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R. Banta and W. R. Cotton

Abstract

In the traditional model of ridge-valley winds, there are typically two wind regimes on a dry day: a downslope, drainage wind at night due to cooling at the surface along the slopes, and an upslope wind during the day due to solar heating of the slopes. This study presents observations from South Park, a broad, flat basin in the Colorado Rockies. The observations consist of time sequences of surface observations, surface mesonet analyses, and vertical atmospheric soundings using a tethered balloon system. On a typical dry day in South Park, three wind regimes were observed: the downslope regime, the upslope regime, and a late morning or afternoon wind which corresponded in direction to the winds above the ridgetops. Because the gradient and ridgetop winds were most frequently from the west, we have called these winds the “afternoon westerues.”

The afternoon westerlies occur in conjunction with a deep (2–3 km or more) afternoon convective boundary layer in which momentum (and other properties) are well mixed all the way down to the surface. The appearance of the westerlies at the surface is thus a consequence of the strong turbulent mixing within the convective boundary layer.

Vertical tethered balloon soundings taken in mid-morning show that the upslope winds form within a shallow convective boundary layer, which develops beneath the nocturnal inversion in response to surface heating. This stable inversion layer inhibits downward mixing of the upper-level westerlies and allows easterly, upslope flow to establish itself near the surface. When the last remnant of the nocturnal inversion is erased by surface heating and other processes, the westerlies are free to mix downward, and afternoon westerlies are observed at the surface.

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M. J. Manton and W. R. Cotton

Abstract

A parameterization of the constant flux surface layer is developed in order to provide boundary conditions for numerical models of the atmospheric boundary layer and moist convective layer. Algebraic expressions are found for the turbulence covariances in the surface layer under all stability conditions.

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W. R. Cotton and G. T. Tripoli

Abstract

No abstract available.

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W. R. Cotton, R. L. George, and K. R. Knupp

Abstract

A detailed, multisensor case study of mesoscale convective storms occurring in summer over the central and eastern Colorado Rockies is presented. This case study uses data obtained during the 1977 South Park Area Cumulus Experiment (SPACE) from surface meteorological stations, rawinsondes and tethered balloons, conventional and Doppler radars, powered aircraft and satellites.

On 19 July 1977, a north–south oriented line of intense convective cells formed and remained within South Park, an elevated plain 2.8 km above sea level located within the Rocky Mountains. Elevated surface heating in South Park created a region of low-level convergence which imported Pacific moisture from west of the Rockies into South Park. The mososcale thunderstorm line formed over this convergence zone. Subsequently, northerly surface flow, having the appearance of a “density current”, penetrated into South Park late in the afternoon, enhancing the intensity of convective storms. Various interactions of the storm system with the mesoscale environment were observed. A single large convective cell was then observed to grow on the southern end of the mesoscale line, exhibiting supercell characteristics and substantial modification of the environmental flow. A detailed description of this quasi-steady storm is given in Parts II and III (Knupp and Cotton, 1982a,b).

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G. Feingold, S. Yang, R. M. Hardesty, and W. R. Cotton

Abstract

This paper explores the possibilities of using Ka-band Doppler radar, microwave radiometer, and lidar as a means of retrieving cloud condensation nucleus (CCN) properties in the stratocumulus-capped marine boundary layer. The retrieval is based on the intimate relationship between the cloud drop number concentration, the vertical air motion at cloud base, and the CCN activation spectrum parameters. The CCN properties that are sought are the C and k parameters in the N = CS k relationship, although activation spectra based on the lognormal distribution of particles is also straightforward. Cloud droplet concentration at cloud base is retrieved from a Doppler cloud radar combined with a microwave radiometer following a previously published technique. Cloud base is determined from a lidar or ceilometer. Vertical velocity just above cloud base is determined from the vertically pointing Doppler cloud radar. By combining the simultaneous retrievals of drop number and vertical velocity, and assuming theoretical relationships between these parameters and the subcloud aerosol parameters, the C parameter can be derived, under the assumption of a fixed k. If a calibrated backscatter lidar measurement is available, retrieval of both C and k parameters is possible. The retrieval is demonstrated for a dataset acquired during the Atlantic Stratocumulus Transition Experiment using a least squares minimization technique. Sensitivity to assumptions used in the retrieval is investigated. It is suggested that this technique may afford the acquisition of long-term datasets for climate monitoring purposes. Further investigation with focused experiments designed to address the issue more rigorously is required.

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Catherine A. Finley, W. R. Cotton, and R. A. Pielke Sr.

Abstract

A nested grid primitive equation model (RAMS version 3b) was used to simulate a high-precipitation (HP) supercell, which produced two weak tornadoes. Six telescoping nested grids allowed atmospheric flows ranging from the synoptic scale down to the tornadic scale to be represented in the simulation. All convection in the simulation was initiated with resolved vertical motion and subsequent condensation–latent heating from the model microphysics; no warm bubbles or cumulus parameterizations were used.

Part I of this study focuses on the simulated storm evolution and its transition into a bow echo. The simulation initially produced a classic supercell that developed at the intersection between a stationary front and an outflow boundary. As the simulation progressed, additional storms developed and interacted with the main storm to produce a single supercell. This storm had many characteristics of an HP supercell and eventually evolved into a bow echo with a rotating comma-head structure. An analysis of the storm's transition into a bow echo revealed that the interaction between convective cells triggered a series of events that played a crucial role in the transition.

