Modeling Condensation in Thunderstorm Clouds

Morrison, H. C., UCAR

Cloud Processes

Convective Processes

Grabowski W and H Morrison. 2017. "Modeling condensation in deep convection." Journal of the Atmospheric Sciences, 74(7), doi:10.1175/JAS-D-16-0255.1.


Values of predicted supersaturation (relative humidity minus 100%) with respect to liquid from the simulation using the explicit condensation method on the horizontal axis versus the quasi-equilibrium supersaturation assuming a local balance between the increase from vertical motion and depletion by condensation on the vertical axis. The blue and red crosses show results for relatively small and large values, respectively, of the bulk hydrometeor masses and concentrations.


Values of predicted supersaturation (relative humidity minus 100%) with respect to liquid from the simulation using the explicit condensation method on the horizontal axis versus the quasi-equilibrium supersaturation assuming a local balance between the increase from vertical motion and depletion by condensation on the vertical axis. The blue and red crosses show results for relatively small and large values, respectively, of the bulk hydrometeor masses and concentrations.

Science

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. 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. This occurred because the relative humidities inside the thunderstorm clouds were larger using the explicit condensation method, with humidities up to 105-110% in the cores of the strongest storms leading to the generation of more ice particles.

Impact

Simulation results differed notably using the saturation adjustment and explicit methods for modeling condensational growth of cloud droplets. Previous studies have shown this result for very intense supercell thunderstorms, while our study focused on moderate strength, scattered thunderstorms. 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.

Summary

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. 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.