The NCAR Clouds in Climate Program presently has three scientific priorities: 1) tropical oceanic convective cloud systems, 2) oceanic convection in cold air outbreaks behind midlatitude cyclones ("cold air outbreaks"), and 3) the application of these results to parameterizations for climate models. Also, these aspects are supported by contributing basic studies in convection and its large-scale interaction. These studies are either primary research within the Division's Cloud Systems Group or collaborative with the NCAR Climate and Global Dynamics Division (CGD). They helped launch the Global Energy and Water-cycle Experiment (GEWEX) Cloud System Study (GCSS) Working Group4: Precipitating Convective Cloud Systems.

These studies make extensive use of cloud-resolving models (CRMs), together with theoretical studies and observational analysis. The numerical codes used were either based on work done by Terry Clark and colleagues or Piotr Smolarkiewicz and colleagues. The latter was specifically designed for massively parallel computers.

This research is supported by a combination of base funding, part of the ROCEW named program support to NCAR, and an inter-agency TOGA COARE grant.

Wojciech Grabowski, Xiaoqing Wu (joint appointment with CGD), and Mitchell Moncrieff continued to study interactions of cloud dynamics and cloud microphysics with radiation and surface processes using GATE (GARP [Global Atmospheric Research Programme] Atlantic Tropical Experiment) sounding data. Experiments using observed conditions simulated the evolution of cloud systems during the period of 1-7 September 1974 in Phase III of GATE. In the last year, the work concentrated on two main topics: 1) further comparisons between two-dimensional and three-dimensional simulations (see figure), and 2) effects of cloud microphysics on bulk properties of simulated convective systems. Comparison between two-dimensional and three-dimensional results shows that the two-dimensional framework offers accurate results as far as bulk properties of convection are concerned (e.g., mean thermodynamic fields, surface precipitation, effects of clouds on radiative fluxes, etc.). However, the two-dimensional simulations produced a much higher temporal variability of quantities directly related to convection (e.g., CaPE, surface precipitation, convective mass flux, etc.). The results support the notion that, as long as high frequency temporal variability is not of primary importance, low resolution two-dimensional simulations can be used as realizations of tropical cloud systems in the climate problem and for improving and/or testing cloud parameterizations for large-scale models. As far as effects of cloud microphysics on simulated GATE systems are concerned, it was shown that effects of microphysics alone are rather weak, but when combined with the influence on radiative fluxes, microphysics can modify the convective response substantially.

Wu, Grabowski, and Moncrieff continue their simulations of the long-term behavior of TOGA COARE cloud systems in both two and three spatial dimensions. Using the observations provided by Gregory McFarquhar and Andrew Heymsfield, the microphysics in the CRM were improved for the better representation of cloud-radiation and atmosphere-ocean interaction. The CRM-simulated cloud radiative properties are in better agreement with observations. In collaboration with William Collins and Jeffrey Kiehl (CGD), a set of offline calculations of cloud radiative properties were performed to evaluate the radiation scheme. The results illustrate the important roles of both cloud condensate content and size of condensate particles in the representation of the interaction between clouds and radiative fluxes. Aided by William Hall, a unique seven-day three-dimensional (840 km by 840 km by 40 km) simulation of TOGA COARE cloud systems was performed. Analysis is underway to understand the behavior of cloud systems during convectively active phases of the intraseasonal oscillation.

Wu, Grabowski, and Moncrieff, collaborating with Ralph Milliff and William Large (CGD), coupled a CRM with an ocean model. Specifically, the 39-day dynamically consistent surface forcing from a two-dimensional TOGA COARE simulation was used to force a one-dimensional (1D) ocean model. The results show that the 1D ocean model with a nonlocal K profile parameterization can simulate the major feature of the warm pool response to the atmospheric forcing, although the difference between model-produced and observed SST (sea surface temperature) still exists. This shows that the vertical oceanic mixing is one of the key physical processes that affects the SST evolution. Observational studies by several ocean groups showed that there is significant oceanic transport during the late period of 39-day simulation. The work is underway to include this process into the ocean model and to fully couple the CRM and the ocean model.

