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  • Author or Editor: Michael J. Weissbluth x
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Michael J. Weissbluth
and
William R. Cotton

Abstract

Currently, there is no adequate cumulus parameterization suitable for use in mesoscale models having horizontal resolutions between 5 and 50 kilometers. Based on the similarity of the temporal and spatial evolution of the vertical variances between a CCOPE supercell and a generic tropical squall line as explicitly simulated by the Regional Atmospheric Modeling System developed at Colorado State University, a convective parameterization scheme is developed that represents microscale turbulence with a modified second-order closure scheme and cumulus draft-scale eddies with a convective adjustment scheme. The microscale turbulence scheme is based upon the Mellor and Yamada 2.5-level closure modified to predict solely on ww and includes Zeman and Lumley's formulation for the buoyancy-driven mixed layer to close the pressure term and the eddy transport term. If deep convection is indicated, the microscale turbulence scheme includes contributions from cumulus draft-scale fluxes determined from a cloud model and uses different length scales to represent the planetary boundary-layer eddies and the in-cloud eddies.

The cumulus draft-scale tendencies of heat, moisture, and hydrometeors are specified by a mesoscale compensation term and a convective adjustment term. The convective adjustment term is the difference between a cloud model-derived properly and its environmental value, and is modulated by a time scale determined through a moist static energy balance. The mesoscale compensation term is a product of the vertical gradient of the appropriate scalar and a convective velocity equal to ( ww)½ . The cloud model is calibrated and generalized by comparisons with conditionally sampled data from the two explicitly simulated storms.

One unique feature of this approach is that the parameterization is not simply a local grid column scheme; ww is transported by the turbulence as well as the mean horizontal and vertical winds. Thus, the scheme responds to shear and is more global in nature than current cumulus parameterizations, and maintains a memory of previous convective activity. Furthermore, the scheme provides explicit cumulus source functions for all hydrometeor species. Results from a simple two-dimensional simulation of deep cumulus indicate the satisfactory performance of this scheme. Part II of this paper will compare explicit simulations of two- and three-dimensional Florida sea-breeze convection with parameterized simulations on various coarser grids.

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Michael J. Weissbluth
and
William R. Cotton

Abstract

Vertical divergence of the mountain wave's momentum flux has recently been hypothesized to be an important contribution to the global momentum budget. Wavebreaking theories and envelope orography have been employed to explain the divergence of the momentum flux. Here, cloud-top radiational cooling is shown to locally destabilize the environment and disrupt the propagation of the mountain wave in idealized two-dimensional simulations, thus drastically altering the expected momentum flux profile. Also, simulations of two-dimensional mountain waves indicate that nonlinearities can increase the wave response if the lower layer is decoupled from the flow aloft or decrease the wave response by providing multiple reflection levels for the incident mountain wave. The onset of wavebreaking and the level at which the wave breaks can be influenced by the ambient thermodynamic profile.

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Melville E. Nicholls
and
Michael J. Weissbluth

Abstract

Results of a simulation of a tropical squall line which allows three dimensionality on the scale of convective elements, shows many similarities with those of a two-dimensional simulation. Differences are 1) The quasi-three-dimensional model produces less front-to-rear acceleration of updraft air and rear-to-front acceleration of downdraft air; 2) The horizontally averaged vertical mass flux and momentum flux profiles show sharper low-level peaks in two-dimensions; 3) The ratio of the maximum to minimum vertical velocities is larger in the quasi-three-dimensional simulation; 4) There is more of a cellular structure in the vertical plane perpendicular to the line in two-dimensions; and 5) The ratio of ice to liquid water is greater in the quasi-three-dimensional simulation.

An unexpected result is that very little of the air feeding the rear low-level downdraft originates from ahead of the system, even in the quasi-three-dimensional simulation. Strong vertical mixing of the inflow air occurs so that the equivalent potential temperature of the mid-level air increases and it ascends rather than feeds the cold pool. The strong vertical mixing is associated with overturning cells, which become more prevalent as the updraft branch of the circulation tilts. Results indicate that at the upper regions of the main downdraft, the pressure force is playing a role in accelerating air downwards. The major mechanism responsible for the downdraft appears to be diabatic cooling.

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Hongli Jiang
,
William R. Cotton
,
James O. Pinto
,
Judy A. Curry
, and
Michael J. Weissbluth

Abstract

The authors’ previous idealized, two-dimensional cloud resolving model (CRM) simulations of Arctic stratus revealed a surprising sensitivity to the concentrations of ice crystals. In this paper, simulations of an actual case study observed during the Beaufort and Arctic Seas Experiment are performed and the results are compared to the observed data.

It is again found in the CRM simulations that the simulated stratus cloud is very sensitive to the concentration of ice crystals. Using midlatitude estimates of the availability of ice forming nuclei (IFN) in the model, the authors find that the concentrations of ice crystals are large enough to result in the almost complete dissipation of otherwise solid, optically thick stratus layers. A tenuous stratus can be maintained in the simulation when the continuous input of moisture through the imposed large-scale advection is strong enough to balance the ice production. However, in association with the large-scale moisture and warm advection, only by reducing the concentration of IFN to 0.3 of the midlatitude estimate values can a persistent, optically thick stratus layer be maintained. The results obtained from the reduced IFN simulation compare reasonably well with observations.

The longwave radiative fluxes at the surface are significantly different between the solid stratus and liquid-water-depleted higher ice crystal concentration experiments.

This work suggests that transition-season Arctic stratus can be very vulnerable to anthropogenic sources of IFN, which can alter cloud structure sufficiently to affect the rates of melting and freezing of the Arctic Ocean.

The authors find that the Hallett–Mossop riming splintering mechanism is not activated in the simulations because the cloud droplets are very small and cloud temperatures are outside the range supporting efficient rime splintering. Thus, the conclusions drawn from the results presented in this paper may be applicable to only a limited class of Arctic stratus.

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