Publications

Export 18 results:
Sort by: Author Title Type [ Year  (Desc)]
2014
Pistone, K, Eisenman I, Ramanathan V.  2014.  Observational determination of albedo decrease caused by vanishing Arctic sea ice. Proceedings of the National Academy of Sciences of the United States of America. 111:3322-3326.   10.1073/pnas.1318201111   AbstractWebsite

The decline of Arctic sea ice has been documented in over 30 y of satellite passive microwave observations. The resulting darkening of the Arctic and its amplification of global warming was hypothesized almost 50 y ago but has yet to be verified with direct observations. This study uses satellite radiation budget measurements along with satellite microwave sea ice data to document the Arctic-wide decrease in planetary albedo and its amplifying effect on the warming. The analysis reveals a striking relationship between planetary albedo and sea ice cover, quantities inferred from two independent satellite instruments. We find that the Arctic planetary albedo has decreased from 0.52 to 0.48 between 1979 and 2011, corresponding to an additional 6.4 +/- 0.9 W/m(2) of solar energy input into the Arctic Ocean region since 1979. Averaged over the globe, this albedo decrease corresponds to a forcing that is 25% as large as that due to the change in CO2 during this period, considerably larger than expectations from models and other less direct recent estimates. Changes in cloudiness appear to play a negligible role in observed Arctic darkening, thus reducing the possibility of Arctic cloud albedo feedbacks mitigating future Arctic warming.

2010
Hadley, OL, Corrigan CE, Kirchstetter TW, Cliff SS, Ramanathan V.  2010.  Measured black carbon deposition on the Sierra Nevada snow pack and implication for snow pack retreat. Atmospheric Chemistry and Physics. 10:7505-7513.   10.5194/acp-10-7505-2010   AbstractWebsite

Modeling studies show that the darkening of snow and ice by black carbon deposition is a major factor for the rapid disappearance of arctic sea ice, mountain glaciers and snow packs. This study provides one of the first direct measurements for the efficient removal of black carbon from the atmosphere by snow and its subsequent deposition to the snow packs of California. The early melting of the snow packs in the Sierras is one of the contributing factors to the severe water problems in California. BC concentrations in falling snow were measured at two mountain locations and in rain at a coastal site. All three stations reveal large BC concentrations in precipitation, ranging from 1.7 ng/g to 12.9 ng/g. The BC concentrations in the air after the snow fall were negligible suggesting an extremely efficient removal of BC by snow. The data suggest that below cloud scavenging, rather than ice nuclei, was the dominant source of BC in the snow. A five-year comparison of BC, dust, and total fine aerosol mass concentrations at multiple sites reveals that the measurements made at the sampling sites were representative of large scale deposition in the Sierra Nevada. The relative concentration of iron and calcium in the mountain aerosol indicates that one-quarter to one-third of the BC may have been transported from Asia.

2009
Ramanathan, V, Feng Y.  2009.  Air pollution, greenhouse gases and climate change: Global and regional perspectives. Atmospheric Environment. 43:37-50.   10.1016/j.atmosenv.2008.09.063   AbstractWebsite

