Mesoscale
and Microscale Meteorology
National Center for Atmospheric Research
National Center for Atmospheric Research
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Mesoscale
and Microscale Meteorology
National Center for Atmospheric Research |
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GCSS COARE, Version 2.0:
GCSS Working Group on Precipitating Convective
Cloud Systems:
Cloud Resolving Model Intercomparisons
Precipitating Cloud Systems in TOGA COARE:
Overview
Mitchell W. Moncrieff, National Center for Atmospheric
Research, USA
David Gregory, European Centre for Medium
Range Forecasts, UK
Steven K. Krueger, University of Utah,
USA
Jean-Luc Redelsperger, CNRM (CNRS and
Meteo-France), France
Wei-Kuo Tao, NASA Goddard Space Flight
Center, USA
The Working Group on Precipitating Convective Cloud Systems of the GEWEX (Global Energy and Water-Cycle Experiment) Cloud System Study (GCSS), will conduct a series of workshops in which cloud resolving models will be intercompared and evaluated against observations. These verified data will, in turn, be used to evaluate and improve single-column models which are an integral part of the development, testing and implementation of parameterization schemes in numerical weather prediction (NWP) models and general circulation models (GCMs). This comprehensive approach to addressing the long-standing problem of the effect of clouds on climate, involves a judicious mix of basic research in convective processes, studies of the interaction of clouds with the environment, and the development of parameterization schemes.
The first intercomparison study (described here) will make use of the comprehensive data sets from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). Since a modeling procedure similar to that in the TOGA COARE study can be used, convection over the tropical eastern Atlantic will also be modeled by some participants. In that case the initializing, forcing and verification data will be from the Global Atmosphere Research Programme Atlantic Tropical Experiment (GATE) conducted July through September 1974. The observational datasets used in the evaluations will range from case studies of deep convection on diurnal time scales, to studies of ensemble effects of convection on spatial scales of order 1000 km on time scales of at least one week. Evaluations will eventually involve remote sensing and long-term monitoring datasets because, with few exceptions, datasets available from field experiments are of limited horizontal extent and duration.
The intercomparison project will be consistent with objectives of both GCSS and TOGA COARE. The main objectives of TOGA COARE are to study (i) "the principal processes responsible for the coupling of the ocean and atmosphere in the western Pacific warm pool system," and (ii) "the principal atmospheric processes that organize convection in the warm pool region." The GCSS principle objective is "to develop better parameterizations of cloud systems for climate and numerical weather prediction models via an improved understanding of coupled physical processes."
The GCSS was established under the auspices of the World Meteorological Organization/World Climate Research Programme (WMO/WCRP). Introduced in GCSS (1993, 1994), with recent progress reported in WCRP (1996), this program has the challenging task of advancing knowledge of the large-scale effects of convective processes and its parameterization in large-scale models. For the most part, the GCSS activities are conducted by four GCSS Working Groups (WGs) centered on four primary cloud system categories: Boundary Layer Clouds (WG 1); Cirrus Cloud Fields (WG 2); Extra-tropical Layer Cloud Systems (WG 3); and Precipitating Convective Cloud Systems (WG 4).
Cloud resolving models (CRMs), verified against observational datasets, are the main approach of the GCSS in its quest to the large-scale role of clouds. These models are based on the nonhydrostatic equations of motion and can therefore properly treat the cloud-scale and mesoscale circulations that couple the physical processes, in contrast to the general circulation models (GCMs) that parameterize them. They evolved over the last two decades from idealized, two-dimensional cloud models having small domains to physically sophisticated, three-dimensional models capable of approximating the evolution of precipitating convective cloud systems in large domains.
The domains of state-of-the-art CRMs span many climate models grid volumes and can therefore meaningfully be applied to parameterization studies. However, since CRMs are limited-area models, boundary conditions place constraints the interaction of cloud systems with the large-scale flow so advances in model design are necessary. Note that CRMs of precipitating cloud systems include (but poorly resolve) boundary layer clouds; the resolution required to realistically approximate the dynamical components of cloud systems is a key issue.
Microphysics and radiation have to be parameterized in all atmospheric models, and existing methods need to be properly evaluated. From the GCSS perspective--focusing on the environmental effects of clouds--it is important to determine the minimum level of sophistication required for CRM microphysical parameterizations. Note that in GCMs and NWP models the prognostic treatment of clouds (in which microphysical processes are considered in a much simpler way than in CRMs) has been identified as one of the pacing issues (WCRP 1995). Since convection is explicitly resolved in CRMs, uncertainties involved with cloud dynamics are minimized, making the treatment of microphysical processes one step less complex.
