Wojciech W. Grabowski

Breaking the Cloud Parameterization Deadlock

A key factor limiting the reliability of simulations of anthropogenic climate change is the inability to accurately represent the various effects of clouds on climate. Despite the best efforts of t...

Coupling Cloud Processes with the Large-Scale Dynamics Using the Cloud-Resolving Convection Parameterization (CRCP)

Abstract A formal approach is presented to couple small-scale processes associated with atmospheric moist convection with the large-scale dynamics. The approach involves applying a two-dimensional cloud-resolving model in each column of a three-dimensional large-scale model. In the spirit of classical convection parameterization, which assumes scale separation between convection and the large-scale flow, the cloud-resolving models from neighboring columns interact only through the large-scale dynamics. This approach is referred to as Cloud-Resolving Convection Parameterization (CRCP). In short, CRCP involves many two-dimensional cloud-resolving models interacting in a manner consistent with the large-scale dynamics. The approach is first applied to the idealized problem of a convective–radiative equilibrium of a two-dimensional nonrotating atmosphere in the presence of SST gradients. This simple dynamical setup allows comparison of CRCP simulations with the cloud-resolving model results. In these tests, t...

The multidimensional positive definite advection transport algorithm: nonoscillatory option

Abstract This paper presents a nonoscillatory option (i.e., free of dispersive ripples) of the advection algorithm described previously in J. Comput. Phys. ( 54 (1984), 325; 67 (1986), 396). The approach adopted merges the flux-corrected transport methodology with the iterative formalism of the algorithm. Further discussion of the algorithms accuracy is included. Theoretical considerations are illustrated through numerical tests and examples of applications to atmospheric fluid dynamics problems.

Cloud-Resolving Modeling of Tropical Cloud Systems during Phase III of GATE. Part I: Two-Dimensional Experiments

Abstract A formal framework is established for the way in which cloud-resolving numerical models are used to investigate the role of precipitating cloud systems in climate and weather forecasting models. Emphasis is on models with periodic lateral boundary conditions that eliminate unrealistic numerically generated circulations caused by open boundary conditions in long-term simulations. Defined in this formalism is the concept of large-scale forcing and the cloud-environment interactions that are consistent with the periodic boundary conditions. Two-dimensional numerical simulations of the evolution of cloud systems during 1–7 September 1974 in Phase III of the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) are conducted. Based on the above formalism, a simple technique is used to force an anelastic cloud-resolving model with evolving large-scale horizontal wind field and large-scale forcing for the temperature and moisture obtained from the GATE data. The 7-day period selected i...

Cloud-Resolving Modeling of Cloud Systems during Phase III of GATE. Part II: Effects of Resolution and the Third Spatial Dimension

Abstract Two- and three-dimensional simulations of cloud systems for the period of 1–7 September 1974 in phase III of the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) are performed using the approach discussed in Part I of this paper. The aim is to reproduce cloud systems over the GATE B-scale sounding array. Comparison is presented between three experiments driven by the same large-scale conditions: (i) a fully three-dimensional experiment, (ii) a two-dimensional experiment that is an east–west section of the three-dimensional case, and (iii) a high-resolution version of the two-dimensional experiment. Differences between two- and three-dimensional frameworks and those related to spatial resolution are analyzed. The three-dimensional experiment produced a qualitatively realistic organization of convection: nonsquall clusters, a squall line, and scattered convection and transitions between regimes were simulated. The two-dimensional experiments produced convective organ...

An intercomparison of cloud-resolving models with the atmospheric radiation measurement summer 1997 intensive observation period data

SUMMARY This paper reports an intercomparison study of midlatitude continental cumulus convection simulated by eight two-dimensional and twothree-dimensional cloud-resolving models (CRMs), driven by observed large-scale advective temperature and moisture tendencies, surface turbulent euxes, and radiative-heating proe les during three sub-periods of the summer 1997 Intensive Observation Period of the US Department of Energy’s Atmospheric Radiation Measurement (ARM) program. Each sub-period includes two or three precipitation events of various intensities over a span of 4 or 5 days. The results can be summarized as follows. CRMs can reasonably simulate midlatitude continental summer convection observed at the ARM Cloud and Radiation Testbed site in terms of the intensity of convective activity, and the temperature and specie c-humidity evolution. Delayed occurrences of the initial precipitation events are a common feature for all three sub-cases among the models. Cloud mass e uxes, condensate mixing ratios and hydrometeor fractions produced by all CRMs are similar. Some of the simulated cloud properties such as cloud liquid-water path and hydrometeor fraction are rather similar to available observations. All CRMs produce large downdraught mass euxes with magnitudes similar to those of updraughts, in contrast to CRM results for tropical convection. Some inter-model differences in cloud properties are likely to be related to those in the parametrizations of microphysical processes. There is generally a good agreement between the CRMs and observations with CRMs being signie cantly better than single-column models (SCMs), suggesting that current results are suitable for use in improving parametrizations in SCMs. However, improvements can still be made in the CRM simulations; these include the proper initialization of the CRMs and a more proper method of diagnosing cloud boundaries in model outputs for comparison with satellite and radar cloud observations.

