Abstract

This paper presents the development and application of a binned approach to cloud-droplet riming within a bulk microphysics model. This approach provides a more realistic representation of collision–coalescence that occurs between ice and cloud particles of various sizes. The binned approach allows the application of specific collection efficiencies, within the stochastic collection equation, for individual size bins of droplets and ice particles; this is in sharp contrast to the bulk approach that uses a single collection efficiency to describe the growth of a distribution of an ice species by collecting cloud droplets. Simulations of a winter orographic cloud event reveal a reduction in riming when using the binned riming approach and, subsequently, larger amounts of supercooled liquid water within the orographic cloud.

Abstract

Air pollution generated in industrial and urban areas can act to suppress precipitation by creating a narrow cloud droplet spectrum, which inhibits the collision and coalescence process. In fact, precipitation ratios of elevated sites to upwind coastal urban areas have decreased during the twentieth century for locations in California and Israel while pollution emissions have increased. Precipitation suppression by pollution should also be evident in other areas of the world where shallow, orographic clouds produce precipitation. This study investigates the precipitation trends for sites along the Front Range of the Rocky Mountains to determine the effect of air pollution on precipitation in this region. The examination of precipitation trends reveals that the ratio of upslope precipitation for elevated sites west of Denver and Colorado Springs, Colorado, to upwind urban sites has decreased by approximately 30% over the past half-century. Similar precipitation trends were not found for more pristine sites in the region, providing evidence of precipitation suppression by air pollution.

Abstract

The microphysics module of the version of the Regional Atmospheric Modeling System (RAMS) maintained at Colorado State University has undergone a series of improvements, including the addition of a large-cloud-droplet mode from 40 to 80 μm in diameter and the prognostic number concentration of cloud droplets through activation of cloud condensation nuclei (CCN) and giant CCN (GCCN). The large-droplet mode was included to represent the dual modes of cloud droplets that often appear in nature. The activation of CCN is parameterized through the use of a Lagrangian parcel model that considers ambient cloud conditions for the nucleation of cloud droplets from aerosol. These new additions were tested in simulations of a supercell thunderstorm initiated from a warm, moist bubble. Model response was explored in regard to the microphysics sensitivity to the large-droplet mode, number concentrations of CCN and GCCN, size distributions of these nuclei, and the presence of nuclei sources and sinks.

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 w′ w′ 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 ( w′ w′)^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; w′ w′ 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.

Abstract

A two-way interactive, nested-grid simulation of a rotating supercell thunderstorm was performed. After 90 min the genesis of a descending incipient tornado vortex initially located aloft was simulated. The associated pressure-deficit tube subsequently built downward into the subcloud layer, where it continually fed upon a low-level source of vertical vorticity possibly introduced by the low-level downdraft. The pressure-deficit tube then drew in the low-level vorticity-rich air, allowing it to descend to the surface. A strong vortex thus formed in the subcloud field.

Abstract

The Regional Atmospheric Modeling System (RAMS), developed at Colorado State University, was used to predict boundary-layer clouds and diagnose fractional cloudiness. The primary case study for this project occurred on 7 July 1987 off the coast of southern California. On this day, a transition in the type of boundary-layer cloud was observed from a clear area, to an area of small scattered cumulus, to an area of broken stratocumulus, and finally, to an area of solid stratocumulus. This case study occurred during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment field study. RAMS was configured as a nested-grid mesoscale model with a fine grid having 5-km horizontal grid spacing covering the transition area.

Various fractional cloudiness schemes found in the literature were implemented into RAMS and tested against each other to determine which best represented observed conditions. The complexities of the parameterizations used to diagnose the fractional cloudiness varied from simple functions of relative humidity to a function of the model's subgrid variability. It was found that some of the simpler schemes identified the cloud transition better, while others performed poorly.

Abstract

The characteristics of a severe squall line are examined using data acquired from the 1981 Cooperative Convective Precipitation Experiment (CCOPE). The case is unusual in that the squall line was decoupled from an immediate, surface-based inflow source due to a mesoβ-scale (20–200 km) outflow pool produced by a separate mesoscale convective system. Both systems participated in the development of a mesoscale convective complex which subsequently produced sustained, severe surface winds throughout its entire life cycle. Despite the absolutely stable, presquall atmospheric boundary layer, the squall line produced radar reflectivity values of 70 dBZ and storm-induced outflow of 30 m s⁻¹. Aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high-valued θ_c air overriding the cold dome produced by the developing MCC.

The strongest surface winds were located upshear from the convective line and consisted of a northerly (alongline) component. Because the middle-level environmental flow was from the southwest, a simple vertical transport of middle-level momentum cannot account for the observed surface flow. The strong surface winds were primarily a result of the local surface pressure gradient associated with a mesohigh–mesolow couplet that accompanied the squall line.

The squall line also maintained a strong, quasi-steady, supercell-like cell that could be tracked by radar for several hours. The kinematic structure, derived from a multiple Doppler radar analysis, shows that significant alongline flow was also generated by the rotational characteristics of the supercell. This feature distinguishes this system from tropical squall lines and many midlatitude squall lines which are composed of transient ordinary cells.

