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Xiaoqing Wu

Abstract

Diagnostic and semiprognostic analyses are performed using OK PRE-STORM (Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central) data to examine the cumulus-environment interaction in midlatitude convective systems. The similarities and differences of the interaction processes between midlatitude and tropical convective systems are also discussed: Analyses of PRE-STORM and GATE (GARP Atlantic Tropical Experiment) data show generally larger vertical wind shear, large-scale forcing, and moist convective instability in midlatitude MCCs (mesoscale convective complexes) and squall lines than in tropical cloud clusters. It is found that the interaction mechanism based on the cumulus-induced subsidence and detrainment is capable of explaining most of the observed heating and drying under widely different environment conditions. Convective- wale downdrafts act to cool and moisten the lower troposphere in the midlatitudes as in the tropics. The quasi- equilibrium assumption between stabilization by convection and destabilization by large-scale forcing is valid and holds better in the midlatitudes since the large-scale forcing is much stronger. Both the cumulus and stratiform cloud effects are stronger in midlatitude than in tropical convective systems.

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Xiaoqing Wu

Abstract

Heat and moisture budgets of two mesoscale convective systems of a characteristic horizontal scale of approximately 450 km are studied using the Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment mesoscale data with a cutoff wavelength of 150 km. The scale dependence of budget residuals is examined by further varying horizontal resolutions of data with low-pass spatial filters. The results show that the horizontal distributions of the vertically integrated heat source 〈Q 1〉 and moisture sink 〈Q 2〉 are dependent on horizontal resolutions of data. For the mesoscale data, the horizontal distributions of 〈Q 1〉 and 〈Q 2〉 show pronounced “dipole” patterns that are closely related to the horizontal fluxes of heat and moisture due to mesoscale circulations. For the larger-scale data, the dipole pattern disappears and the horizontal distributions of 〈Q 1〉 and 〈Q 2〉 show positive values in the area of convection. However, the vertical profiles of the observed heat source and moisture sink averaged over a large domain (600 km × 600 km) are not significantly dependent on horizontal resolutions of data.

The observed budget residuals are compared with the cumulus-induced heating and drying obtained semi-prognostically using the Arakawa-Schubert parameterization with the mesoscale and larger-scale data. The major features of the budget residuals are reproduced by the cumulus-induced heating and drying. When the larger-scale data are used, additional contributions from the condensation and evaporation due to mesoscale stratiform clouds and precipitation are needed to explain the budget residuals. When the mesoscale data are used, the representation of hydrometeors generated and transported from the region of cumulus convection to the region of stratiform clouds is required to accurately reproduce the budget residuals. The Arakawa-Schubert quasi-equilibrium assumption becomes more accurate for the data-resolving mesoscale circulations.

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Xiaoqing Wu

Abstract

The effects of ice microphysics on the mean state of tropical atmosphere and ocean are quantified using a coupled cloud–ocean model. The cloud-resolving model (CRM) treats explicitly the cloud-scale dynamics instead of using parameterization as is necessary in a general circulation model (GCM). The ocean model is a one-dimensional (1D) mixed layer model with a nonlocal K-profile parameterization to represent the vertical mixing in the oceanic surface boundary layer. Two sets of 40-day simulations attain radiative–convective–oceanic quasi-equilibrium states, one is a coupled simulation, the other has a fixed sea surface temperature (SST). Each set consists of two simulations, with a larger and smaller ice fall speed, respectively.

The two coupled simulations (T0C and M2C) yield dramatically different radiative–convective–oceanic quasi-equilibrium states demonstrating the profound impact of ice microphysics on the water vapor, cloud, and radiation fields. The mean SST and mixed layer depth in M2C is 0.67 K colder and 33 m deeper than those in T0C, and the net surface solar radiation in M2C is 88 W m−2 smaller than that in T0C. The simulation associated with the larger ice fall speed achieves a quasi-equilibrium state characterized by a colder and drier atmosphere, less cloudiness, stronger convection and precipitation, and warmer SST. On the other hand, a quasi-equilibrium state associated with the smaller ice fall speed has a warmer and moister atmosphere, more cloudiness, weaker convection and precipitation, and colder SST. The key mechanism is cloud–radiation interaction: the more (less) cloudiness the ice microphysics produces, the weaker (stronger) the radiative cooling.

The upper-ocean mixing and entrainment of the oceanic deep water play an important role in establishing the quasi-equilibrium SST. The comparison between two coupled and two fixed-SST simulations illustrates that the water vapor variation induced by the change of ice microphysics is reduced by the SST feedback, while the cloud and radiation variation is enhanced in the coupled simulation.

