Radiation Weakens Idealized Midlatitude Cyclones

Midlatitude cyclones are strongly affected by diabatic processes. While the importance of latent heating is well established, the role of radiation has received little attention. Here we address this question for idealized cyclones by performing baroclinic life cycle simulations in the global atmosphere model ICON with and without radiation, and with transparent clouds. Radiation substantially weakens the simulated cyclone: peak eddy kinetic energy reduces by 50%, and minimum storm central pressure increases by 17 hPa. An analysis of the Lorenz energy cycle shows that the radiative weakening is not due to changes in the large‐scale environment alone but involves radiative processes within the cyclone. In fact, radiation warms the lower tropospheric part of the cyclone's warm conveyor belt and cools the upper tropospheric part. We hypothesize that radiation weakens the cyclone by destroying midtropospheric potential vorticity in the warm conveyor belt. Plain Language Summary Midlatitude storms can cause strong rain and wind damage. Forecasting their strength and location is important for society but is challenging because of the many physical factors that influence their behavior. It has been shown that energy released when water vapor condenses into liquid water or ice is crucial for heating the air, allowing it to rise and strengthening air motion in the storm. Another mechanism for heating and cooling is still neglected: thermal radiation from the surface and the air itself. We simulate storms in an idealized model of Earth completely covered with ocean. Removing the effects of land allows us to isolate the impact of radiation in the atmosphere. We compare simulations with and without radiation effects and find that radiation weakens the storm significantly. One reason could be that in deep clouds, the lowest layers absorb radiation from the surface and heat, while the highest layers emit thermal radiation to space and cool. This pattern of heating and cooling could influence air motion and slow storm rotation. Storm weakening due to radiation is as important as strengthening by water condensation effects and should be included in models in order to predict storm strength correctly. Key Points Impact of radiation is studied in baroclinic life cycle simulations with global atmosphere model Radiation weakens midlatitude cyclones, likely by destroying midtropospheric potential vorticity Radiative weakening is comparable in magnitude to cyclone strengthening