Pollution Is Leading to Atmospheric Warming, Larger Clouds and Bigger Storms

“Global cli­mate mod­els don’t see this effect because thun­der­storm clouds sim­u­lated in those mod­els do not include enough detail,” said Fan. “The large amount of heat trapped by the pollution-enhanced clouds could poten­tially impact regional cir­cu­la­tion and mod­ify weather systems.”

Clouds are one of the most poorly under­stood com­po­nents of Earth’s cli­mate sys­tem. Called deep con­vec­tive clouds, thun­der­storm clouds reflect a lot of the sun’s energy back into space, trap heat that rises from the sur­face, and return evap­o­rated water back to the sur­face as rain, mak­ing them an impor­tant part of the cli­mate cycle.

To more real­is­ti­cally model clouds on a small scale, such as in this study, researchers use the physics of tem­per­a­ture, water, gases and aerosols — tiny par­ti­cles in the air such as pol­lu­tion, salt or dust on which cloud droplets form.

In large-scale mod­els that look at regions or the entire globe, researchers sub­sti­tute a stand-in called a para­me­ter­i­za­tion to account for deep con­vec­tive clouds. The size of the grid in global mod­els can be a hun­dred times big­ger than an actual thun­der­head, mak­ing a sub­sti­tute necessary.

However, thun­der­heads are com­pli­cated, dynamic clouds. Coming up with an accu­rate para­me­ter­i­za­tion is impor­tant but has been dif­fi­cult due to their dynamic nature.

Inside a thun­der­storm cloud, warm air rises in updrafts, push­ing tiny aerosols from pol­lu­tion or other par­ti­cles upwards. Higher up, water vapor cools and con­denses onto the aerosols to form droplets, build­ing the cloud. At the same time, cold air falls, cre­at­ing a con­vec­tive cycle. Generally, the top of the cloud spreads out like an anvil.

Previous work showed that when it’s not too windy, pol­lu­tion leads to big­ger clouds . This occurs because more pol­lu­tion par­ti­cles divide up the avail­able water for droplets, lead­ing to a higher num­ber of smaller droplets that are too small to rain. Instead of rain­ing, the small droplets ride the updrafts higher, where they freeze and absorb more water vapor. Collectively, these events lead to big­ger, more vig­or­ous con­vec­tive clouds that live longer.

Now, researchers from PNNL, Hebrew University in Jerusalem and the University of Maryland took to high-performance com­put­ing to study the invig­o­ra­tion effect on a regional scale.

To find out which fac­tors con­tribute the most to the invig­o­ra­tion, Fan and col­leagues set up com­puter sim­u­la­tions for two dif­fer­ent types of storm sys­tems: warm sum­mer thun­der­storms in south­east­ern China and cool, windy frontal sys­tems on the Great Plains of Oklahoma. The data used for the study was col­lected by dif­fer­ent DOE Atmospheric Radiation Measurement facilities.

The sim­u­la­tions had a res­o­lu­tion that was high enough to allow the team to see the clouds develop. The researchers then var­ied con­di­tions such as wind speed and air pollution.

Fan and col­leagues found that for the warm sum­mer thun­der­storms, pol­lu­tion led to stronger storms with larger anvils. Compared to the cloud anvils that devel­oped in clean air, the larger anvils both warmed more — by trap­ping more heat — and cooled more — by reflect­ing addi­tional sun­light back to space. On aver­age, how­ever, the warm­ing effect dominated.

The spring­time frontal clouds did not have a sim­i­larly sig­nif­i­cant warm­ing effect. Also, increas­ing the wind speed in the sum­mer clouds damp­ened the invig­o­ra­tion by aerosols and led to less warming.

This is the first time researchers showed that pol­lu­tion increased warm­ing by enlarg­ing thun­der­storm clouds. The warm­ing was sur­pris­ingly strong at the top of the atmos­phere dur­ing the day when the storms occurred. The pollution-enhanced anvils also trapped more heat at night, lead­ing to warmer nights.

“Those num­bers for the warm­ing are very big,” said Fan, “but they are cal­cu­lated only for the exact day when the thun­der­storms occur. Over a longer time-scale such as a month or a sea­son, the aver­age amount of warm­ing would be less because those clouds would not appear everyday.”

Next, the researchers will look into these effects on longer time scales. They will also try to incor­po­rate the invig­o­ra­tion effect in global cli­mate models.

Source: DOE/Pacific Northwest National Laboratory Release