Greenhouse gases (GHGs) warm the Surface and the atmosphere with significant implications for rainfall, retreat of glaciers and sea ice, sea level, among other factors. About 30 years ago, it was recognized that the increase in tropospheric ozone from air pollution (NO(x), CO and others) is an important greenhouse forcing term. In addition, the recognition of chlorofluorocarbons (CFCs) on stratospheric ozone and its climate effects linked chemistry and climate strongly. What is less recognized, however, is a comparably major global problem dealing with air pollution. Until about ten years ago, air pollution was thought to be just an urban or a local problem. But new data have revealed that air pollution is transported across continents and ocean basins due to fast long-range transport, resulting in trans-oceanic and trans-continental plumes of atmospheric brown Clouds (ABCs) containing sub micron size particles, i.e., aerosols. ABCs intercept Sunlight by absorbing as well as reflecting it, both of which lead to a large surface dimming. The dimming effect is enhanced further because aerosols may nucleate more cloud droplets, which makes the clouds reflect More solar radiation. The dimming has a Surface cooling effect and decreases evaporation of moisture from the surface, thus slows down the hydrological cycle. On the other hand, absorption of solar radiation by black carbon and some organics increase atmospheric hearing and tend to amplify greenhouse warming of the atmosphere. ABCs are concentrated in regional and mega-city hot spots. Long-range transport from these hot spots causes widespread plumes over the adjacent oceans. Such a pattern Of regionally concentrated Surface dimming and atmospheric Solar heating, accompanied by widespread dimming over the oceans, gives rise to large regional effects. Only during the last decade, we have begun to comprehend the surprisingly large regional impacts. In S. Asia and N. Africa, the large north-south gradient in the ABC dimming has altered both the north-south gradients in sea Surface temperatures and land-ocean contrast in surface temperatures, which in turn slow down the monsoon circulation and decrease rainfall over the continents. On the other hand, heating by black carbon warms the atmosphere at elevated levels from 2 to 6 kin, where most tropical glaciers are located, thus strengthening the effect of GHGs on retreat of snow packs and glaciers in the Hindu Kush-Himalaya-Tibetan glaciers. Globally, the surface cooling effect of ABCs may have masked as Much 47% of the global warming by greenhouse gases, with an uncertainty range of 20-80%. This presents a dilemma since efforts to curb air pollution may unmask the ABC cooling effect and enhance the surface warming. Thus efforts to reduce GHGs and air pollution should be done under one common framework. The uncertainties in our understanding of the ABC effects are large, but we are discovering new ways in which human activities are changing the climate and the environment. (C) 2008 Elsevier Ltd. All rights reserved.

2007
Hadley, OL, Ramanathan V, Carmichael GR, Tang Y, Corrigan CE, Roberts GC, Mauger GS.  2007.  Trans-Pacific transport of black carbon and fine aerosols (D < 2.5 ┬Ám) into North America. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd007632   AbstractWebsite

[1] This study presents estimates of long-range transport of black carbon (BC) and aerosol fine mass (diameter less than 2.5 mm) across the Pacific Ocean into North America during April 2004. These transport estimates are based on simulations by the Chemical Weather Forecast System (CFORS) model and evaluated across 130 degrees W, (30 degrees N-60 degrees N) from 26 March through 25 April 2004. CFORS calculates BC transport into North America at 25-32 Gg of which over 75% originates from Asia. Modeled fine aerosol mass transport is between 900 and 1100 Gg. The BC transport amounts to about 77% of the published estimates of North American BC emissions. Approximately 78% of the BC and 82% of the fine aerosol mass transport occur in the midtroposphere above 2 km. Given the relatively large magnitude of the estimated BC transport, we undertake a detailed validation of the model simulations of fine aerosol mass and BC over the west coast of North America. In situ aircraft data were available for the month of April 2004 to assess the accuracy of model simulations of aerosols in the lower troposphere. Aircraft data for aerosol mass collected in the eastern Pacific Ocean during April 2004 as part of the Cloud Indirect Forcing Experiment, as well as surface measurements of fine mass and BC at 30 west coast locations, are compared to CFORS predictions. These surface sites are part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Both the aircraft and the IMPROVE data sets reveal similar patterns of good agreement near and above the boundary layer accompanied by large overprediction within the boundary layer. The observational data validate the CFORS simulations of BC and fine aerosol mass above the boundary layer. The near-surface overprediction does not impair the major conclusions of this study regarding long-range aerosol and BC transport, as most of the long-range transport occurs above 2 km. From this we conclude that the transport of BC from Asia and other regions west is a major source of BC at high elevations over North America. The simulated concentrations of BC between 1 and 3 km, as well as the measured BC concentrations over the elevated IMPROVE sites, range from 0.1 to 0.3 mu g/m(3). Direct radiative forcing over North America due to the modeled BC concentration between 1 and 15 km is estimated at an additional 2.04-2.55 W/m(2) absorbed in the atmosphere and a dimming of-1.45 to-1.47 W/m(2) at the surface. The impact of transported BC on the regional radiation budget through direct and indirect effects of the transported BC and other aerosols warrants further study.

