Modeling Condensation in Thunderstorm Clouds
Simple model of condensation may not be sufficient for moderate thunderstorms.
Condensation of water vapor to form and grow cloud droplets is one of the most fundamental processes of cloud and precipitation formation. It drives cloud-scale winds and vertical air motion through the release of latent heat and determines the strength of storm updrafts, the upward winds that enable cumulus clouds to grow into thunderstorms.
Two methods of cloud droplet condensation in cloud models were compared for simulations of scattered thunderstorms. The more detailed explicit condensation method led to stronger vertical motion in the thunderstorm cores and more upper-level cloudiness. Adding the effects of explicit condensation to weather and Earth system models will improve their ability to predict details of thunderstorms, including rain amount.
Cloud models simulate condensation using two different methods. The first is the simple “saturation adjustment” method in which exactly water-saturated conditions are assumed inside liquid clouds–that is, relative humidity is assumed to be 100%. The second method is more detailed and calculates condensation explicitly using the model’s predicted relative humidity, allowing humidity to be larger than 100% inside clouds. These two methods were compared in model simulations of scattered thunderstorms using the “piggybacking” approach that robustly separates dynamical from cloud microphysical effects. The saturation adjustment method produced larger thunderstorm vertical velocities due to its greater latent heating during condensation, leading to the clouds having more buoyancy. There was also a large impact on upper-level anvil clouds. Simulations using the explicit condensation method had more numerous, smaller ice particles that fell out more slowly compared to the simulations using saturation adjustment, leading to thicker, longer-lasting upper-level clouds. These findings have implications for simulating convective storms since most models use the simpler saturation adjustment approach, while the explicit condensation method is believed to be more accurate. However, results depend somewhat on details of the aerosol particles ingested into storms that activate cloud droplets, and more detailed observations are needed to confirm the existence of these high relative humidities inside thunderstorm clouds simulated by the explicit condensation method. This suggests a need for improvements in our ability to measure humidity accurately inside clouds. These results also suggest that the simpler saturation adjustment method can be modified in a straightforward way to capture the effects of explicit condensation, which will be explored in future work.
National Center for Atmospheric Research
This work was partially supported by the U.S. DOE ASR grant DE-SC0016476. WWG was also partially supported by the Polish National Science Center (NCN) “POLONEZ 1” grant 2015/19/P/ST10/02596. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
Grabowski, W.W. and H. Morrison. “Modeling condensation in deep convection.” Journal of the Atmospheric Sciences, 74: 2247-2267 (2017). [DOI: 10.1175/JAS-D-16-0255.1]