Transport of ozone between boundary layer and cloud layer by cumulus clouds

Abstract

Turbulence data obtained from over 100 aircraft penetrations of isolated, nonprecipitating cumulus clouds are used to derive statistical parameters associated with the transport of ozone to and from the underlying boundary layer. The parameters are determined by the difference in ozone concentration between the cloud layer and boundary layer, AO 3. The total cloud flux of ozone, defined as the transport across a horizontal plane within a cloud due to mean and turbulent cloud motions, is given by (-1.2 ___ 0.2)AO3(•), where (•) is the cloud vertical velocity averaged over an observing period. Cloud turbulence contributes about 30% to the total cloud flux. INTRODUCTIONAbove the boundary layer a major source of vertical transport is cloud motions, which are generally a combination of direct large-scale transport and smaller-scale turbulence. In the summertime, fair-weather cumulus clouds tend to form over mixed layers that are sufficiently moist. The cumulus clouds become particularly active in the afternoon, when the height of the capping inversion exceeds the height of the lifting condensation level. In general, when the scalar properties of the mixed layer differ from those of the cloud layer, then cloud venting, that is, net removal from the mixed layer, is possible by transport of the mixed layer scalar by the convective cells that form the roots of "active" cumulus clouds [Stull, 1985]. The depth of penetration into the cloud layer is presumably dependent on the strength of convection and the latent heat flux in the mixed layer and on the intensity of stratification at and above the inversion. Using an airborne ultraviolet differential absorption lidar system, Chin•l et al. [1985] found that significant vertical transport of ozone and aerosols occurs during cumulus cloud convective activity. In Ching et al.'s experiment, the ozone excess above the boundary layer was approximately equal to the concentration difference between the mixed layer and the free troposphere, as determined by aircraft vertical soundings.The presence of thermals and their associated cumulus clouds can affect the photochemistry of the atmosphere within and above the mixed layer [Lamb, 1982]. Air pollution chemistry will be affected by the sudden presence of water and reduced sunlight within the clouds. As the clouds continue to grow, decreasing sunlight at the surface lowers the heat flux to a point where growth can be sustained only by the release of latent heat within the clouds. In the mature stage, cloud roots no longer originate in the lowest levels of the mixed layer, and instead, the cloud is fed from upper mixed layer air, usually containing older pollutants than those found near the ground.There is evidence that cloud transport is an important factor in acid rain. It has been found that a large fraction of the sulfur transported out of the Midwest and present at any time over weastern North America has passed through cloud [Bornstein and Thompson, 1980; Acid Deposition Modeling7 Project (ADMP), 1983]. Cumulus clouds can remove s...

In situ vertical profiles of aerosol extinction, mass, and composition over the southeast United States during SENEX and SEAC<sup>4</sup>RS: observations of a modest aerosol enhancement aloft

“…Several preceding studies have observed vertical transport of trace gases and aerosol from the mixed layer into the cloudinfluenced transition layer during shallow cumulus convection (Angevine, 2005;Ching and Alkezweeny, 1986;Greenhut, 1986;Langford et al, 2010). Our observations are consistent with this earlier work.…”

“…The transition layer is intermittently mixed by thermal plumes that originate in the mixed layer and form cumulus clouds that release latent heat within the layer. There have been several observations of vertical transport and redistribution of trace gases by shallow cumulus convection (Angevine, 2005;Ching and Alkezweeny, 1986;Ching et al, 1988;Greenhut, 1986), and a few studies have investigated the vertical transport and aerosol formation during cumulus convection (Ching et al, 1988;Sorooshian et al, 2006Sorooshian et al, , 2007Wonaschuetz et al, 2012).…”

Vertical ozone fluxes and related deposition parameters over agricultural and forested landscapes

“…During the first field study, estimates of Zi were determined as the base of the elevated inversion from temperature profiles obtained by another aircraft in the same region. During the second field study, Zi values for the cases prior to 16 August were taken to be the height of the largest negative aerosol gradient derived from a stationary lidar system, while values of Zi for the last four experimental cases in Ohio were taken from analyses of the aircraft data by Greenhut (1986).…”

Influence of fair‐weather cumulus clouds on isoprene chemistry

“…Because shallow cumulus cloud processes are not fully resolved in global or regional atmospheric chemistry models, it is important to quantify the influence of these clouds (via enhanced vertical transport, modified photolysis rates, modified emission rates, and cloud chemistry) on CBL chemistry. There have been only a few studies that have examined the role of fair weather clouds on the transport and chemistry of chemical constituents [ Greenhut , 1986; Ching and Alkezweeny , 1986; Ching et al , 1988; Thompson et al , 1994; Angevine , 2005; Vila‐Guerau de Arellano et al , 2005, hereinafter VKBP05; Karl et al , 2007]. One‐dimensional models (or single column models) of the cloud topped boundary layer have been useful to examine the influence of shallow cumulus clouds on pollutant transport [e.g., Angevine , 2005].…”