Convective Organization

Project S5 deals with the question of the organization of convective clouds. Convective clouds are a typical occurrence in the sky of warm sunny days. They exhibit a large variety in shape and size, ranging from the isolated and randomly distributed cumuli to organized cloud clusters that can span thousands of kilometers (see left and right figure).

Random distribution of cumulus clouds.
A large convective cell.

But do these different appearances of convection matter for the climate of a particular region? And if so, are some biases in state-of-the-art global climate models associated with a misrepresentation of convective organization on the meso- or sub-synoptic scales? 

To answer these questions, observational signatures of convective organizations are identified and their representation is evaluated in the ICON_LEM model as well as in coarser resolution simulations using parametrized convection. The role of convective organizations is quantified for the European Summer climate and the tropical Atlantic climate. The consideration of two different climatic regimes helps us isolating the signature of convective organization and assessing the generality of the results.

Characteristics of Organization

Work packages 1 and 2 are based on a close collaboration between the University of Bonn, the Leibniz Institute for Tropospheric Research (TROPOS) and the Max Planck Institute for Meteorology. Signatures of convective organizations are investigated using weather radars, geostationary satellites and ICON_LEM simulations. In a first step, a robust index is developed to quantify convective organization using radar and satellite data as well as a combination of the two. This definition is then applied to synthetic observations derived from simulation results to evaluate the representation of convective organization in the ICON model at various resolutions. In a second step, the relationship between the organization and observational flow features is studied. Considered flow features include precipitation efficiency and mesoscale circulations. The overall aim is to infer relationships between organization and environmental properties, indicative of the potential effects of organization on climate.

Origin and Effects of Convective Organization

A distinction of causes and effects based on diagnostic relationships between organization and flow features, as in work packages 1 and 2, is difficult. Work packages 3 and 4 thus perform targeted sensitivity experiments with the ICON_LEM to better decipher the effects of organization on climate.

Role of Radiation for Convective Organization

Work package 3 revolves around the question if and how radiative transfer influences the formation of clouds on local and larger scales. Explorations with idealized Large-Eddy simulations revealed that radiative heating and cooling, stimulates shallow cumulus cloud dynamics, leading to the formation or destruction of cloud streets. The strength of convective organization depends on surface conditions, the background wind speed and the sun's azimuth and zenith angle. Particularly the shortcomings of one dimensional radiative transfer (shadow always directly beneath clouds) has a detrimental influence on the development of cloud-radiative feedback mechanisms. The goal in Phase II of the HD(CP)² project is to further examine the role of 1D and 3D radiative transfer on the generation of organization for shallow and deep convection. Particularly the realistic setup of the ICON_LEM will shed some light if radiatively induced organization persists in the vicinity of large scale forcing or surface heterogeneity.
The first step towards the above mentioned experiments is to adapt the 3D radiative transfer solver developed in Phase I to the ICON_LEM. An improved characterization of radiative processes may furthermore be useful for simulations conducted in work package 4 or prove to be an additional factor in the development of convective parametrization schemes in work package 5 and 6.

Shallow Cumulus cloud simulations driven with 1D radiative transfer (Twostream, top panel) and 3D radiative transfer (TenStream, bottom). Shown are surface fluxes(sensible and latent heat, L+H) and
cloud liquid water content (LWC). The position of the sun in the case of
3D radiative transfer organizes the convection in cloud streets.

In work package 4, the role of sub-synoptic scale organization for the large-scale circulation is investigated. This is done on the one hand by relating the dynamical properties of the convective cells, such as mass flux or vorticity, to the degree of convective organization and to the large-scale circulation. On the other hand, sensitivity experiments are performed where convective organization is inhibited or forced over specific regions, and the effects of such modifications on the large-scale circulation is studied.

Parameterization of Convective Organization

Work packages 5 and 6 aim at developing new parameterization approaches for the representation of convective organization in low-resolution global climate models, where convective processes are parametrized. Work package 5  focuses on shallow cumulus clouds (see figure).

It studies the effect of the organized structures and motions on exchange and transport in the cloudy boundary layer, as well as between the Earth's surface and the free atmosphere. To do this, the effect of precipitation and organized motions on the second-moment budget equations are investigates based on the results of the ICON_LEM simulations, and subsequently parameterized.

Work package 6 focuses on deep convection. It aims at developing a theoretical model to account for the clustering of convection and its potential effects. The theoretical model is implemented in an existing convective parameterization of the ICON nodel and the resulting effects are investigated.

Work package 6 focuses on deep convection. It aims at developing a theoretical model to account for the clustering of convection and its potential effects, in particular on the convective variability. The effects of organization are included in an existing deep convection scheme of the ICON model, where the number and properties of the triggered clouds will be affected by the existing clouds. The ability of the scheme to reproduce the convective variability seen in the high-resolution HD(CP)² simulations is tested and results are compared with the theoretical model. In collaboration with work package 3, simple ways to include the effects of convective organization on the radiation will be explored and the climatic effects of such changes documented.