Why are shallow cumuli bigger and stronger over land than over ocean?

Credit: Traute Crueger, MPI-M

In a new study appearing in the Journal of Atmospheric Sciences, Dr. Mirjana Sakradzija and Dr. Cathy Hohenegger, both scientists in the group "Clouds and Convenction" at Max Planck Institute for Meteorology (MP-M) in the department "The Atmosphere in the Earth System", identified key processes influencing the size of clouds. The questions why clouds are larger or smaller under different conditions and how that affects the vertical transport in a convective region has long puzzled scientists. Finding the answers to these questions is a step forward in understanding the physical processes that determine the cloud cover and the spatial and temporal variability of convection.

In their study, a large-eddy model with a grid spacing of 25 m was used to simulate two typical cases of shallow convection, one representing conditions as encountered over the tropical ocean and another one representing a convective summer day as over mid-latitude continental regions. In these two cases clouds develop different characteristics. Compared to the ocean conditions, clouds cover larger areas and develop higher vertical mass fluxes in average and in the extremes over the continental conditions. The outcome of the sensitivity experiments that were conducted to explain these differences is rather surprising - the distribution of cloud mass fluxes is controlled by the partitioning of the surface heat fluxes into sensible and latent heating rather than by the magnitude of the surface fluxes, their variation over the diurnal cycle, or the level of organization in the cloud field.

The next question arises naturally: why is the partitioning of the surface heat fluxes into sensible and latent heating, characterized by the ratio of these fluxes called the Bowen ratio, the most important control parameter in these simulation? The Bowen ratio controls the efficiency of the moist convective heat cycle in the atmosphere, and thus determines the portion of the surface flux input that can be converted into mechanical work of the convective overturning. This itself determines the convective mass flux of individual clouds and sets the shape of their distribution. The results not only allow us to explain the different shapes of the cloud size distribution but can be used to parameterize this distribution in coarse-resolution models.

This research was conducted as part of cooperation between DWD and MPI in the form of the Hans-Ertel Centre Research group (HErZ).

Original publication
Sakradzija, M. and C. Hohenegger, 2017: What determines the distribution of shallow convective mass flux through cloud base?. J. Atmos. Sci., doi.org/10.1175/JAS-D-16-0326.1


Dr Mirjana Sakridzija
Max Planck Institute for Meteorology
Phone: +49 (0) 40 41173 266
Email: mirjana.sakridzija@we dont want spammpimet.mpg.de

Dr Cathy Hohenegger
Max Planck Institute for Meteorology
Phone: +49 (0) 40 41173 302
Email: cathy.hohenegger@we dont want spammpimet.mpg.de