil (precipitating, extended, detached) representing three stages of cloud evolution outward from the turret. The cloud particle size distributions, as a function of altitude, were used as input to the radiative transfer model using indices of refraction for pure water and pure ice and equivalent sphere Mie theory. The radiative transfer model was used to calculate the ratio of cloud forcing at the surface to cloud forcing at the top of the atmosphere, both for the broadband shortwave and as a function of wavelength. Recent empirical studies have placed this cloud forcing ratio at around 1.5, and our coupled model results approach this value for small solar zenith angles, when the cloud contains large (>100 mu m) ice particles that absorb significantly in the near infrared (primarily the 1.6-mu m window). However, the empirical studies are based on diurnal averages, and our plane-parallel radiative transfer model yields an area and diurnally averaged cloud forcing ratio of only 1.18 for a tropical cumulonimbus and cirrus anvil system, primarily because of the rapid decrease of the ratio with solar zenith angle. The ratio decreases because of the increase in albedo with solar zenith angle, which is a characteristic feature of plane-parallel clouds. Adding dust or aerosol to the cloud layers, to make them absorb at visible wavelengths, makes the instantaneous cloud forcing ratio larger for an overhead Sun but also makes the solar zenith angle dependence in the cloud forcing ratio more pronounced. These two effects cancel, eliminating interstitial aerosol as a possible explanation for the excess cloud absorption in plane-parallel radiative transfer modeling. The strong dependence of the surface/top of the atmosphere cloud forcing ratio on solar zenith angle may be a fundamental defect with the plane-parallel approach to solar radiative transfer in a cloudy atmosphere.

1991
Ramanathan, V, Collins W.  1991.  Thermodynamic regulation of ocean warming by cirrus clouds deduced from observations of the 1987 El NiƱo. Nature. 351:27-32.   10.1038/351027a0   AbstractWebsite

Observations made during the 1987 El Nino show that in the upper range of sea surface temperatures, the greenhouse effect increases with surface temperature at a rate which exceeds the rate at which radiation is being emitted from the surface. In response to this 'super greenhouse effect', highly reflective cirrus clouds are produced which act like a thermostat, shielding the ocean from solar radiation. The regulatory effect of these cirrus clouds may limit sea surface temperatures to less than 305 K.