Understanding the responses of deep convective clouds to a changing thermodynamic environment.

Deep convective clouds play a key role in regulating the Earth’s water and energy cycles and are therefore an important component of the Earth’s climate system. Much work has been done on investigating factors such as how aerosols affect these clouds but much less emphasis has been on how the future changes in thermodynamic structure of the atmosphere will affect these clouds. It is vitally important to know how clouds will respond to these changed thermodynamic conditions.

This study explores how modelled deep convective clouds respond to the projected future warmer climate and compares it with past (current) climate. Atmospheric thermodynamic profiles are taken from NCAR CCSM3 global climate model and the modelling study was based on simulations of idealised deep convective clouds using the Weather Research Forecast (WRF) model with a double-moment cloud microphysics scheme.

The modelling study showed that the future thermodynamic environment produced deep convective clouds that are on average 32% lower in cloud base height, 8.5% higher in freezing level and 9% lower in cloud-top height compared to the past climate results. This results in the future clouds having an average of 15% deeper warm-phase cloud layer and 18% shallower cold-phase cloud layer than in the past climate, signifying a strong warm rain process and reduced cold rain process, where the cold rain process is traditionally known to be the dominant precipitation-forming process in the current climate. The strong warm rain process leads to intense heavy rainfall, which reduces the availability of water in the updraught reaching the ice phase, thus reducing the clouds’ vertical and horizontal extent.

An in-depth investigation was performed to assess the factors associated with the changes of the future thermodynamic environment influencing the cloud development. Such factors include assessment of the significance of temperature structure, relative humidity structure, both structural changes, and increased moisture due to the warmer mean temperature. It was found that the structural effects of temperature and relative humidity have greater impact on the cloud development compared to the increase in the mean temperature. The temperature structure factor significantly reduces the average total water content and cloud vertical ex- tent by 75% and 8 km, respectively. On the other hand, the relative humidity structure significantly increases the average total water content and cloud vertical extent by 80% and 1.8 km, respectively. Separately assessing these two factors gives significant opposing effects on the cloud’s total water content and vertical height. When combined, these two effects cancels out to some extent, but they still produce weaker and smaller deep convective clouds than the effects that arise from the mean temperature increase.

Following from the differences in the cloud development between the past and future environments, the strong warm rain process and weak cold rain process results in the cloud fraction in the future reduced by half of that in the past environment. This reduced cloud horizontal extent results in cloud radiative forcing at the top of the atmosphere that is positive, indicating an increased warming in the future environment. The average local cloud radiative forcing was evaluated to be 43.7 and 54.7 W m−2 for the whole cloud and anvil cloud, respectively. If this is extended to midlatitude deep convective region over land with a coverage of 3% or global coverage of deep convective clouds of less than 1%, a simplistic approximation of the cloud radiative forcing is evaluated to be +1.31 and +0.44 W m−2, respectively. This is opposite to the radiative forcing exerted by aerosol- cloud interaction reported in the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5), which was estimated at -0.55 W m−2.

The results from this study highlight that the changes in the vertical thermodynamic structure affect the cloud development significantly. This study was based on the output of one global climate model, however, it does suggest that it is important that climate modelling groups pay particular attention to the way their models forecast the thermodynamic structure in both temperature and moisture of the future climate.