Deep Convective Cloud Systems

Goal: To understand the physics of convective cloud systems on time scales up to intraseasonal, how they influence large scales, and how they can be parameterized.

Deep convective cloud systems can now be numerically simulated for long periods over large domains with high resolution. These simulations provide a way of understanding large-scale circulations of which deep convection is an integral part, and they provide a consistent basis for the development of parameterizations of the effects of deep convection cloud systems in larger-scale motions. However, microphysical processes, cloud-radiation interaction, and sub-grid turbulent processes have inherent uncertainties that feedback to produce uncertainties in predicted large-scale motions. The work described in this section is part of the NCAR Clouds in Climate Program (CCP), which is a concerted effort to bring together process studies and parameterization of deep convection relevant to climate modeling and numerical weather prediction.

Cloud systems on long time scales

Current emphasis is on cloud systems in the tropics, on time scales from a week or so up to the intraseasonal, and on space scales from about a kilometer toplanetary. The focus on the tropics is motivated by the fact that the tropics play a key role in the climate system in terms of the energy and water cycle. In spite of its importance, the coupling between moist convection and large-scale dynamics in the tropics lacks a fundamental basis. The difficulty has been that large-scale processes in the tropics depend directly on the continued and systematic action of small-scale processes. For example, the concerted effect of deep cumulus clouds may influence a slow, large-scale tropical oscillation, but the tropical oscillation equally affects the cumulus convection. Hence the small and large-scale motions must be solved for together. The need for long-time-and-space-scale integrations means that processes that can usually be neglected for short-time-scale weather prediction (e.g., cloud-radiation and air-sea interaction) must be accounted for in models of climate.

In simulating convection on a time scale of a few hours, the primary issue is how to approximate the rates of change between the three phases of water. On time scales longer than a day or so, the evaporation rate and fall velocity of the hydrometeors, and the interaction of water in any of its three phases with solar and long-wave radiation becomes progressively more important. When cloud-resolving models are integrated for long times a key issue is the nonlinear coupling between the parameterized microphysics and radiation through explicitly resolved dynamics. We will continue to use the single-column-type approach in which the model (i.e., the cloud-resolving model (CRM), or the single-column model) is driven by large-scale conditions derived from the data gathered in major observational campaigns (e.g., GATE and TOGA COARE). This approach is a valuable testbed to evaluate various parameterizations (cloud microphysics in particular) used in the CRM and to assess the impact of cloud systems on surface processes and on the radiative transfer.

Building on our experience with GATE and TOGA COARE cloud systems, we aim to couple the cloud-resolving model with an ocean model. Multiscale modeling of a coupled system of clouds and the ocean has only very recently become computationally feasible. This kind of multiscale modeling is necessary to achieve realistic Hadley and Walker circulations in domains of order 10,000 km, and will build on ongoing idealized prototype simulations.

Idealized modeling is also used to study cloud systems and their impact on the large-scale tropical dynamics. We will continue to study convection organization in long-term simulations of convective-radiative equilibrium, applying two- and three- dimensional domains within the equatorial waveguide using both cloud-resolving and parameterized convection. A nonhydrostatic global model is being used in a series of studies of convection organization on a rotating planet, starting with a constant sea surface temperature (SST) aquaplanet, and proceeding to a planet with idealized distribution of the SST and landmasses, including topography. Using such an idealized setup we study the role of convection in monsoons.

Finally, we study the influence of organized convection (e.g., mesoscale convective systems) on the large-scale momentum budget in the tropics. Convection organization, how it is modulated by the large-scale dynamics, and how it feeds back into the large-scale flow are key issues. MMM has a strong heritage in the dynamics of the organized convection that will be essential in theoretical and observational studies of the large-scale impacts of organized convection.

Convectively generated tropical ice clouds

Tropical cirrus covers a significant part of the tropics and has a key effect on the Earth's radiation budget and dynamics. Because anvils occupy an area much greater than the deep convection producing them, studies are needed to understand better the interactions among radiation, dynamics, and microphysics. Modeling studies and satellite data will be used to determine the complete life cycle of tropical anvils and factors responsible for their persistence.