2006
Ramana, MV, Ramanathan V.  2006.  Abrupt transition from natural to anthropogenic aerosol radiative forcing: Observations at the ABC-Maldives Climate Observatory. Journal of Geophysical Research-Atmospheres. 111   10.1029/2006jd007063   AbstractWebsite

[1] Using aerosol-radiation observations over the north Indian Ocean, we show how the monsoon transition from southwest to northeast flow gives rise to a similar transition in the direct aerosol radiative forcing from natural to anthropogenic forcing. These observations were taken at the newly built aerosol-radiation-climate observatory at the island of Hanimaadhoo (6.776 degrees N, 73.183 degrees E) in the Republic of Maldives. This observatory is established as a part of Project Atmospheric Brown Clouds (ABC) and is referred to as the ABC-Maldives Climate Observatory at Hanimaadhoo (ABC_MCOH). The transition from the southwest monsoon during October to the northeast monsoon flow during early November occurs abruptly over a period of few weeks over ABC-MCOH and reveals a dramatic contrast between the natural marine aerosols transported from the south Indian Ocean by the southwest monsoon and that of the polluted aerosols transported from the south and Southeast Asian region by the northeast monsoon. We document the change in the microphysical properties and the irradiance at the surface, to identify the human signature on aerosol radiative forcing. We first establish the precision of surface radiometric observations by comparing simultaneous observations using calibrated Kipp & Zonen and Eppley pyrheliometers and pyranometers for direct, diffuse and global solar radiation. We show that the direct, diffuse and global radiation can be measured within a precision of about 3 to 5 Wm(-2). Furthermore, when we include the observed aerosol optical properties as input into the Monte Carlo Aerosol Cloud Radiation (MACR) model (developed by us using Indian Ocean Experiment data), the simulated fluxes agree with the observed direct, diffuse and global fluxes within the measurement accuracy. A steady southwest monsoon flow of about 5 to 7 ms(-1) persists until middle of October which switches to an abrupt change in direction to northeast flow of similar speeds bringing in polluted air from south Asia. However, it is not until end of November that a steady northeasterly flow is well established. The abrupt transition is accompanied by a large increase in aerosol optical depth from about 0.1 in October to as high as 0.4 during January, the SSA decreases from 1 to about 0.9, and the Angstrom coefficient increases from about 0.5 (suggesting large particles > 1 micron) to about 1.2 in January (submicron particles) and an increase in aerosol extinction below 3 km altitude. These changes are consistent with the transport of continental pollution from south and Southeast Asia (about 1000 to several 1000 km away from ABC_MCOH) to the north Indian Ocean during the northeast monsoon. The direct aerosol forcing, determined solely from radiometric observations without resorting to models, changes from -5 Wm(-2) during October to -22 Wm(-2) during January. About 50% of this forcing occurs in the photosynthetically active part of the solar spectrum (0.4 to 0.7 micron). MACR shows that the decrease in SSA from 1 to 0.9 changes the aerosol forcing efficiency by a factor of about 2 from about -40 Wm(-2) (per AOD) in October to -80 Wm(-2) (per AOD) in January. Thus the arrival of the brown clouds from south and Southeast Asia has a large seasonal dimming effect over remote parts of the north Indian Ocean. The observational results presented here should be used for validating climate models that attempt to simulate the anthropogenic effects of aerosol forcing on climate. The observational and model results presented in his study shows how near continuous surface based observations can be used to differentiate the human impact on aerosol forcing which is a major challenge for models.

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.

2004
Ramana, MV, Ramanathan V, Podgorny IA, Pradhan BB, Shrestha B.  2004.  The direct observations of large aerosol radiative forcing in the Himalayan region. Geophysical Research Letters. 31   10.1029/2003gl018824   AbstractWebsite

We show here that absorbing aerosols have led to a large reduction of surface solar radiation during winter over the Himalayan region. Our results are based on radiometric, aerosol and Lidar observations made at three sites in Nepal during winter 2003. The monthly mean aerosol optical depth (AOD) ranged from 0.2 to 0.34 and the TERRA satellite MODIS data reveal that AODs measured over these sites were typical of the entire Himalayan region. The near-surface aerosol single scattering albedo was in the range from 0.7 to 0.9. The presence of strongly absorbing aerosols resulted in a relatively large diurnal mean aerosol surface radiative forcing efficiency of - 73 Wm(-2) (per unit optical depth). The seasonal mean reduction in solar flux was as high as 25 Wm(-2) and aerosol heating as much as 1 K per day within the first two kilometers.