Both cloud feedback and water vapor feedback are climatically important. Wu and Moncrieff quantified the relationship between SST variations and tropical convection. Observations show conflicting relationships between SST and deep convection in the tropics. They found that an increase or decrease of SST does not significantly affect the convective intensity and cloud amount under a given large-scale forcing. However, the effects on the water vapor distribution are strong. The warmer (colder) SST results in a warm and moist (cold and dry) equilibrium climate, larger (smaller) precipitation and higher (lower) upper-tropospheric relative humidity. It is only when SST variations are positively correlated with the large-scale forcing (ascent) that a positive correlation between SST variations and convection exists. Large-scale dynamics is therefore paramount to both convective intensity and cloud amount.

Using CRM-produced cloud-scale properties and field observations, Wu and Margaret LeMone investigated the structure of cloud clusters within convectively active phases of intraseasonal oscillation. During the westerly wind burst periods in December 1992 and February 1993, both model-produced cloud-scale properties and observations showed that within the cloud clusters dynamically active precipitating clouds propagated eastward, while the upper-tropospheric clouds propagate westward. These findings suggest that the satellite-measured movement of cloud clusters is not necessarily directly tied to the movement of dynamically active precipitating clouds in the lower troposphere.

The validation of the cloud distributions in the CRMs is an essential step toward using these simulations to develop cloud parameterizations for the NCAR Community Climate Model (CCM). Collins (CGD) compared the radiative fluxes simulated by the cloud-resolving modeling of cloud systems over the western Pacific warm pool (reported above) with observations from TOGA COARE and CEPEX. A special top-of-atmosphere radiation dataset that contains the explicit effects of clouds was used. The cloud population and its temporal evolution, with corresponding properties derived from satellite observations of the COARE region, was also compared. The interaction of enhanced short-wave absorption in clouds with cloud dynamics was also tested. Previous model studies of this interaction were inconclusive because of the overly simplified dynamics in the model physics. Modified cloud optical properties that reproduce recent observations of short-wave absorption were tested in the CRM to examine the effects on cloud formation and lifetime.

Timothy Schneider and Graeme Stephens (Colorado State University), in collaboration with Wu, Moncrieff, and Grabowski, used CRM-produced cloud properties to simulate spaceborne cloud radar reflectivity. Critical issues for the observation of tropical cloud systems with cloud radars, from space or otherwise, include attenuation in precipitating regions and the ability to detect thin ice clouds. Estimates of attenuation based on the CRM-produced data from 22 December during TOGA COARE were found to be comparable to observations made with ground-based radars in precipitating tropical cloud systems. Radar reflectivity-height probability density functions of pristine cloud ice derived from the CRM data were favorably compared to observations as well. These analyses were used as part of a proposal for a spaceborne cloud-observing system.

Ralph Milliff (CGD), Christopher Wikle and Mark Berliner (Geophysical Statistics Project), in collaboration with Moncrieff, Wu, and Grabowski, compared near-surface wind component spectra between the three-dimensional CRM results for TOGA COARE and coincident ERS-1 scatterometer data. This collaboration is evolving in mutually beneficial ways to enhance the development of the CRM, as well as validate and place in context new surface wind datasets over the global ocean at mesoscale resolution. One aspect being more fully investigated is the role of the mesoscale organization of deep convection.

