Air pollution unable to intensify storms via warm-phase invigoration
Submitter
Romps, David — Lawrence Berkeley National Laboratory
Area of research
Cloud-Aerosol-Precipitation Interactions
Journal Reference
Science
Of the mechanisms proposed for how aerosols might invigorate storms, the observational evidence on supersaturation makes the warm-phase mechanism unlikely.
An actively debated hypothesis is that air pollution from human activities regularly makes storms around the world more intense. In the "warm-phase invigoration mechanism," extra particles of air pollution (aerosols) release latent heat sooner, making updrafts more buoyant and faster. But is the supersaturation (excess of relative humidity beyond 100%) in clouds big enough to drive this effect?
Impact
We measure supersaturation in convective clouds during the GOAmazon 2014/15 campaign and find a typical supersaturation of ~0.2%, which is nearly two orders of magnitude lower than the ~10% supersaturation required for practically significant warm-phase invigoration.
Summary
An actively debated hypothesis is that air pollution from human activities regularly makes storms around the world more intense, with intensity measured, for example, by the speed of storm updrafts. There are three main proposed mechanisms for how aerosols might invigorate storms, one of which is the so-called "warm-phase invigoration mechanism". In this mechanism, extra particles of air pollution (aerosols) make storm updrafts more buoyant and therefore faster, with "warm-phase" referring to the lower altitudes where clouds are composed of liquid water (as opposed to ice). But this mechanism requires the water vapor in cloud updrafts to be far out of equilibrium, as measured by the supersaturation (the amount by which the relative humidity exceeds 100%). We show that observed supersaturations in storm clouds over the Amazon are much too low for this mechanism to operate. In other words, the warm-phase mechanism is unlikely to have any practically significant effect on storm intensity.
This work was supported by the U.S. Department of Energy’s (DOE) Atmospheric System Research (ASR), an Office of Science, Office of Biological and Environmental Research program; Lawrence Berkeley National Laboratory is operated for the DOE by the University of California under Contract No. DE-AC02-05CH11231.
Figure 1. The maximum potential increase in updraft vertical velocity from air pollution due to warm-phase invigoration during the GOAmazon campaign, calculating using in situ observations. Image courtesy of David M. Romps.