CRCP: a cloud resolving convection parameterization for modeling the tropical convecting atmosphere

Abstract A new computational approach, CRCP, is proposed in which both the large-scale (LS) tropical dynamics and cloud-scale (CS) dynamics are captured explicitly. The leading idea is to represent subgrid scales of the LS model by imbedding a 2D CS model in each column of the 3D LS model – the approach tailored for distributed memory architectures. The overall philosophy underlying CRCP is the reinvestment of efforts from large-eddy simulation to elaborate yet ‘embarrassingly parallel’ turbulence models. Similar as in the traditional ‘convection parameterization’, the LS model provides ‘ambient forcings’ for the CS model imbedded inside each LS column, and the CS model feeds back a ‘convective response’ for every column of the LS model. Furthermore, availability of the cloud-scale data allows for explicit coupling of moist convection with radiative and surface processes. Following our experience with cloud-resolving modeling of the tropical convection, the CS model is oriented along the E–W direction inside each LS model column. A simple strategy for the coupling the LS and CS models derives from physical understanding of interactions between LS flow and moist tropical convection. Theoretical considerations are illustrated with an example of application to observational data from the Phase III of the Global Atmospheric Research Programme Atlantic Tropical Experiment (GATE).

MJO-like Coherent Structures: Sensitivity Simulations Using the Cloud-Resolving Convection Parameterization (CRCP)

Abstract Interaction between equatorially trapped disturbances and tropical convection is investigated using a nonhydrostatic global model that applies the cloud-resolving convection parameterization (CRCP). The CRCP represents subgrid scales of the global model by embedding a 2D cloud-resolving model in each column of the global model. The modeling setup is a constant-SST aquaplanet, with the size and rotation of earth, in radiative–convective quasi equilibrium. The global atmosphere is assumed to be initially at rest. No large-scale organization of convection is present outside the equatorial waveguide. Inside the waveguide, on the other hand, the model simulates spontaneous formation of coherent structures with deep convection on the leading edge and strong surface westerly winds to the west, the westerly wind bursts. These coherent structures resemble the Madden–Julian oscillation (MJO) observed in the terrestrial Tropics and are present in simulations applying prescribed and interactive radiation, an...

Toward Cloud Resolving Modeling of Large-Scale Tropical Circulations: A Simple Cloud Microphysics Parameterization

Abstract This paper discusses cloud microphysical processes essential for the large-scale tropical circulations and the tropical climate, as well as the strategy to include them in large-scale models that resolve cloud dynamics. The emphasis is on the ice microphysics, which traditional cloud models consider in a fairly complex manner and where a simplified approach is desirable. An extension of the classical warm rain bulk parameterization is presented. The proposed scheme retains simplicity of the warm rain parameterization (e.g., only two classes of condensed water are considered) but introduces two important modifications for temperatures well below freezing:1) the saturation conditions are prescribed based on saturation with respect to ice, not water; and 2) growth characteristics and terminal velocities of precipitation particles are representative for ice particles, not raindrops. Numerical tests suggest that, despite its simplicity, the parameterization is able to capture essential aspects of the ...

Long-Term Behavior of Cloud Systems in TOGA COARE and Their Interactions with Radiative and Surface Processes. Part I: Two-Dimensional Modeling Study

Abstract Two-dimensional cloud-resolving modeling of tropical cloud systems was performed for a 39-day period (5 December 1992 through 12 January 1993) during the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE). This period contains strong convective episodes within a major westerly wind burst. A cloud-resolving model combined with a cloud-interactive radiation scheme was driven by the observed evolving large-scale forcing for temperature and moisture, evolving large-scale horizontal winds, and evolving sea surface temperature. These all represent averaged conditions over the Intensive Flux Array of TOGA COARE. Model-produced quantities were evaluated against the observational data and used to quantify interactions of the simulated cloud systems with radiative and surface processes. Focus is on quantities of value to cloud–climate research and to an improved physical basis for the parameterization of cloud systems per se. During convectively disturbed periods (...