Liu and Moncrieff conducted idealized two-dimensional cloud-resolving numerical modeling to investigate the diurnal variability of deep tropical oceanic convection affected by evolving large-scale forcing. The underlying surface was an open ocean with a constant sea surface temperature. Emphasis was on two distinct regimes: 1) highly organized squall-line-like convection in strong ambient shear, and 2) less organized non-squall cloud clusters without ambient shear. A pronounced diurnal cycle was simulated for the highly organized case; convective activity and intensity attained a maximum around the pre-dawn time and a minimum in the late afternoon. The modeled diurnal variation was primarily attributed to the direct interaction between radiation and convection, whereas the cloud–cloud-free differential heating mechanism played a secondary role. When the radiative effect of clouds was excluded, a diurnal cycle was still present. Moreover, the cloud radiative forcing had a negative influence on precipitation/convective activity, in contrast with general circulation modeling results. A similar diurnal variability was obtained for the less organized case and was characterized by more precipitation during the night and early morning and less precipitation in the afternoon and evening.

Observational studies demonstrated that the large-scale forcing (i.e., cooling and moistening due to large-scale horizontal and vertical advections) vary greatly from place to place and from time to time in both vertical distribution and magnitude. To better understand how tropical convection responds to various kinds of large-scale forcing, Liu, Moncrieff, and Grabowski conducted a series of idealized cloud-resolving numerical simulations with different external cooling and moistening. Some preliminary simulations revealed that not only the forcing intensity but also the vertical structure exerts an important influence on the convective development. Further simulations and detailed analyses are underway to systematically examine how large-scale forcing regulates tropical convection, such as the cloud cover, cloud mass flux, cloud optical properties, precipitation, and heat and moisture budgets.

William Anderson (now with SCD) and Smolarkiewicz continued their work on high performance computing strategies for atmospheric models. They finished the message-passing variant of the massively parallel semi-Lagrangian/Eulerian nonhydrostatic model, completing the model. It includes moist and subgrid-scale modules, which makes it a useful tool for atmospheric applications. It runs efficiently on Cray T3D, which is an additional powerful computational resource. Vanda Grubišic (visitor, Yale University) and Smolarkiewicz comprehensively tested the code's overall performance in computationally intensive LES experiments for numerous PBL scenarios (dry, moist, convective, shear, cloud topped, past complex terrain, etc.). This model is now used extensively in the studies on the role of clouds in climate conducted in the division's Cloud System Group/CCP, for example, cold-air outbreak convection and some of the tropical convective system initiatives described herein.

The focus of this new CCP initiative is the organization of convection in mid- and high-latitude cold air outbreaks and their role in convective momentum transport. Grubišic, Moncrieff, and Smolarkiewicz instituted a study on this shallow form of convection which is important from a climatological point of view but whose momentum transport properties are relatively poorly understood. The multiplicity of processes and scales dictates the use of uniform high resolution in relatively large domains, requiring massively parallel (MPP) computers. High-resolution three-dimensional simulations with the MPP version of the Eulerian/semi-Lagrangian anelastic cloud-resolving model were performed of an idealized late-stage, mid-latitude, cold-air outbreak case. Results of these experiments revealed a strong sensitivity of the organization of convective updrafts with respect to the environmental shear. A study is underway to establish simple dynamical explanations for such organization. More complex three-dimensional simulations including all the key physical processes (surface fluxes, radiation, liquid and ice microphysics, subgrid-scale turbulence) were performed using a case observed during ARKTIS93 experiment, a case study of the European Cloud Resolving Model (EUCREM) initiative. Sensitivity studies are being performed to infer a role for different physical processes in fostering a transition between observed regimes of convective organization (rolls, open, and closed cells).

The development of parameterization methods is integral to the objectives of CCP. Until now the main emphasis has been on cloud system. With the finding that convective processes have an important effect on the CSM (reported elsewhere in this report), CCP has assigned the highest priority to evaluating and improving convective parameterization in the CCM3, and work has started along these lines.