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Xiaoqing Wu and Michio Yanai

Abstract

Dynamical effects of organized cumulus convection on its environment with vertical wind shear are studied. Analyses of the wind field and momentum budget residual for mesoscale convective systems observed during SESAME and PRE-STORM reveal systematic differences in the vertical transport of horizontal momentum between mesoscale convective complex (MCC) and squall line cases. In the MCC, a distinct minimum in wind speed appears over the area of intense convection and the momentum budget residual acts to decelerate the environmental wind and to reduce the upper-level vertical shear. Therefore, the inferred vertical transport of momentum in the MCC is downgradient in the upper layer. On the other hand, in the squall line, there is no wind speed minimum and the upper-level vertical shear of the line-normall component of the environmental wind increases as convection develops. Thus, the vertical transport of momentum normal to the squall line is upgradient in the upper layer, although the transport of momentum parallel to the line is downgradient.

The effects of cumulus convection on the environmental flow are through the subsidence of environmental air that compensates the cloud mass flux, the detrainment of momentum from clouds, and the convective-scale horizontal pressure gradient force acting on the environment. A cumulus momentum parameterization including the convective-scale pressure gradient force is formulated. The pressure gradient force is related to the vertical wind shear, cloud mass flux, and orientation of organized convection. The parameterization is capable of reproducing both the upgradient and downgradient transports of horizontal momentum, provided the mode of organization of cumulus convection is known.

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Liping Deng and Xiaoqing Wu

Abstract

Weak temporal variability in tropical climates such as the Madden–Julian oscillation (MJO) is one of major deficiencies in general circulation models (GCMs). The uncertainties in the representation of convection and cloud processes are responsible for these deficiencies. With the improvement made to the convection scheme, the Iowa State University (ISU) GCM, which is based on a version of the NCAR Community Climate Model, is able to simulate many features of MJO as revealed by observations. In this study, four 10-yr (1979–88) ISU GCM simulations with observed sea surface temperatures are analyzed and compared to examine the effects of the revised convection closure, convection trigger condition, and convective momentum transport (CMT) on the MJO simulations. The modifications made in the convection scheme improve the simulations of amplitude, spatial distribution, eastward propagation, and horizontal and vertical structures, especially for the coherent feature of eastward-propagating convection and the precursor sign of convective center. The revised convection closure plays a key role in the improvement of the eastward propagation of MJO. The convection trigger helps produce less frequent but more vigorous moist convection and enhance the amplitude of the MJO signal. The inclusion of CMT results in a more coherent structure for the MJO deep convective center and its corresponding atmospheric variances.

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Liping Deng and Xiaoqing Wu

Abstract

The kinetic energy budget is conducted to analyze the physical processes responsible for the improved Madden–Julian oscillation (MJO) simulated by the Iowa State University general circulation models (ISUGCMs). The modified deep convection scheme that includes the revised convection closure, convection trigger condition, and convective momentum transport (CMT) enhances the equatorial (10°S–10°N) MJO-related perturbation kinetic energy (PKE) in the upper troposphere and leads to a more robust and coherent eastward-propagating MJO signal. In the MJO source region, the Indian Ocean (45°–120°E), the upper-tropospheric MJO PKE is maintained by the vertical convergence of wave energy flux and the barotropic conversion through the horizontal shear of mean flow. In the convectively active region, the western Pacific (120°E–180°), the upper-tropospheric MJO PKE is supported by the convergence of horizontal and vertical wave energy fluxes. Over the central-eastern Pacific (180°–120°W), where convection is suppressed, the upper-tropospheric MJO PKE is mainly due to the horizontal convergence of wave energy flux. The deep convection trigger condition produces stronger convective heating that enhances the perturbation available potential energy (PAPE) production and the upward wave energy fluxes and leads to the increased MJO PKE over the Indian Ocean and western Pacific. The trigger condition also enhances the MJO PKE over the central-eastern Pacific through the increased convergence of meridional wave energy flux from the subtropical latitudes of both hemispheres. The revised convection closure affects the response of mean zonal wind shear to the convective heating over the Indian Ocean and leads to the enhanced upper-tropospheric MJO PKE through the barotropic conversion. The stronger eastward wave energy flux due to the increase of convective heating over the Indian Ocean and western Pacific by the revised closure is favorable to the eastward propagation of MJO and the convergence of horizontal wave energy flux over the central-eastern Pacific. The convection-induced momentum tendency tends to decelerate the upper-tropospheric wind, which results in a negative work to the PKE budget in the upper troposphere. However, the convection momentum tendency accelerates the westerly wind below 800 hPa over the western Pacific, which is partially responsible for the improved MJO simulation.