2003
Novakov, T, Ramanathan V, Hansen JE, Kirchstetter TW, Sato M, Sinton JE, Sathaye JA.  2003.  Large historical changes of fossil-fuel black carbon aerosols. Geophysical Research Letters. 30   10.1029/2002gl016345   AbstractWebsite

Anthropogenic emissions of fine black carbon (BC) particles, the principal light-absorbing atmospheric aerosol, have varied during the past century in response to changes of fossil-fuel utilization, technology developments, and emission controls. We estimate historical trends of fossil-fuel BC emissions in six regions that represent about two-thirds of present day emissions and extrapolate these to global emissions from 1875 onward. Qualitative features in these trends show rapid increase in the latter part of the 1800s, the leveling off in the first half of the 1900s, and the re-acceleration in the past 50 years as China and India developed. We find that historical changes of fuel utilization have caused large temporal change in aerosol absorption, and thus substantial change of aerosol single scatter albedo in some regions, which suggests that BC may have contributed to global temperature changes in the past century. This implies that the BC history needs to be represented realistically in climate change assessments.

Tian, BJ, Ramanathan V.  2003.  A simple moist tropical atmosphere model: The role of cloud radiative forcing. Journal of Climate. 16:2086-2092.   10.1175/1520-0442(2003)016<2086:asmtam>2.0.co;2   AbstractWebsite

A simple moist model for the large-scale tropical atmospheric circulation is constructed by combining the simple models of Gill and Neelin and Held. The model describes the first baroclinic mode of the moist troposphere with variable "gross moist stability'' in response to given thermodynamic forcing from surface evaporation and atmospheric cloud radiative forcing (CRF), which is a measure of the radiative effects of clouds in the atmospheric radiative heating. When the present model is forced solely by the observed atmospheric CRF, quantitatively reasonable Hadley and Walker circulations are obtained, such as the trades, the ascending branches in the intertropical convergence zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), as well as the descending branches in the cold tongue and subtropics. However, when the model is forced only by the observed surface evaporation, the Walker circulation totally disappears, and the Hadley circulation reverses. These results indicate that, in the context of a moist dynamic model, the spatial variations of atmospheric CRF are more important in terms of driving and maintaining the Hadley and Walker circulations than the spatial variation of surface evaporation.

2002
Rajeev, K, Ramanathan V.  2002.  The Indian Ocean experiment: Aerosol forcing obtained from satellite data. Remote Sensing of Trace Constituents in the Lower Stratosphere, Troposphere and the Earth's Surface: Global Observations, Air Pollution and the Atmospheric Correction. 29( Burrows JP, Takeucki N, Eds.).:1731-1740., Oxford: Pergamon-Elsevier Science Ltd   10.1016/s0273-1177(02)00086-8   Abstract

The tropical Indian Ocean provides an ideal and unique natural laboratory to observe and understand the role of anthropogenic aerosols in climate forcing. Since 1996, an international team of American, European and Indian scientists have been collecting aerosol, chemical and radiation data from ships and surface stations, which culminated in a multi-platform field experiment conducted during January to March of 1999. A persistent haze layer that spread over most of the northern Indian Ocean during wintertime was discovered. The layer, a complex mix of organics, black carbon, sulfates, nitrates and other species, subjects the lower atmosphere to a strong radiative heating and a larger reduction in the solar heating of the ocean. We present here the regional distribution of aerosols and the resulting clear sky aerosol radiative forcing at top-of-atmosphere (TOA) observed over the Indian Ocean during the winter months of 1997, 1998 and 1999 based on the aerosol optical depth (AOD) estimated using NOAA14-AVHRR and the TOA radiation budget data from CERES on board TRMM. Using the ratio of surface to TOA clear sky aerosol radiative forcing observed during the same period over the Indian Ocean island of Kaashidhoo (Satheesh and Ramanathan, 2000), the clear sky aerosol radiative forcing at the surface and the atmosphere are discussed. The regional maps of AVHRR derived AOD show abnormally large aerosol concentration during the winter of 1999 which is about 1.5 to 2 times larger than the AOD during the corresponding period of 1997 and 1998. A large latitudinal gradient in AOD is observed during all the three years of observation, with maximum AOD in the northern hemisphere. The diurnal mean clear sky aerosol forcing at TOA in the northern hemisphere Indian Ocean is in the range of -4 to -16 Wm(-2) and had large spatio-temporal variations while in the southern hemisphere Indian Ocean it is in the range of 0 to -6Wm(-2). The importance of integrating in-situ data with satellite data to get reliable picture of the regional scale aerosol forcing is demonstrated. (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