Grubišic contributed to the GCSS Working Group on Boundary Layer Clouds intercomparison study of non-precipitating shallow cumuli based on observations from BOMEX. The focus of this study is on using detailed information on dynamics of shallow convective clouds, provided by the high resolution three-dimensional numerical simulations, to improve parameterizations of turbulent fluxes, and entrainment and detrainment rates associated with shallow convection in large-scale models. The massively parallel version of the Eulerian/semi-Lagrangian non-hydrostatic model was used to perform two- and three-dimensional large-eddy simulations (LES). This was an important test for the performance of this nonoscillatory forward-in-time fluid code against other LES models, which has shown it to be highly competitive in terms of both accuracy and the computational expense. Results of the sensitivity studies indicate the shear in the lowest few hundred meters of the boundary layer is very important for the organization of clouds into more linear features, and confirmed the importance of a proper two-dimensional domain orientation in simulating such three-dimensional convective flows. Work is underway to further quantify these conclusions.

Chin Hoh Moeng and Bjorn Stevens (ASP) continue to be involved with the GEWEX Cloud System Study (GCSS) Boundary-Layer Cloud Working Group effort in examining the accuracy of the large-eddy simulations technique for PBL clouds. The fourth intercomparison study, which took place in 1997, focused on simulation of the BOMEX experiment. The results of the intercomparison indicated that details of the cloud simulations (such as cloud patterns and organization) varied considerably among LES but the average energetics and turbulence statistics were rather insensitive to SGS physics and numerics. All LES tended to produce a small cloud fraction (less than 20 percent), in accord with observations. Current work within the GCSS framework is directed toward understanding what processes control the cloud fraction downstream of stratocumulus, a poorly represented regime in climate models. Stevens is now organizing the fifth GCSS Boundary-Layer Cloud intercomparison study based on an ATEX observational case in which the cumulus regime is associated with a larger cloud fraction than in the BOMEX case.

CRM simulations provide a means of testing parameterizations using a complete, high-resolution description of the atmosphere not available in observational data; for example, convective mass fluxes as shown in the figure. A series of studies is being conducted not only within NCAR but also in collaboration with the broader community. The most comprehensive community effort has been through Moncrieff's chairmanship of the Global Energy and Water-cycle Experiment (GEWEX) Cloud System Study (GCSS) Working Group (now handed over to Steven Krueger, University of Utah). To facilitate the development of physically based parameterizations, cloud-resolving model results are being intercompared and also compared with single-column models. Two model intercomparison workshops were held, with both hosted by CCP. The intercomparison results can be accessed here.

Xin-Zhong Liang and Wei-Chyung Wang (State University of New York at Albany), in collaboration with Wu and Moncrieff, used the CRM simulations to develop probability distribution functions (PDFs) that represent the statistical cloud geometric characteristics. These PDFs are required for the "mosaic" treatment of cloud-radiation interactions that incorporate subgrid variability in GCMs. Since observations are not sufficient to objectively describe the cloud geometric association, CRM simulations provide important supplementary data to the ARM measurements. Progress is being made in deriving various cloud PDFs using both ARM measurements and CRM simulations.

Within NCAR collaborative studies involving CGD and MMM have evaluated the performance of the convective parameterization scheme used in the Community Climate Model (CCM3), the atmospheric component of the Climate System Model (CSM). This is prior to using the CRM results to improve the performance of the parameterization scheme. Another more statistically based study, joint between CCP and the Geophysical Statistics Project (GSP) at NCAR, uses the CRM realizations of tropical convection.

Stevens (ASP) and Moeng analyzed the representation of the subtropical planetary boundary layer (PBL) in the Climate System Model (CSM). They found that the CSM represents well some structural aspects of the subtropical marine PBL regimes, such as the frequency and approximate height of the trade inversion. However, their analysis revealed several shortcomings in the current PBL cloud formulation:

