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 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 set of organization indices has been assessed using radar and satellite data as well as a combination of the two. Two indices were investigated to characterize the degree at which the convective objects are organized in space, namely SCAI and I.org. Examples are sketched in Figure 1. The SCAI index counts the number of objects (clouds or precipitation cores) within a certain distance and combines this number within the typical distance of all objects in the fixed neighborhood. The lower the SCAI the more organized are the objects. The I.org, which is nearest neighbor cumulative distribution function (NNCDF) – based assesses the distribution of nearest neighbor distances between objects and is able to distinguish among random (I.org =0.5), regular (I.org <0.5) and organized (I.org>0.5) object arrangements.
A third index, namely I.shape, which is based on the area and on the perimeter of the objects is employed to characterize the dominant shape of the organizations. This index varies from 0 (line form) to 1 (circular form). Figure 2 shows an example of the above-mentioned indices applied to radar observations.
The same indices were then applied to synthetic observations derived from simulation results to evaluate the representation of convective organization in the ICON model at various resolutions. An example of synthetic and observed satellite-based brightness temperatures is shown in Figure 3. Convection starts to develop along the North-to-South oriented convergence line which strongly contributes to the initial organization of deep convective structures. The simulations at different grid spacings show differences in the way they capture the start and spatial structure of developing convection.
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.
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 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 (see the coresponding article).
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.
The role of sub-synoptic scale organization for the large-scale circulation
In work package 4, the role of sub-synoptic scale organization for the large-scale circulation is investigated. This is done by relating the dynamical properties of the convective cells to the degree of convective organization and to the large-scale circulation. Therefore Fourier and wavelet spectra of different quantities are computed to distinguish between different scales and directions of convective cells.
Organized convection leads to updrafts with a horizontal extent about 20 to 100 km along the cold pools and a negative slope (-0.4 between 2.5 and 100 km), while randomly distributed cells influence smaller scales (below 20 km) with a positive slope (Figure 1). Also spectra of other (dynamical) properties like horizontal wind velocity, divergence, vorticity, potential temperature, cloud top temperature or PV differ with the degree of convective organization.
Furthermore, from the rain rate wavelet spectra we can obtain the orientation of the cells (Figure 2). For the 05. July 2015, a day of the high-resolution simulations done in HD(CP)², most energy exits in north-south direction for scales between 19.2 and 76.8 km. This is due to a cold front with a foregoing convergence line moving from west to east. In case of unorganized scattered cells (e.g., 15. August 2014) the spectra are isotropic with most energy at smallest scales.
In addition to these analyses, 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 are studied.
Parameterization of Convective Organization
Work package 5 addresses the organization characteristics of shallow precipitating convection and its impact on the boundary layer turbulence. The precipitation and the associated organized motions influences the higher order moments of scalar quantities such as the moisture and potential temperature. However, the exact link between the organization of the cloud and the formation of precipitation during shallow convection and their feedback to the boundary layer turbulence is less clear and is the focus of the present work. Different idealized large eddy simulations have been carried out for the study and figure 1 gives an example of the evolution of the liquid water path during one of the simulation. As a first step, the second and third turbulent moments and their budgets are studied for the scalar quantities (see figure 2). The budget analysis reveals the importance of microphysical effects in the cloud layer. The study also highlights the role of the pressure redistribution term in maintaining the second and third order budgets. Further we aim to understand the source mechanism that is responsible for the increase of moisture variance during the precipitation and associated cloud organization. The analysis ultimately helps to develop parametrization approaches to include the effect of precipitation on the turbulence.
In the future, we are interested in investigating different modes of organization of precipitating shallow convection and associated feedbacks on the turbulence by addressing the following aspects:
- Can the different modes of organization (random, cluster, arc) be controlled?
- If so, what will be the role of cold pools and mid-level moisture for maintaining different modes of cloud organization?
- Can the mode of organization of shallow convection be diagnosed from the higher-order moments?
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.
Anurose Theethai Jacob