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Xiaoqing Wu and Stephen Guimond

Abstract

Two-dimensional (2D) and three-dimensional (3D) cloud-resolving model (CRM) simulations are conducted to quantify the enhancement of surface sensible and latent heat fluxes by tropical precipitating cloud systems for 20 days (10–30 December 1992) during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The mesoscale enhancement appears to be analogous across both 2D and 3D CRMs, with the enhancement for the sensible heat flux accounting for 17% of the total flux for each model and the enhancement for the latent heat flux representing 18% and 16% of the total flux for 2D and 3D CRMs, respectively. The convection-induced gustiness is mainly responsible for the enhancement observed in each model simulation. The parameterization schemes of the mesoscale enhancement by the gustiness in terms of convective updraft, downdraft, and precipitation, respectively, are examined using each version of the CRM. The scheme utilizing the precipitation was found to yield the most desirable estimations of the mean fluxes with the smallest rms error. The results together with previous findings from other studies suggest that the mesoscale enhancement of surface heat fluxes by the precipitating deep convection is a subgrid process apparent across various CRMs and is imperative to incorporate into general circulation models (GCMs) for improved climate simulation.

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Sunwook Park and Xiaoqing Wu

Abstract

The relationship among the surface albedo, cloud properties, and radiative fluxes is investigated for the first time using a year-long cloud-resolving model (CRM) simulation with the prescribed evolving surface albedo. In comparison with the run using a fixed surface albedo, the CRM with the observed surface albedo represents the shortwave radiative budget closer to the observations in the winter. The greater surface albedo induces weaker instability in the low troposphere so that the amount of low clouds decreases during the winter. This reduces the shortwave and longwave cloud radiative forcing at the surface. The analysis of the CRM simulations with the evolving surface albedo reveals that there is a critical value (0.35) of the surface albedo. For albedos greater than the critical value, the upward shortwave flux at the top of the atmosphere (TOA) is positively proportional to the surface albedos when optically thin clouds exist, and is not much affected by reflection on the cloud top. If optically thick clouds occur and the surface albedo is greater than the critical value, the upward shortwave flux at the TOA is significantly influenced by the reflection of cloud top, but not much affected by the surface albedo. In addition, for albedos larger than the critical value, the downward shortwave flux at the surface is primarily influenced by the surface albedo and the reflection from the cloud base if optically thick clouds occur. However, the downward shortwave flux at the surface is not significantly affected by the surface albedo when optically thin clouds exist because the reflection on the cloud base is weak. When surface albedos are less than the critical value, those relationships among surface albedo, shortwave flux, and cloud properties are not obvious. The surface albedo effect on shortwave flux increases as solar zenith angle (SZA) decreases, but its dependence on the SZA is negligible when optically thick clouds exist.

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Xiaoqing Wu and Liping Deng

Abstract

The moist static energy (MSE) anomalies and MSE budget associated with the Madden–Julian oscillation (MJO) simulated in the Iowa State University General Circulation Model (ISUGCM) over the Indian and Pacific Oceans are compared with observations. Different phase relationships between MJO 850-hPa zonal wind, precipitation, and surface latent heat flux are simulated over the Indian Ocean and western Pacific, which are greatly influenced by the convection closure, trigger conditions, and convective momentum transport (CMT). The moist static energy builds up from the lower troposphere 15–20 days before the peak of MJO precipitation, and reaches the maximum in the middle troposphere (500–600 hPa) near the peak of MJO precipitation. The gradual lower-tropospheric heating and moistening and the upward transport of moist static energy are important aspects of MJO events, which are documented in observational studies but poorly simulated in most GCMs. The trigger conditions for deep convection, obtained from the year-long cloud-resolving model (CRM) simulations, contribute to the striking difference between ISUGCM simulations with the original and modified convection schemes and play the major role in the improved MJO simulation in ISUGCM. Additionally, the budget analysis with the ISUGCM simulations shows the increase in MJO MSE is in phase with the horizontal advection of MSE over the western Pacific, while out of phase with the horizontal advection of MSE over the Indian Ocean. However, the NCEP analysis shows that the tendency of MJO MSE is in phase with the horizontal advection of MSE over both oceans.

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Xiaoqing Wu and Mitchell W. Moncrieff

Abstract

The collective effects of organized convection the environment were estimated using a two-dimensional, two-way nested cloud-resolving numerical model with a large outer domain (4500 km). As initial conditions, the authors used an idealized environment of the onset stage of the December 1992 westerly wind burst that occurred during the Tropical Oceans Global Atmosphere Coupled Ocean-Atmosphere Response Experiment.

Two key aspects relating to convective parameterization were examined. First, the transports, sources, and sinks of heat, moisture, and momentum were derived using the model-produced dataset. In particular, the total momentum flux compares well with Moncrieff's dynamical theory. Second, the bulk energetics of the cloud system were examined using the model-produced dataset. The authors found that the shear generation of kinetic energy is comparable to the buoyancy generation and dominates the sum of the buoyancy and water-loading generation. This means that, in addition to the thermodynamic generation of kinetic energy, shear generation should be included in the closure condition for the parameterization of organized convection in large-scale models.

A simple mass-flux-based parameterization scheme is outlined for organized convection that consistently treats dynamical and thermodynamical fluxes.

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