Aerosol modulation of atmospheric and surface solar heating over the tropical Indian Ocean

Abstract

The major finding of this study is that aerosols over the tropical Indian Ocean enhance clear sky atmospheric solar heating significantly and decrease the surface solar heating by even a larger amount. The results presented here are based on aerosol chemical, microphysical, and optical and radiometric data collected at the island of Kaashidhoo (4.97°N, 73.47°E) during February and March of 1998, as part of the first field phase of the Indian Ocean experiment (INDOEX). The aerosol optical properties were integrated with a multiple scattering Monte Carlo radiative transfer model which was validated at the surface with broadband flux measurements and at the top of the atmosphere (TOA) with the clouds and earth's radiant energy system (CERES) radiation budget measurements. We consider both externally and internally mixed aerosol models with very little difference between the two models in the estimated forcing. For the February-March period, the aerosols increase the monthly mean clear sky atmospheric solar heating by about 12 W/m2 (about 15% of the total atmospheric solar heating) and decrease the sea surface clear sky solar heating by about 16 W/m2 with a daily range from 5 to 23 W/m2. The net aerosol forcing at the top of the atmosphere is about −4 W/m2 with a daily range from −2 to −6 W/m2. Although the soot contributes only about 10% to the aerosol optical thickness, it contributes more than 50% to the aerosol induced atmospheric solar heating. The fundamental conclusion of this study is that anthropogenic aerosols over the tropical Indian Ocean are altering the clear sky radiation budget of the atmosphere and surface in a major manner.

Seasonal changes in aerosol characteristics over Arabian Sea and their consequence on aerosol short-wave radiative forcing: Results from ARMEX field campaign

“…8. In the same figure, the DF and Z f reported for this region for the winter (DecemberFebruary 1998) season during INDOEX (Podgorny et al, 2000) are also given for comparison. The figure brings out two important aspects:…”

Long term studies on characteristics of black carbon aerosols over a tropical urban station Pune, India

“…Panicker et al (2010) have reported that the relative contribution of BC aerosols to the total aerosol radiative forcing over Pune was about 55% even though BC contributed only 2.2 to 6% to the total aerosol load. During the INDOEX studies, it was observed that a mere 6% of soot contributes 11% to the aerosol optical depth (Satheesh et al, 1999); a 35% reduction in total solar radiation over the ocean surface and an increase of about 50% in atmospheric heating (Podgorny et al, 2000). BC is stated to be the second most important component of global warming after CO 2 in terms of direct forcing (Jacobson, 2001).…”

Persistent, Widespread, and Strongly Absorbing Haze Over the Himalayan Foothills and the Indo-Gangetic Plains

“…2d) led to column (surface-atmosphere) heating because the absorbing aerosols absorbed the solar radiation reflected by clouds, whereas, when the aerosols are confined below low clouds, they led to net cooling. The data shown in Figures 2a to 2d were incorporated in a Monte Carlo AerosolCloud Radiation (MACR) model (PODGORNY et al, 2000) and the simulated solar radiation fluxes were validated with observed fluxes (using Pyranometers and Biospherical instruments) as in Figure 2e. In addition, the observed surface and TOA forcing (reduction of absorbed solar radiation at the surface and at the top-of-the atmosphere) by aerosols were compared with computed forcing as in Figure 2f.…”

“…The excellent agreement with observed fluxes and forcing validated the MACR model, which was then used to estimate direct forcing for clear and cloudy skies (see RAMANATHAN et al, 2001a for details). The aerosol forcing efficiency (forcing per unit optical depth at 0.5 micron) at the surface ranged from )70 to )75 W m )2 and about )22 to )23 W m )2 at the TOA (SATHEESH and PODGORNY et al, 2000;CONANT, 2000;VALERO et al, 1999;RAMANATHAN, 1999, 2002).…”