In-situ and remote-sensing measurements of cloud microphysical and radiative properties help derive distributions of cloud microphysical properties as a function of altitude as well as their relationship to cloud radiative properties. Convectively generated cirrus have sufficiently high optical depths near cloud top to produce localized areas of bright or optically thick cirrus, reflecting more than 40% of the incoming solar radiation. However, the upper parts of cirrus cannot alone account for the high albedos. The lower parts sometimes extend down to the melting layers in the so-called stratiform cloud regions that are usually necessary to produce high albedos. These aspects need to be further quantified and our involvement in field programs in subtropical and tropical locations will allow us to obtain data that can be used in parameterizataion development.

Using observational microphysical data past and forthcoming tropical field programs, parameterizations will be developed of the tropical cirrus microphysical properties in terms of diagnostic or prognostic variables for GCMs and CRMs. For example, expressions for the ice-particle effective radius, extinction coefficient, absorption coefficient, and mean terminal velocity in terms of cloud ice water content and temperature will be developed. Additionally, a characterization of ice particle shapes will be provided.

Aside from the difficulty of parameterizing convection per se, the calculation of convective cloud-system areal extent, or cloud fraction, is very important for radiative transfer in GCMs. A primary area of research is based upon satellite data and analyses, and progress will be accelerated by using cloud-resolving models. In turn, observational data will aim to develop new microphysical schemes to be used in CRMs and provide important validation for CRM simulations.

Impact of tropical cloud systems on radiative transfer

Clouds impact radiative processes in a complicated way. Large-scale weather and climate models apply a plane-parallel approach to deal with the transfer of shortwave (solar) and longwave (thermal) radiation. Such an approach can be argued appropriate for models with grids featuring large aspect ratio (i.e., the ratio between the horizontal and vertical grid spacings). Cloud-resolving models, featuring grids with aspect ratios close to one, usually apply a similar approach because of the lack of affordable alternatives. However, the validity of the plane-parallel approach can be questioned when vertical columns with a few-kilometer horizontal extent are treated independently, and only vertical radiative fluxes are considered. We will evaluate the effects of detailed three-dimensional radiative transfer on the energy budget and evolution of the resolved convection models. The traditional "independent pixel" radiative parameterizations adapted from GCMs will be replaced by multidirectional quasi-exact calculations. The objectives are to (1) determine the changes in the surface and top-of-atmosphere radiative fluxes when radiation is allowed to interact with the 3D structure of the cloud field; (2) determine the relative importance of changes to cloud microphysics and changes to radiative parameterizations for the energy budget; and (3) quantify the effects of three-dimensional radiative transfer on the structure of radiative equilibrium solutions for tropical convection. The principal objective is to determine what aspects of radiative interactions with cloud geometry and
inhomogeneity are important for large-scale model parameterizations.

Radiative processes have long been postulated to strongly influence tropical deep convection (e.g., the early morning maximum of convective intensity over tropical oceans). Such an influence was reproduced using cloud-resolving models. It is not clear, however, what influence radiative transfer has on the large-scale tropical dynamics and on the SST through the interaction of radiation with water vapor and clouds. For instance, idealized studies suggest that convective-radiative equilibrium is unstable in the sense that self-maintaining circulations have to develop to balance the differential radiative cooling between dry/cloud-free and moist/cloudy large-scale regions. It remains to be seen if this "moisture-radiation instability" is relevant for the intraseasonal variability in the tropics. A need for a cloud-resolving approach to address the large-scale impacts of radiative transfer is apparent.

Microphysical parameterizations used in cloud-resolving models have been developed to represent phase changes of water substance and precipitation fallout. They are not designed to predict parameters relevant for the radiative transfer (such as effective radius or single scattering albedo of cloud particles). Moreover, impact of some hydrometers (e.g., graupel or rain) is often neglected in radiative transfer models. One can argue, however, that parameterizations of cloud microphysics and of radiation transfer should be closely coupled in order to address the cloud-radiation interaction in a meaningful way. We will attempt to develop such microphysical parameterizations.

Parameterization of deep convection

Current parameterizations do not account for the effects of convective organization, which influences the life cycle and spatial coherence of large-scale circulations in the tropics. We have a hierarchy of models to tackle such problems ranging from idealized process models, to cloud-resolving models, to an intermediate model in which convection is parameterized but dynamical interactions are resolved, and finally to a large-scale "cloud-resolving parameterization" model which applies a cloud-resolving model instead of a parameterization scheme. This unique suite of models will allow comprehensive study of not only the parameterization problem, but also the underlying fundamental problem of understanding the interaction between convection and the mean flow.