2001
Podgorny, IA, Ramanathan V.  2001.  A modeling study of the direct effect of aerosols over the tropical Indian Ocean. Journal of Geophysical Research-Atmospheres. 106:24097-24105.   10.1029/2001jd900214   AbstractWebsite

The microphysical, chemical, optical, and lidar data collected during the Indian Ocean Experiment (INDOEX) resulted in a self-consistent aerosol formulation for a multiple-scattering Monte Carlo radiation model. The model was used to simulate the direct aerosol radiative forcing, cloud radiative forcing, and heating rates for typical winter monsoon conditions over the tropical Indian Ocean. The focus of the study is to understand how the anthropogenic and natural aerosols partition the incoming solar energy between the ocean mixed layer and the overlying cloudy atmosphere. The observed aerosol single-scattering albedo, omega, was in the range 0.8-0.9 at 500 Tim, mean aerosol visible optical thickness, tau (A), was in the range 0.1-0.8 at 500 nm, and the low-level clouds had horizontal scales of few kilometers and a cloud fraction of about 25%, typical of low-level clouds in the tropical oceans. The aerosol layer extended well above the low-level clouds in many instances, which has a significant impact on the radiative forcing. Although contributing only about 10% to the aerosol optical thickness, the soot transported from Asia and the Indian subcontinent significantly affects the aerosol direct forcing of the cloudy atmosphere. For monthly mean conditions (tau (A) = 0.4, omega = 0.9 and 25% low-cloud fraction), the diurnal mean surface radiative forcing is about -25 W m(-2) and the atmospheric forcing ranges from +22 to +25 W m(-2). The top-of-the-atmosphere direct aerosol forcing is in the range of zero to -3 W m(-2). The aerosol enhances the cloud atmospheric forcing by 0.5 and by 2.5 W m(-2) when aerosol is mostly below and above the clouds, respectively. Furthermore, the trade wind boundary layer is subject to a heating of about 1 to 1.5 K/d which might burn off the trade cumulus themselves. Thus the major impact of the predominantly anthropogenic aerosol over the tropical Indian Ocean is a substantial redistribution of the solar energy between the atmosphere and the ocean mixed layer.

Eck, TF, Holben BN, Dubovik O, Smirnov A, Slutsker I, Lobert JM, Ramanathan V.  2001.  Column-integrated aerosol optical properties over the Maldives during the northeast monsoon for 1998-2000. Journal of Geophysical Research-Atmospheres. 106:28555-28566.   10.1029/2001jd000786   AbstractWebsite

Measurements made during the Indian Ocean Experiment (INDOEX) have shown the presence of large aerosol loadings over the region of the northern Indian Ocean and Arabian Sea. In recent years there has been significant interannual variability in the magnitude of this aerosol loading during the NE monsoon months of. January-April. Monitoring of the integrated atmospheric column effective aerosol optical properties was initiated in early 1998 and continued in 2000 on the island of Kaashidhoo in the Republic of Maldives. An Aerosol Robotic Network Sun-sky radiometer at the Kaashidhoo Climate Observatory made spectral measurements of the direct Sun and directional sky radiances which were utilized to infer spectral aerosol optical depths tau (a), single scattering albedos, asymmetry factors, and aerosol size distributions. Monthly average aerosol optical depths at 500 nm varied by more than a factor of 2 during January through April for the 3 years that were investigated, 1998-2000. Interannual variations in the monthly mean Angstrom wavelength exponent were also observed, resulting from differences in the bimodal aerosol size distributions. Spectral variations in the Angstrom wavelength exponent were observed, especially at high aerosol optical depths when fine mode aerosols dominated over the optical influence of coarse-mode aerosols. Some differences in spectral single scattering albedo and asymmetry factor were observed for 1999 versus 2000 in the infrared wavelengths, but with relatively little change in the visible wavelengths. The spectral variation in the retrieved single scattering albedo was large, with approximately linear wavelength dependence averaging from 0.91 at 440 nm to 0.83 at 1020 nm for January-March 1999 for observations where tau (a) at 440 nm >0.4.