Abstract

Using a simplified thermodynamic sounding, and variable vertical wind shear, we investigate the role of gravity waves on the structure and propagation of a simulated two-dimensional squall line. Based on an observed squall line environment, the modeled troposphere has been divided into three distinct thermodynamic layers. These consist of an absolutely stable atmospheric boundary layer (ABL), an elevated well-mixed layer, and an upper tropospheric layer of intermediate stability. We find the mixed layer to have a dual role; it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts, and it provides an ideal layer to trap mesoβ-scale (20–200 km) wave energy generated in the stable layers. The generated waves thus have a significant and lasting impact on the simulation.

We also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long-lived squall lines. Experiments that vary the vertical wind shear profile demonstrate that the most vigorous and long-lived squall lines arise with a deep layer of strong vertical wind shear. This result is dependent on the changes in the phase speed and magnitude of the stable layer waves that occur in the sheared versus nonsheared environments. Without flow, waves generated by an initial heat pulse split into symmetric leftward and rightward moving disturbances. Waves generated in the upper tropospheric stable layer are found to move relative to the lower tropospheric waves resulting in a decoupling of deep tropospheric vertical motion and a decrease in strength of the simulated system. With vertical wind shear, the magnitude of the simulated waves is enhanced and an opportunity for sustained coupling between the upper and lower waves exists. It is shown that the upper and lower tropospheric waves in a sheared environment account for many of the circulation features typically associated with two-dimensional squall lines.

A simple mechanism for the rear-to-front middle-level jet and surface wake low is also presented.

Abstract

A midlatitude mesoscale convective complex (MCC), which occurred over the central United States on 23–24 June 1985, was simulated using the Regional Atmospheric Modeling System (RAMS). The multiply nested-grid simulation agreed reasonably well with surface, upper-air, and satellite observations and ground-based radar plots. The simulated MCC had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall. Several other characteristic MCC circulations were also simulated: a divergent cold pool in the lower troposphere, midlevel convergence coupled with a relatively cool descending rear-inflow jet, and relatively warm updraft structure, and a cold divergent anticyclone in the tropopause region. Early in the MCC simulation, a mesoscale convectively induced vortex (MCV) formed on the eastern edge of the convective line. While frequently associated with MCCs and other mesoscale convective systems (MCSs), MCVs are more typically reported in the mature and decaying stages of the life cycle. Several hours later, a second MCV formed near the opposite end of the convective line, and by the mature phase of the MCC, these MCVs were embedded within a more complex system-wide vortical flow in the lower troposphere.

Analysis of the first MCV during its incipient phase indicates that the vortex initially formed near the surface by convergence/stretching of the large low-level ambient vertical vorticity in this region. Vertical advection appeared largely responsible for the upward extension of this MCV to about 3.5 km above the surface, with tilting of horizontal vorticity playing a secondary role. This mechanism of MCV formation is in contrast to recent idealized high-resolution squall line simulations, where MCVs were found to result from the tilting into the vertical of storm-induced horizontal vorticity formed near the top of the cold pool.

Another interesting aspect of the simulation was the development of a banded vorticity structure at midtropospheric levels. These bands were found to be due to the apparent vertical transport of zonal momentum by the descending rear-to-front circulation, or rear-inflow jet. An equivalent alternative viewpoint of this process, deformation of horizontal vorticity filaments by the convective updrafts and rear-inflow jet, is discussed.

Part II of this work presents a complementary approach to the analysis presented here, demonstrating that the circulations seen in this MCC simulation are, to a large degree, contained within the nonlinear balance approximation, the related balanced omega equation, and the PV as analyzed from the PE model results.

Abstract

A nonlinear balance condition, which permits the diagnosis of both balanced divergent and nondivergent flows, is presented. This analysis approach is applied to the results of a numerical simulation of a midlatitude mesoscale convective complex (MCC) to assess the degree of balance of these and similar convective weather systems.

It is found that, to a large extent, the simulated MCC represents a highly balanced fluid system. The nondivergent component of the MCC wind field was found to be largely balanced from the time of initial convection to dissipation. Perhaps more surprisingly, the storm-induced divergent model winds are also balanced to a fair degree, though certainly less so than the nondivergent flow. Further, the balanced divergent flow makes up a significant portion of the total balanced flow in some regions of the MCC. System-scale divergence profiles of the model and balanced winds are compared and found to agree reasonably well, especially in the growth and mature stages of the MCC.

Within a stationary averaging volume enclosing the MCC, the greatest disparity between the model and balanced circulations is found in the downward vertical motion. The model downward mass flux significantly exceeds the balanced downward flux at most times during the simulation, suggesting that the process of mass adjustment due to convective heating is largely dominated by unbalanced fast-manifold processes, such as inertia–gravity waves. The unbalanced flow is found to be composed largely of divergent circulations of periodic nature (i.e., gravity waves). The appearance and characteristics of these features are found to be in good agreement with current theoretical predictions regarding the atmospheric response to convective heating and associated compensating subsidence.

The (modified) Rossby radii (λ _R ) for two lowest-order gravity wave modes are calculated. The mesoscale convective vortex (MCV) within the storm is larger than λ _R for all but the gravest mode. The λ _R ⁿ for the mature storm as an ensemble also indicates a good degree of balance with λ _R ⁿ⁼¹ scaling similar to the MCC and larger values of n scaling smaller than the system as a whole. These λ _R values strongly suggest that this simulated MCC represents an inertially stable balanced mesoscale convective system.