A Short Course in Cloud Physics

[1] We explore the effects of increases in aerosol concentration on cloud lifetime for warm convective clouds using a two-dimensional single cloud model and three-dimensional large eddy simulations (LES). The models include size-resolved treatment of drop size distributions and warm microphysical processes. It is shown using a variety of soundings representing marine trade cumulus, and continental convective clouds that contrary to expectation, an increase in aerosol concentration from very clean to very polluted does not increase cloud lifetime, even though precipitation is suppressed. Cloud lifetimes are statistically similar although individual clouds may experience decreases in lifetime of 10–40%. An evaporation-entrainment feedback that tends to dilute polluted clouds more than clean clouds is identified. It is proposed that the small changes in cloud lifetime are due to competing effects of precipitation suppression and enhanced evaporation, with the latter tending to dominate in these shallow clouds.

Aerosol effects on the lifetime of shallow cumulus

We investigated trace gas emissions from a biomass fire near Otavi, northern Namibia, on 13 September 2000 as part of the Southern African Regional Science Initiative 2000 (SAFARI 2000). Observations included fast measurements of carbon monoxide (CO), ozone (O 3 ), acetone (propanone, CH 3 COCH 3 ), and acetonitrile (ethanenitrile or methyl cyanide, CH 3 CN). Additionally, flask samples were taken and analyzed for nonmethane hydrocarbons (NMHCs). Measurements close to the fire were used to quantify the emissions of the different compounds. Several transects were flown through the plume downwind to study the chemical changes in the plume during the first 2 hours after the emission. Fast production of O 3 was observed, with the O 3 /CO molar enhancement ratio ER(O 3 /CO) reaching ∼0.10 in the plume after 2 hours. Acetone molar enhancement ratios (ER(acetone/CO) were ∼5 x 10 -3 in the very young plume but ∼9 x 10 -3 after 1-2 hours of aging, pointing to a substantial fast secondary acetone formation. To understand the chemical processes occurring in the plume, we simulated the plume chemistry with a dilution box model. The fast O 3 production is well reproduced, whereas acetone mixing ratios in the aging plume are underestimated by the model. This points to additional yet unidentified acetone precursors. The model suggests an overall molar enhancement ratio of these compounds with respect to CO of 7.5 x 10 -3 and a reaction rate coefficient of 6 x 10 -11 cm 3 s -1 for the reaction with HO radicals. The secondary acetone production may cause the net acetone source from biomass burning to be underestimated when only data from observations immediately near the fires are considered. The atmospheric implications of these findings are discussed.

Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations : SAFARI 2000-Southern African Regional Science Initiative

Aerosol cloud interactions in southeast Pacific stratocumulus: satellite observations, in situ data and regional modeling

Some of the global warming from anthropogenic greenhouse gases is offset by increased reflection of solar radiation by clouds with smaller droplets that form in air polluted with aerosol particles that serve as cloud condensation nuclei. The resulting cooling tendency, termed the indirect aerosol forcing, is thought to be comparable in magnitude to the forcing by anthropogenic CO2, but it is difficult to estimate because the physical processes that determine global aerosol and cloud populations are poorly understood. Smaller cloud droplets not only reflect sunlight more effectively, but also inhibit precipitation, which is expected to result in increased cloud water. Such an increase in cloud water would result in even more reflective clouds, further increasing the indirect forcing. Marine boundary-layer clouds polluted by aerosol particles, however, are not generally observed to hold more water. Here we simulate stratocumulus clouds with a fluid dynamics model that includes detailed treatments of cloud microphysics and radiative transfer. Our simulations show that the response of cloud water to suppression of precipitation from increased droplet concentrations is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air. Only when the overlying air is humid or droplet concentrations are very low does sufficient precipitation reach the surface to allow cloud water to increase with droplet concentrations. Otherwise, the response of cloud water to aerosol-induced suppression of precipitation is dominated by enhanced entrainment of overlying dry air. In this scenario, cloud water is reduced as droplet concentrations increase, which diminishes the indirect climate forcing.

The impact of humidity above stratiform clouds on indirect aerosol climate forcing

Exploring Meteorological and Biomass Burning Aerosol Influences on Marine Stratocumulus in the Southeast Atlantic using WRF-Chem

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Exploring Meteorological and Biomass Burning Aerosol Influences on Marine Stratocumulus in the Southeast Atlantic using WRF-Chem