The simulated storm structure and evolution differed significantly from that of classic supercells produced by idealized simulations. Several vertical vorticity and condensate maxima along the flanking line moved northward and merged into the mesocyclone at the northern end of the convective line during the bow echo transition. Vorticity budget calculations in the mesocyclone showed that vorticity advection from the flanking line into the mesocyclone was the largest positive vorticity tendency term just prior to and during the early phase of the transition in both the low- and midlevel mesocyclone, and remained a significant positive tendency in the midlevel mesocyclone throughout the bow echo transition. This indicates that the flanking line was a source of vertical vorticity for the mesocyclone, and may explain how the mesocyclone was maintained in the HP supercell even though it was completely embedded in heavy precipitation.

The simulated supercell also produced two weak tornadoes. The evolution of the simulated tornadoes and an analysis of the tornadogenesis process will be presented in Part II.

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G. G. Carrió, H. Jiang, and W. R. Cotton

Abstract

The objective of this paper is to assess the impact of the entrainment of aerosol from above the inversion on the microphysical structure and radiative properties of boundary layer clouds. For that purpose, the Los Alamos National Laboratory sea ice model was implemented into the research and real-time versions of the Regional Atmospheric Modeling System at Colorado State University.

A series of cloud-resolving simulations have been performed for a mixed-phase Arctic boundary layer cloud using a new microphysical module that considers the explicit nucleation of cloud droplets. Different aerosol profiles based on observations were used for initialization. When more polluted initial ice-forming nuclei (IFN) profiles are assumed, the liquid water fraction of the cloud decreases while the total condensate path, the residence time of the ice particles, and the downwelling infrared radiation monotonically increase. Results suggest that increasing the aerosol concentrations above the boundary layer may increase sea ice melting rates when mixed-phase clouds are present.

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G. G. Carrió, W. R. Cotton, D. Zupanski, and M. Zupanski

Abstract

A cloud-nucleating aerosol retrieval method was developed. It allows the estimation of ice-forming nuclei and cloud condensation nuclei (IFN and CCN) for regions in which boundary layer clouds prevail. The method is based on the assumption that the periodical assimilation of observations into a microscale model leads to an improved estimation of the model state vector (that contains the cloud-nucleating aerosol concentrations). The Colorado State University Cloud Resolving Model (CRM) version of the Regional Atmospheric Modeling System (RAMS@CSU) and the maximum likelihood ensemble filter algorithm (MLEF) were used as the forecast model and the assimilation algorithm, respectively. On the one hand, the microphysical modules of this CRM explicitly consider the nucleation of IFN, CCN, and giant CCN. On the other hand, the MLEF provides an important advantage because it is defined to address highly nonlinear problems, employing an iterative minimization of a cost function. This paper explores the possibility of using an assimilation technique with microscale models. These initial series of experiments focused on isolating the model response and showed that data assimilation enhanced its performance in simulating a mixed-phase Arctic boundary layer cloud. Moreover, the coupled model was successful in reproducing the presence of an observed polluted air mass above the inversion.

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G. G. Carrió, H. Jiang, and W. R. Cotton

Abstract

The potential impact of intrusions of polluted air into the Arctic basin on sea ice melting rates and the surface energy budget is examined. This paper extends a previous study to cloud-resolving simulations of the entire spring season during the 1998 Surface Heat Budget of the Arctic (SHEBA) field campaign. For that purpose, the Los Alamos National Laboratory sea ice model is implemented into the research and real-time versions of the Regional Atmospheric Modeling System at Colorado State University (RAMS@CSU). This new version of RAMS@CSU also includes a new microphysical module that considers the explicit nucleation of cloud droplets and a bimodal representation of their spectrum. Different aerosol profiles based on 4 May 1998 observations were used to characterize the polluted upper layer and the 2–3 daily SHEBA soundings were utilized to provide time-evolving boundary conditions to the model. Results indicate that entrainment of ice-forming nuclei (IFN) from above the inversion increases the sea ice melting rates when mixed-phase clouds are present. An opposite although less important effect is associated with cloud condensation nuclei (CCN) entrainment when liquid-phase clouds prevail.

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Graham Feingold, W. R. Cotton, Bjorn Stevens, and A. S. Frisch

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This paper considers the production of drizzle in statocumulus clouds in relation to the boundary-layer turbulent kinetic energy and in-cloud residence times. It is shown that drizzle production in statocumulus of the order of 400 m in depth is intimately related to the vertical velocity structure of the cloud eddies. In a series of two dimensional numerical experiments with fixed cloud condensation nucleus concentrations, the effect on drizzle production of enhanced or diminished vertical velocities is simulated. Rather than do this by simulating clouds exhibiting more or less energy, we modify drop terminal velocities in a manner that conserves the fall velocity relative to the air motions and allows droplet growth to occur in a similar dynamical environment. The results suggest that more vigorous clouds produce more drizzle because they enable longer in-cloud dwell times and therefore prolonged collision-coalescence. In weaker clouds, droplets tend to fall out of the cloud before they have achieved significant size, resulting in smaller amounts of drizzle. In another series of experiments, we investigate the effects of the feedback of drizzle on the boundary-layer dynamics. Results show that when significant amounts of drizzle reach the surface, the subcloud layer is stabilized, circulations are weaker, and the boundary layer is not well mixed. When only small amounts of drizzle are produced, cooling tends to be confined to the region just below cloud base, resulting in destabilization, more vigorous circulations, and a better mixed boundary layer. The results strongly suggest that a characteristic time associated with collision-coalescence be incorporated into drizzle parameterizations.

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