Aerosols and Convection: Global scale, MJO Scale and Regional Scale Analyses

Abstract:

We have investigated interactions between atmospheric thermodynamics, boundary layer aerosol (CCN) concentrations, convective intensity and lightning flash rates (from the TRMM LIS and the Vaisala GLD 360 global network) on three distinct scales, including the global tropical ocean and land masses, the Madden Julian Oscillation genesis region over the central Indian Ocean (CIO) region, and four regions in the U.S., Washington D.C., northern Alabama, central Oklahoma and eastern Colorado. The U.S. locations are each supported by VHF Lightning Mapping Arrays.

Total lightning density is shown to increase by a factor of 2-3 as a function of CCN concentration over tropical land and ocean regions. The greatest sensitivity in the lightning vs. aerosol relationship was found in more unstable environments and where warm-cloud depth was intermediate (deep) over land (ocean). The maximum height of 30 dBZ echo tops in lightning producing convective features was found to be insensitive to changes in CCN concentration. However, the vertical profile of radar reflectivity (VPRR) showed a consistent increase of 2-4 dBZ for convective features that developed in more polluted environments, suggesting that aerosols may act to intensify the convection, but not necessarily make the convection deeper. These findings are consistent with the hypothesis that aerosols act to invigorate convection by influencing the evolution of a cloud's hydrometeor populations.

For the regional scale analysis, storms in Colorado have favorable thermodynamics (high cloud bases, shallow warm cloud depths and large CAPE’s) that aerosols (CCN) appear to have little effect in a bulk sense. For the three remaining regions, storms forming in environments with CCN concentrations between 700 and 1200 cm^-3 have notably stronger VPRR and larger flash rates. For aerosol concentrations below and above this range, storms have less vigor and reduced flash rates, consistent with the Rosenfeld et al. (2008) study.

Finally we show how continental aerosols transported by Rossby gyres following propagating MJO’s over the CIO may act to modify convection north of the Equator. This effect is minimal south of the Equator where the gyres transport pristine aerosol concentrations from the southern Indian Ocean towards the Equator.