GCSS WG 4 Action Groups (see GCSS Science Plan 1994; A6-7) reported on issues involving the physics and parameterization of precipitating convective cloud systems in global models. Consideration of these reports led to the selection of tropical cloud systems as the priority. This choice was based on their key importance in climate models, and on the comprehensive manner in which they can be studied using cloud resolving models. Emphasis was put on cirrus-generating deep convection over the open ocean and islands in the Maritime Continent (the name given to the large region centering on Indonesia) partly because new datasets are available from TOGA COARE and from MCTEXAt the time of writing of the Action Group's report there was a prospect that in situ measurements by a high-flying aircraft would be available to evaluate remotely sensed data from surface-based instrumentation (e.g., two profilers, a lidar, a millimeter wavelength radar, a dual-polarization 5-cm wavelength Doppler radar). In actuality, however, the high-flying aircraft was not deployed. The MCTEX field phase ran from mid-November through mid-December 1995 and produced unique datasets on convection initiation, storm-scale dynamics and the physics of convectively generated cirrus (Maritime Continent Thunderstorm Experiment), respectively.
Continental precipitating cloud systems (tropical and midlatitude) were identified for early study. In particular, midlatitude cloud systems occurring during the warm season over the central USA will be first considered because data sets to evaluate the models are available from the DOE Atmospheric Radiation Measurement (ARM) program and the GEWEX Continental International (GCIP).
Midlatitude marine cloud systems also identified for intensive study because unexplained systematic errors in lower troposphere in NWP models have been traced to these cloud systems. Of special interest is the lagrangian evolution of boundary layer clouds into deep convection in baroclinic environments, notably organized convection in cold air outbreaks behind midlatitude depressions and in the trade wind regions.
While a better understanding of all the above cloud systems is required from the viewpoint of properly representing them in general circulation models, there are compelling reasons for selecting cloud systems observed in TOGA COARE for the first GCSS WG 4 model intercomparison project.
The TOGA COARE field phase took place November 1992 to February 1993 in the tropical western Pacific (Webster and Lukas 1992). One of the conclusions of the TOGA COARE Data Workshop was the need for TOGA COARE to develop and organize modeling activities to address scientific issues relating to convection in TOGA COARE. For example: "One area where cloud and mesoscale models are expected to bring valuable help is in the explanation of the convective organization...two major related issues are (i) to classify convective events observed during the TOGA COARE Intensive Observation Periods, and (ii) to examine the convection predictability. Modeling strategies will need to be developed given the hierarchical structure and modulation of convection over the broad range of time and scales from order 100 km to synoptic scales" (Redelsperger 1994).
Cloud systems in the western Pacific region occur on a hierarchy of scales ranging from about 1 km to more than 1000 km and are challenging the fundamental assumptions underpinning convective parameterization. Not only are individual convective clouds organized into mesoscale convective systems but, in turn, these systems are sometimes organized into yet larger-scale "super clusters." These are beginning to explicitly appear (but are not properly resolved) in high-resolution (e.g., spectral models at T213 resolution) global numerical weather prediction models and are causing problems (Moncrieff and Klinker 1996).
Other issues relating to mesoscale cloud systems in general circulation models require study. For example, organized convection is the principal generator of tropical cirrus that is well known to be climatically important because of its interaction with radiative processes. It is also responsible for heavy rainfall that affects the thermal stability of the oceanic mixed layer and its response to solar heating. The generation of convective and mesoscale downdrafts within these organized systems strongly affects the surface energy budget and therefore the coupling between ocean and atmosphere. Finally, it produces momentum fluxes of a complex form whose large-scale role is poorly understood.
The principal objectives of the model intercomparisons are to:
The methodology is to:
The model intercomparison activities are organized by a Scientific Steering Committee composed of representatives from GCSS, TOGA COARE and the GCM communities, namely David Gregory, Mitchell Moncrieff (chair), Steven Krueger (leader, Case 1), Jean-Luc Redelsperger (leader, Case 2); and W.-K. Tao. To meet the above objectives, this committee is charged with:
Precipitating cloud systems are the most complex forms of atmospheric convection because interactions among all the physical processes are strong.
Key questions are:
After considering several options, the steering committee opted for the following two complementary TOGA COARE convection studies:
In Case 1 the initial conditions will be given by soundings obtained as part of an intensive observation period; in Case 2 grid-scale averages over the entire TOGA COARE Intensive Flux Array (IFA) will be used as initial conditions in keeping with the nature of parameterization studies.
Details of Case 1 are presented in Redelsperger et al. (1996). During recent years, a great deal of modeling and observational analysis has been devoted to squall lines. Studies have ranged from idealized dynamical models to numerical simulations containing quite sophisticated parameterizations of microphysics. However, intercomparisons of numerical simulations have not yet been conducted in a comprehensive way, and in few instances have they been properly evaluated against data obtained from field programs.
This is a simulation and evaluation against observational data of a squall line observed on 22 February 1993. It will include an evaluation of sub-grid scale parameterizations presently used in cloud resolving models--an issue arguably best addressed on a case study basis. For example, the strengths and weaknessess of different microphysical parameterization schemes will be evaluated, as well as other issues such as downdraft and updraft mass fluxes, effects of convection on surface fluxes, and interaction between convective and stratiform regions. Observations to be used include those obtained by airborne Doppler radars, from which 3D fields of wind and reflectivity can be derived. The initial conditions will use the sounding analyses by LeMone et al. (1994). The simulations will help determine to what extent 3D CRMs can reproduce the structure and evolution of the observed mesoscale convective systems.
