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Christopher A. Davis

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Christopher A. Davis

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The dynamics of a cyclone development over the midwestern United States on 15 December 1987 are investigated with a focus on the relationship between cyclone structure and condensational heating. Low-level cyclogenesis is initiated by a large-amplitude tropopause perturbation that develops over western North America. Using potential-vorticity (PV) inversion diagnostics, we show how the near-surface winds associated with this upper disturbance create a localized, warm, thermal anomaly within a surface baroclinic zone. The distribution of precipitation and the diabatic generation of a positive low-level PV feature near the cyclone center are also controlled by the tropopause perturbation. Development culminates in a superposition of positive anomalies of tropopause PV, moisture-induc6d PV, and surface potential temperature θ, with contributions to the total low-level circulation being about 30%, 20%, and 50%, respectively.

This case is compared with a different cyclogenesis event (4–5 February 1988), characterized by an initially small-amplitude upper-level wave and relatively fixed structure during growth. The vertical structure in the February 1988 case allowed the ascent induced by the tropopause and surface anomalies to reinforce. The nearly fixed structure and long development period led to a diabatically produced PV perturbation that was twice as intense as the low-level PV in the December cyclone. While comparable precipitation and PV generation rates were present in the December case, structural transience limited the intensity of the moisture-induced PV perturbation.

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Christopher A. Davis

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The treatment of the potential vorticity (PV) distribution as a composite of individual perturbations is central to the diagnostic and conceptual utility of PV. Nonlinearity in the inversion operator for Ertel's potential vorticity renders quantitative piecewise inversion (inversion of individual portions of the potential vorticity field) ambiguous. Several methods of piecewise inversion are compared for idealized and observed potential vorticity anomalies of varying strengths. Even as the Rossby number of the balanced solutions increases well past unity, relative differences among the more plausible methods do not increase significantly near the anomaly. These relative differences are also found to be smaller than those obtained by comparing any of the methods to quasigeostrophic inversion. However, differences above and below anomalies increase with increasing Rossby number, suggesting that one cannot uniquely diagnose the interaction of large amplitude PV anomalies.

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Christopher A. Davis

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The present study considers a variety of cyclone developments that occur in an idealized, baroclinic channel model featuring full condensation heating effects over an ocean with prescribed sea surface temperature variation. The geostrophic basic-state jet is specified by the tropopause shape, and horizontal shear is included by specifying the meridional variation of zonal wind on the lower boundary. The horizontal shear induces anticyclonic wave breaking of baroclinic waves. Normal mode perturbations are computed using a “fake-dry” version of the model but integrated forward using full physics.

Low-latitude moist convection is particularly strong in simulations with strong surface easterlies that destabilize the troposphere through water vapor fluxes from the ocean surface. Deep convection produces a locally elevated dynamic tropopause and an associated anticyclone. This modified zonal flow supports moist baroclinic instability. The resulting cyclones, identified as subtropical cyclones, occur in deep westerly vertical wind shear but are nearly devoid of lower-tropospheric baroclinicity initially. These systems are distinguished from baroclinically dominated secondary cyclones that also form at relatively low latitudes in the simulations.

For weak jets and strong subtropical surface easterlies, subtropical cyclone development dominates formation on the midlatitude jet. For strong westerly jets or weak horizontal shear, the situation is reversed and the midlatitude baroclinic wave can help or hinder the ultimate intensification of the subtropical cyclone. The similarity of this cross-latitude influence to the extratropical transition of tropical cyclones is noted.

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Christopher A. Davis

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A transient feature marked primarily by a sudden increase in wind speed and brief period of heavy snow was observed over northeastern Colorado on 11-12 March 1993. Little, if any, abrupt changes in surface pressure or temperature were noted, such as might accompany a gravity current. Gradual temperature falls and pressure rises were observed behind the velocity surge; however, there was no cyclonic wind shift nor did the feature last for more than a few hours, suggesting that it was not likely a classical front. Data indicate that the surge originated near the peak of the Cheyenne Ridge, a 400-m, east-west elongated hill near the border between Colorado and Wyoming.

Simulations using an idealized, two-dimensional isentropic model show that a transient resembling observed surge results from impulsively started flow over heated terrain. Heating creates convergence that is in phase with the velocity surge at the leading edge of the topographic wave. The result is a coherent disturbance in velocity, which moves downstream at nearly the mean flow speed.

Simulations with The Pennsylvania State University-National Center for Atmospheric Research nonhydrostatic model (MM5) using a horizontal resolution of 6.7 km capture the evolution of the observed surge. These simulations support the identification of the surge as a topographic wave modified by heating, but they also indicate an important role played by latent heating in the updraft at the leading edge of the surge. Because the latent heating occurs mainly downstream from the Cheyenne Ridge, the formation of the topographic wave is unaltered by it, however, the updraft at the leading edge of the surge it greatly intensified, leading to a narrow, propagating band of heavy snow.

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Christopher A. Davis

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The author diagnoses two observed cases of Rocky Mountain lee cyclogenesis and perform several idealized simulations to understand the effect of the mountains on incident baroclinic waves. Several issues are examined: 1) in what sense is the effect of the mountains cyclolytic and why; 2) what is the effect of adding a mean surface wind; 3) what sorts of behavior may result in differing mean flows; and 4) what is a useful conceptual framework in which to view lee cyclogenesis? The dynamical underpinning for analysis of observations and idealized simulations is the quasigeostrophic (QG) equations.

