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Effects of aerosols on trade wind cumuli over the Indian Ocean: Model simulations
Author(s) -
Mcfarquhar Greg M.,
Wang Hailong
Publication year - 2006
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.04.179
Subject(s) - environmental science , atmospheric sciences , liquid water content , liquid water path , cloud cover , cloud forcing , radiative cooling , cloud albedo , aerosol , sea salt aerosol , longwave , water vapor , parametrization (atmospheric modeling) , radiative forcing , radiative transfer , meteorology , cloud computing , geology , physics , quantum mechanics , sea salt , computer science , operating system
Abstract An Eulerian non‐hydrostatic anelastic fluid model using a warm cloud bulk parametrization is used to study responses of trade wind cumuli to increased cloud drop concentrations produced by enhanced aerosol numbers and to absorption from aerosols containing soot. Simulations are initialized using temperature and moisture profiles measured during the Indian Ocean Experiment (INDOEX) and sea surface temperatures retrieved from the Tropical Rainfall Measuring Mission microwave imager. Cloud covers between 4 and 10% and cloud‐averaged liquid water contents of between 0.01 and 0.6 g m −3 more closely match values retrieved and measured in situ during INDOEX than those modelled in past studies; previous studies found larger cloud coverage associated with cloud spreading below a strong inversion not present in the INDOEX soundings. As in previous studies, a diurnal cycle with reductions of cloud cover and liquid water path (LWP) after sunrise is seen, with absorbing aerosols within cloud augmenting the decrease in cloud cover. Variations in aerosol vertical profiles can change semi‐direct radiative forcing at the surface (top of the atmosphere) from +1.6(+1.3)W m −2 when aerosols are confined to cloud compared to −0.4(−0.6)W m −2 when aerosols occur below cloud. Low‐level water‐vapour mixing ratios decrease as vertical motions increase, the vertical motions being enhanced by below‐cloud solar heating and cloud‐top radiative cooling and suppressed by in‐cloud heating. Cloud cover and LWP are larger for stronger vertical motions, explaining the dependence of radiative forcing on aerosol profile. In the late afternoon and evening, increases in radiative cooling and sensible and latent heat flux, occurring due to reductions in mixed‐layer temperature and vapour, allow cloud cover and LWP to recover to values like those found in simulations without absorbing aerosols. Copyright © 2006 Royal Meteorological Society.