This case, detailed in Krueger et al. (1996), will evaluate the ability of CRMs to treat the interaction of cloud ensembles with large-scale variables and, specifically, to describe time-averaged statistics of cumulus ensembles throughout many episodes of deep convection. It is thus focused on the bulk effects of deep convection, the accompanying mesoscale circulations, and the interaction of cloud ensembles with the environment. Statistical aspects of parameterization will be included in the study. The results will be evaluated against the average properties over the entire TOGA COARE IFA. In this way, the evaluation of single-column models will also be facilitated.
This case will focus on a six-day period in the onset phase of a December 1992 westerly wind burst that has much relevance to atmosphere-ocean interaction. The cloud-scale response to observed large-scale forcing during the period will be simulated by two-dimensional models with large domains and a resolution of order a kilometer. (Those participants with the models and computational capability are encouraged to run 3D experiments). Specifically, the simulations will be forced by time-dependent temperature and moisture advection and horizontal winds averaged over the IFA (Lin and Johnson 1995). The feasibility of such a calculation has been demonstrated by Wu et al. (1996) and Grabowski et al. (1995), who simulated the life cycle of convective systems during an easterly wave episode in GATE. Participants are encouraged to run the GATE case as well as the TOGA COARE one.
To facilitate a link to the parameterization community single-column models will be compared against CRMs and observations. A comprehensive approach using a hierarchy of models and observations can thus be brought to bear on the parameterization of convection over the western Pacific as presently represented in GCMs.
Results will be presented, discussed and collated in the 1st GCSS WG 4 Cloud Resolving Model Intercomparison Workshop during 21-23 October 1996 in Annapolis, Maryland, co-hosted by NASA Goddard Space Flight Center and NCAR. This will be a forum for addressing models, evaluations of models against observations and parameterization issues alike.
CSS, GEWEX Cloud System Science Team, 1993: The GEWEX Cloud System Study, Bull. Amer. Met. Soc., 74, 387-400.
GCSS, GEWEX Cloud System Study Science Plan, 1994: IGPO Publication Series, 11, K.A. Browning (ed.), World Climate Research Programme International GEWEX Project Office, 84 pp.
Grabowski, W.W., X. Wu, and M.W. Moncrieff, 1995: Cloud resolving modeling of tropical cloud systems during Phase III of the GATE. J. Atmos. Sci., in press.
Krueger, S.K., D. Gregory, M.W. Moncrieff, J.-L. Redelsperger, and W.-K. Tao, 1996: GCSS Working Group 4 Cloud Resolving Model Intercomparison Project. Case 2: Large-scale effects of cloud systems, working manuscript.
LeMone, M.A., D.P. Jorgensen, and B.F. Smull, 1994: The impact of two convective systems of sea-surface stresses in COARE. Preprint Volume, 6th Conference on Mesoscale Processes, Amer. Meteor. Soc., Portland, 40-44.
Lin, X., and R.H. Johnson, 1995: Kinematic and thermodynamic characteristics of the flow over the western Pacific warm pool during TOGA COARE. J. Atmos. Sci., submitted.
Randall, D.A., K.-M. Xu, R.C. J. Sommerville, and S. Iacobellis, 1995: Single-column and Cloud Ensemble models as links between observations and climate models. J. Climate, submitted.
WCRP, 1995: Report on the "Workshop on Cloud Microphysics Parameterizations in Global Atmospheric General Circulation Models," Kananaskis, Alberta, Canada, 23-25 May, 1995, WCRP-90, WMO/TD-No. 713.
WCRP, 1996: Report on the "Fourth Session of the GEWEX Cloud System Study (GCSS) Science Panel," 11-15 December, Washington, D.C., USA, in press.
Moncrieff M.W., 1994: A Proposed GCSS & Joint Modelling Project Based on Cloud Systems Observed in TOGA COARE.
Moncrieff, M.W., and E. Klinker, 1996: Mesoscale cloud systems in the tropical western Pacific as a process in General Circulation Models. Quart, J. Roy. Met. Soc., conditionally accepted.
Redelsperger, J.-L., 1994: Report on "Cloud and Mesoscale Modelling Session of the Toulouse COARE Workshop," TOGA COARE International Data Workshop, Toulouse, France.
Redelsperger, J.-L., D. Gregory, S.K. Krueger, M.W. Moncrieff, and W.-K. Tao, 1996: GCSS Working Group 4 Cloud Resolving Model Intercomparison Project. Case 1: A squall line, working manuscript.
Webster, P.J., and R. Lukas, 1992: TOGA COARE: The Coupled Ocean-Atmosphere Response Experiment. Bul Amer. Met. Soc., 73, 1377-1416.
Wu, X., W.W. Grabowski, and M.W. Moncrieff, 1996: Cloud resolving modeling of TOGA COARE cloud systems and their interactions with radiative and surface processes. Part I: Long-term two-dimensional experiments. To be submitted to J. Atmos. Sci.
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