Ramanathan, V, Crutzen PJ, Kiehl JT, Rosenfeld D.  2001.  Atmosphere - Aerosols, climate, and the hydrological cycle. Science. 294:2119-2124.   10.1126/science.1064034   AbstractWebsite

Human activities are releasing tiny particles (aerosols) into the atmosphere. These human-made aerosols enhance scattering and absorption of solar radiation. They also produce brighter clouds that are less efficient at releasing precipitation. These in turn lead to Large reductions in the amount of solar irradiance reaching Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in the atmospheric temperature structure, suppression of rainfall, and less efficient removal of pollutants. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to availability and quality of fresh water, a major environmental issue of the 21st century.

Rajeev, K, Ramanathan V.  2001.  Direct observations of clear-sky aerosol radiative forcing from space during the Indian Ocean Experiment. Journal of Geophysical Research-Atmospheres. 106:17221-17235.   10.1029/2000jd900723   AbstractWebsite

This study presents the regional estimates of the seasonal and diurnal mean broadband (0.3-5.0 mum) clear-sky aerosol radiative forcing at the top of atmosphere (TOA) due to both the natural and the anthropogenic aerosols over the tropical Indian Ocean from 25 degreesN to 25 degreesS. We propose two new methods, the slope method and the differencing method, to obtain clear sky aerosol forcing from solely satellite measurements. The focus of the study is January to March 1997, 1998, and 1999. The TOA clear-sky aerosol forcing was obtained by integrating satellite data for aerosol optical depth (AOD) and the broadband radiation budget. Over 30,000 pixels were collocated to estimate that the diurnal and seasonal mean reflected broadband solar radiation at TOA increases by about 24 W m(-2) per unit increase in AOD at the wavelength of 500 nm. The observed TOA clear-sky aerosol forcing varied between -4 and -14 W m(-2) in the Northern Hemisphere (NH) and between 0 and -6 W m(-2) in the Southern Hemisphere. Assuming a ratio of surface to TOA clear-sky aerosol forcing of 3 which was observed over Kaashidhoo Climate Observatory (4.96 degreesN, 73.46 degreesE) during the same period [Satheesh and Ramanathan, 2000], this leads to a clear-sky aerosol forcing of -12 to -42 Wm(-2) at the surface in the NH. The difference between the TOA forcing and the surface forcing is the atmospheric forcing. As a result, the atmosphere is subject to a large net forcing of about 8-28 Wm(-2) in the NH, largely due to the presence of black carbon. Of equal importance is the fact that the Indian Ocean aerosols introduce a large interhemispheric gradient in the solar heating during the wintertime. The implications for climate and monsoonal circulation may be major and need to be explored with coupled model studies.

1998
Conant, WC, Vogelmann AM, Ramanathan V.  1998.  The unexplained solar absorption and atmospheric H2O: a direct test using clear-sky data. Tellus Series a-Dynamic Meteorology and Oceanography. 50:525-533.   10.1034/j.1600-0870.1998.t01-1-00010.x   AbstractWebsite

