Joint Centre for Mesoscale Meteorology (JCMM)


JCMM Internal Report No.90:

The effects of convection on chemical reactions and precipitation scavenging

by Neil R Gimson

The effects of cumulus convection on the dispersion and removal of chemically active species are considered in the framework of a mesoscale model. The subgrid-scale clouds are parametrized, according to a "mass-flux" scheme, with chemical species having both in-cloud and environmental components.

A simple reaction scheme is applied, consisting of a localized NOx emission, which is oxidized in the presence of ozone and the hydroxyl radical to N(v) species, which themselves are scavenged by precipitation and deposited at the surface.

A case of maritime convection around the United Kingdom is studied (23 October, 1992), which was widespread over the Atlantic Ocean, Irish Sea and the North Sea, but also penetrating inland during the daytime. The dispersion from idealized NOx emissions is studied, and subsequent transformations and removal examined, in terms of the lifetimes of chemical species, under the influence of convection, diffusion, reaction and removal. A mid-level source of NOx produces a plume which travels eastwards over the British Isles, oxidizing at first at its leading edge - the oldest portion. Despite the variation in oxidation rate with height, domain totals of NOx and N(v) are altered only slightly by the presence of convective effects, since convective time-scales are generally shorter than the times of the chemical reactions used here. However, significant reduction in the rate of oxidation occurs in some locations of the computational domain in the final few hours of the integration.

Scavenging of N(v) is affected by convection, chiefly because stratiform and convective rain fall at different locations. More N(v) is washed out when convective effects on the chemical species are included, much over the sea, and the sub-gridscale removal concentrates the deposition into a smaller area.

Though somewhat idealized, the numerical simulations demonstrate important interactions between chemistry and convection, and show that convective effects should be incorporated into an atmospheric chemistry model for the realistic representation of these interactions.

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