The author finds that the most important effect of the mountains is to alter the mean distribution of surface potential temperature and hence change the propagation characteristics of the incident baroclinic wave. The presence of a mountain enhances the gradient of θ*, the QG approximation to surface potential temperature, to the north of the peak and decreases it to the south. Thus the component of the baroclinic wave that is identified with surface potential temperature perturbations propagates around the north side of the mountains and accelerates. This leads to a change in vertical structure of the incident wave that, for the wavelengths considered, systematically results in a smaller growth rate than one would expect without the mountain. The addition of a mean flow extends the influence of the mountain upstream and downstream from the obstacle and causes the waves to deviate to the north well upstream from the mountain, following the largest gradients of θ*.

The structure of the baroclinic waves over and downstream from the mountain varies substantially depending on the location of the upper-level jet. For a jet to the north of the mountain, a strong, synoptic-scale “cold surge” develops in the lee, governed by QG dynamics. Upslope cooling reinforces horizontal temperature advection, and an anticyclone intensifies, moving southward along the contours of θ*. With the jet to the south, the anticyclone weakens and the cyclone in the lee dominates.

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Christopher A. Davis

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Composite analyses of terrain-forced, mesoscale anticyclonic circulations over southern Wyoming and northern Colorado are constructed. These suggest two different types of circulations, based on upstream flow direction. The air within type 1 circulations originates to the west of the Continental Divide and contains little moisture. Type 2 circulations form in more moist, northerly flow and are sometimes associated with snowbands. Both types tend to form during the afternoon and dissipate after sunset, although type 2 events may follow frontal passages and occur at night.

Case studies of one event of each type suggest that anticyclonic vorticity generation occurs within the lowest kilometer above ground level when that layer is nearly vertically mixed in both potential temperature and velocity. An analogy with vorticity generation in mixed-layer models is considered, and it is shown that the conditions for generating negative vorticity in those models are satisfied in each observed case. The mixed-layer mechanism is also favored as it naturally explains the diurnal tendency of the circulations and may therefore explain the observed, late-day snowfall maximum along the Front Range of Colorado.

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Christopher A. Davis

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The upscale aggregation of convection is used to understand the emergence of rotating, coherent midtropospheric structures and the subsequent process of tropical cyclone formation. The Cloud Model, version 1 (CM1), is integrated on an f plane with uniform sea surface temperature (SST) and prescribed uniform background flow. Deep convection is maintained by surface fluxes from an ocean with uniform surface temperature. Convection begins to organize simultaneously into moist and dry midtropospheric patches after 10 days. After 20 days, the patches begin to rotate on relatively small scales. Moist cyclonic vortices merge, eventually forming a single dominant vortex that subsequently forms a tropical cyclone on a realistic time scale of about 5 days. Radiation that interacts with clouds and water vapor aids in forming coherent rotating structures. Using the path to genesis provided by the aggregated solution, the relationship between thermodynamic changes within the vortex and changes in the character of convection prior to genesis is explored. Consistent with previous studies, the approach to saturation within the midtropospheric vortex accelerates the genesis process. A novel result is that, prior to genesis, downdrafts become widespread and somewhat stronger. The increased downdraft mass flux leads to stronger and larger surface cold pools. Shear–cold pool dynamics promote the organization of lower-tropospheric updrafts that spin up the surface vortex. It is inferred that the observed inconsistency between convective intensity and thermodynamic stabilization prior to genesis results from sampling limitations of the observations wherein the important cold pool gradients are unresolved.

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Frederick Sanders and Christopher A. Davis

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Hemispheric anomaly patterns of 1000–500 mb thickness were obtained for 67 cases of explosive cyclogenesis over the western North Atlantic Ocean in December-February during 1962–77, beginning between latitudes 30°–40°N and between longitudes 70°–80°W.

Composite patterns for the 26 strongest cases of cyclogenesis differed markedly from those for the 22 weakest. After a filtering to remove the shortest waves, those for the strongest developments showed a prominent negative anomaly area of large scale, centered over western Canada 5 days before the event, moving southeastward to the western Atlantic days after cyclogenesis. No such pervasive anomaly pattern was seen for the weakest cases.

The most intense cyclogenesis occurred when the air over the region of development was slightly colder than the 15-year average, while the least intense occurred in slightly anomalous warmth.

In the zonal average from 25° to 125°W, the strongest cases occurred with warmth in polar latitudes, coldness in middle latitudes and anomalously strong westerly thermal wind in the cyclogenetic area. The weakest cases occurred with cold polar latitudes, warmth in upper middle latitudes, and slightly cold anomalies but no excessive thermal wind in the latitudes of cyclogenesis.

It is implied that both baroclinic forcing and heat and moisture flux from the sea surface were enhanced in the strongest cases, but neither effect was obviously dominant.

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Christopher A. Davis and David A. Ahijevych

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Conditional composites of dropsondes deployed into eight tropical Atlantic weather systems during 2010 are analyzed. The samples are conditioned based on cloud-top temperature within 10 km of the dropsonde, the radius from the cyclonic circulation center of the disturbance, and the stage of system development toward tropical cyclogenesis. Statistical tests are performed to identify significant differences between composite profiles. Cold-cloud-region-composite profiles of virtual temperature deviations from a large-scale instantaneous average indicate enhanced static stability prior to genesis within 200 km of the center of circulation, with negative anomalies below 700 hPa and larger warm anomalies above 600 hPa. Moist static energy is enhanced in the middle troposphere in this composite mainly because of an increase in water vapor content. Prior to genesis the buoyancy of lifted parcels within 200 km of the circulation center is sharply reduced compared to the buoyancy of parcels farther from the center. These thermodynamic characteristics support the conceptual model of an altered mass flux profile prior to genesis that strongly favors convergence in the lower troposphere and rapid increase of circulation near the surface. It is also noted that the air–sea temperature difference is greatest in the inner core of the pregenesis composite, which suggests a means to preferentially initiate new convection in the inner core where the rotation is greatest.

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