Midweek increase in U.S. summer rain and storm heights suggests air pollution invigorates rainstorms

Abstract

[1] Tropical Rainfall Measuring Mission (TRMM) satellite estimates of summertime rainfall over the southeast U.S. are found on average to be significantly higher during the middle of the work week than on weekends, attributable to a midweek intensification of afternoon storms and an increase in area with detectable rain. TRMM radar data show a significant midweek increase in the echo-top heights reached by afternoon storms. Weekly variations in model-reanalysis wind patterns over the region are consistent with changes in convection implied by the satellite data. Weekly variations in rain gauge averages are also consistent with the satellite estimates, though possibly smaller in amplitude. A midweek decrease of rainfall over the nearby Atlantic is also seen. EPA measurements of surface particulate concentrations show a midweek peak over much of the U.S. These observations are consistent with the theory that anthropogenic air pollution suppresses cloud-drop coalescence and early rainout during the growth of thunderstorms over land, allowing more water to be carried above the 0°C isotherm, where freezing yields additional latent heat, invigorating the storms and producing large ice hydrometeors. The enhanced convection induces regional convergence, uplifting and an overall increase of rainfall. Compensating downward air motion suppresses convection over the adjacent ocean areas. Pre-TRMM-era data suggest that the weekly cycle only became strong enough to be detectable beginning in the 1980's. Rain-gauge data also suggest that a weekly cycle may have been detectable in the 1940's, but with peak rainfall on Sunday or Monday, possibly explained by the difference in composition of aerosol pollution at that time. This ''weekend effect'' may thus offer climate researchers an opportunity to study the regional climate-scale impact of aerosols on storm development and monsoon-like circulation.

Role of Water Vapor Content in the Effects of Aerosol on the Electrification of Thunderstorms: A Numerical Study

“…Our results showed that the previously reported [1][2][3][4][5] enhancement in the lightning activity of thunderstorms as the concentration of aerosols increases is not universal. Under conditions of ample amounts of water vapor, increasing the aerosol concentration resulted in the formation of more small cloud droplets, reducing the collision coalescence efficiency of the cloud droplets and suppressing the warm rain process.…”

“…At the lowest water vapor content (85% of the baseline sounding), the mean flash rate remained almost constant with increasing aerosol concentrations. Previous studies [1][2][3][4] have also indicated that an increase in the concentration of aerosol particles enhances the intensity of electrical activity. However, this does not apply to all thunderstorms under different conditions, and there are still uncertainties in the effects of aerosol on thunderstorm microphysics and electrification processes.…”

“…More ice particles participate in the electrification process under polluted conditions, resulting in an enhancement of this process. Observational studies [4][5][6][7][8][9] have indicated that anthropogenic aerosol particles produced from metropolitan regions, highways, and factories increase the intensity of thunderstorms, and the density and frequency of lightning. The emission of smoke from forest fires also has a marked impact on the electric processes of thunderstorms, leading to an increase in lightning activity around and downwind of forest regions, an increase in the percentage of positive cloud-to-ground lighting flashes, and a reduction in the percentage of negative cloud-to-ground lighting flashes [10][11][12][13].…”

Observations of aerosol‐induced convective invigoration in the tropical east Atlantic

“…Since then others [e.g., Lin et al, 2006;Ten Hoeve et al, 2012] have also found evidence of higher cloud tops, enhanced heavy precipitation, and increased ice amounts in the same region. Similar evidence has also been found for convective invigoration (e.g., increased cloud tops, more frequent lightning, and heavier precipitation events) in polluted environments over the North Atlantic Ocean [Koren et al, 2005], in the southeast United States [Bell et al, 2008], and in China [Wang et al, 2011]. Wang et al [2009] analyzed both observational and modeling data and found evidence that smoke from biomass burning in Central America may lead to an enhancement of severe weather in the south central United States.…”

The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds

“…The aerosols were found even to affect the potential of deep convective clouds to produce large hail and tornadoes, where clouds with added pollution aerosols become potentially more damaging (Rosenfeld and Bell, 2011 and references therein). The impact of aerosols on cloud invigoration and the vertical distribution of latent heating can also modulate regional and large scale circulation systems, as observed by Bell et al (2008) and simulated by Fan et al (2012).…”