This paper is motivated by several recent studies that have shown that observations of atmospheric solar absorption systematically and significantly exceed model estimates. This paper tests whether uncertainties in the physics of water vapor absorption in clear skies are directly responsible for this unexplained excess absorption. Radiative transfer calculations of clear-sky solar fluxes are compared to measurements in the tropical Pacific at the surface and the tropopause. We find that the atmospheric absorption in excess of that predicted by radiative transfer models, if it exists, is less than the experimental uncertainty of 7 W m(-2) (diurnally averaged). Furthermore, the difference between observed and modeled absorption is essentially independent of water vapor amounts between 35 and 51 kg m(-2). A more direct test of the accuracy of modeled water vapor absorption is conducted with two independent multi-spectral radiometers at the Atmospheric Radiation Measurement site in Oklahoma, each providing over 16000 surface measurements of direct solar radiation in the 0.94 mu m region. These spectral data confirm state-of-the-art radiation model computations of water vapor line absorption to within 5% for the wavelength region tested. The model-observation agreement for both tropical and Oklahoma data strongly suggests that uncertainties in the physics of water vapor absorption in clear skies are not a source for any significant excess solar absorption, thus narrowing the search to other atmospheric constituents or water vapor in cloudy skies.

1997
Conant, WC, Ramanathan V, Valero FPJ, Meywerk J.  1997.  An examination of the clear-sky solar absorption over the Central Equatorial Pacific: Observations versus models. Journal of Climate. 10:1874-1884.   10.1175/1520-0442(1997)010<1874:aeotcs>2.0.co;2   AbstractWebsite

Measurements of downward surface solar radiation (global radiation) and albedo taken during the Central Equatorial Pacific Experiment (CEPEX) are used to obtain baseline estimates for two quantities concerning the radiation budget of the tropical oceans: 1) surface absorption of solar radiation in the central equatorial Pacific under cloud-free conditions. and 2) the corresponding absorption by the atmosphere. These values are then compared to two state-of-the-art radiative transfer models to determine if the models are accurately partitioning solar absorption between the atmosphere and the ocean. The paper develops an independent approach to obtain a clear-sky signal from 10-s resolution surface pyranometer data that is in excellent agreement with upper envelope methods. Over a diurnal average, the ocean absorbs 70.9% +/- 1.3% of the solar radiation incident at the top of the atmosphere (TOA). The data, measured from ship and low-flying aircraft platforms, also yield the zenith angle dependence of the surface absorption. The clear-sky data are representative of dry regions east of the date line during March 1993, Likewise, a combination of tropopause albedo measurements from the ER-2 aircraft and Earth Radiation Budget Experiment (ERBE) clear-sky TOA albedos are used to find the absorption of solar radiation by the atmosphere (integrated from the surface to the TOA). Clear-sky TOA albedo is computed from the ER-2 tropopause measurements using a radiative transfer model and measurements of stratospheric aerosol and ozone. The computed TOA albedos agree with ERBE at about 6% for overhead sun, The diurnal average fractional atmospheric column absorption is 20.2% +/- 1.6%. Two multispectral radiation models agree to within 5 W m(-2) of the observed daily average clear-sky oceanic solar absorption when the atmospheric profile is constrained by measurements and the observed TOA albedo is used as a boundary condition.

1995
Ramanathan, V, Subasilar B, Zhang GJ, Conant W, Cess RD, Kiehl JT, Grassl H, Shi L.  1995.  Warm Pool Heat Budget and Shortwave Cloud Forcing: A Missing Physics? Science. 267:499-503.   10.1126/science.267.5197.499   AbstractWebsite

Ship observations and ocean models indicate that heat export from the mixed layer of the western Pacific warm pool is small (<20 watts per square meter). This value was used to deduce the effect of clouds on the net solar radiation at the sea surface. The inferred magnitude of this shortwave cloud forcing was large (approximate to-100 watts per square meter) and exceeded its observed value at the top of the atmosphere by a factor of about 1.5, This result implies that clouds (at least over the warm pool) reduce net solar radiation at the sea surface not only by reflecting a significant amount back to space, but also by trapping a large amount in the cloudy atmosphere, an inference that is at variance with most model results. The excess cloud absorption, if confirmed, has many climatic implications, including a significant reduction in the required tropics to extratropics heat transport in the oceans.

1993
Ramanathan, V, Collins W.  1993.  A thermostat in the tropics? Nature. 361:410-411.   10.1038/361410b0   AbstractWebsite
n/a