Moeng, Peter Sullivan, and Stevens developed a generalized entrainment-rate formula that includes the buoyancy forcing due to cloud-top longwave radiative cooling. This formula is derived by assuming that the entrainment buoyancy flux is a fraction of the vertically-averaged buoyancy flux, which has been shown to be a rational entrainment closure for the clear, convective PBL. This leads to an entrainment rate that depends linearly on both the inverse of the bulk Richardson number and the radiative-flux jump above the mean cloud-top height. This radiative-flux jump is particularly significant in smoke-cloud cases since smoke extends into the capping inversion, and the amount of smoke concentration may be significant above the mean smoke-cloud-top height. This formula was verified through several LES of radiatively cooled smoke-cloud-topped PBLs with different optical thickness, inversion strength, or downward flux from aloft. The smoke-cloud LES show that the contribution to entrainment rate from the radiative-flux-jump term is larger than that from the bulk-Richardson-number term. In other words, entrainment rate is not a simple function of bulk Richardson number, contrary to the suggestion of many studies. The radiative-flux jump above the mean cloud-top height is shown to depend on the variance in cloud-top heights and the optical thickness of the cloud.

Jun-Ichi Yano (visitor, Monash University, Australia) used the discrete Meyer-type wavelet to analyze the three-dimensional simulation of the GATE period by the Cloud System Group CRM in collaboration with Wu and Moncrieff. The wavelet spectrum is found to be useful to objectively classify the various dynamical regimes in the mesoscale convective systems. The wavelet decomposition is further used to objectively decompose the simulated field into the various dynamical components, e.g., the cumulus convective elements and the mesoscale component by using an objective threshold in the wavelet space.

N. Andrew Crook (joint appointment with RAP) has continued his study of convection initiation over the Tiwi Islands (located just north of Darwin, Australia) using the Clark nonhydrostatic numerical model. A number of case studies were examined in both large-scale easterly and westerly regimes. In the simulations, the strongest convection generally develops when sea breezes from the north and south coastlines converge. However, observations from the Maritime Continent Thunderstorm Experiment (MCTEX) indicated that strong convection often developed along a single sea breeze front without interaction from the "other" sea breeze. A number of sensitivity studies were conducted to determine the causes of these different modes of convective development. Currently, the sensitivity of the simulations to large-scale lifting/subsidence has been examined.

Liu and Moncrieff conducted a comprehensive investigation of the effects of stratifications on density currents with a two-dimensional nonhydrostatic model. They focused on three particular situations: 1) a stable-stratified layer underlying a deep neutrally stratified atmosphere; 2) a neutrally stratified layer underlying a deep stable-stratified atmosphere; and 3) a continuously stratified atmosphere. In the case of a stable low-level atmosphere, the simulated system is largely similar to the laboratory-produced density current when the stratification is weak or moderate; the dynamically induced lifting decreases whereas the propagation speed increases with the enhancement of stratification. Multiple-headed density currents or fast-propagating solitary-wave-like disturbances occur in the strongly stratified situation. In a continuously stratified atmosphere or in a neutrally stratified boundary layer underneath a stratified layer, the height of the density current head (i.e., dynamical lifting) and the propagation speed are deduced. The height and translation speed are relatively insensitive to stratification if it is strong.

It has been known for some time that convective storms often form along boundary layer convergence lines. In order to improve our understanding of this mechanism, Crook and Joseph Klemp developed analytical models to predict the depth that boundary layer air is lifted at a convergence line. These models include the effects of varying stability and flow above the convergence line. The predictions of the analytical model were compared with time-dependent nonlinear simulations and in general the agreement was reasonable.

A two-dimensional cloud-resolving model was used by Grabowski to study Walker-like tropical circulations. The 4000-km horizontal domain is used, in which a wavenumber-one SST anomaly is placed. The convective activity with various time-scales was found to be generated in association with the Walker-type circulation. Particularly remarkable is a quasi-two-day oscillation of the Walker circulation itself. A simple box model was constructed by Yano to quantify this dynamic aspect. The linear analysis predicted the periodicity of this oscillation well.

Andrew Majda (Courant Institute for Mathematical Sciences, New York University) and Moncrieff examined the dynamics of steady convective states previously theorized by Moncrieff. By using special shear profiles and finite amplitude excitation they formulated a numerical approach to determine which states are actually realizable. Preliminary results are stressing the importance of jet-like shear profiles, a result that is in keeping with previous numerical modeling results.

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