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Wendell A. Nuss

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Wendell A. Nuss

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Coastally trapped wind reversals that occur along the U.S. West Coast have been described in numerous other studies. The synoptic-scale environment and the forcing of a coastally trapped Kelvin wave are highly linked in the development of these wind reversals. However, not all wind reversals appear to behave like propagating Kelvin waves and the analysis of coastal buoy observations for three years indicates that different types of disturbances occur. Both propagating disturbances and nonpropagating disturbances occur with similar frequencies. While the synoptic-scale characteristics associated with propagating and nonpropagating wind reversals are sometimes rather subtle, several distinct differences occur that suggest a direct link between the coastal dynamics and the synoptic-scale forcing. Synoptic forcing characterized by persistent low-level offshore flow favors the development of propagating disturbances, while weak, nonsustained offshore flow characterizes nonpropagating disturbances. These differences support the idea that propagating events represent a favorable interaction between the synoptic-scale forcing and the excitation and propagation of Kelvin waves, whereas nonpropagating events either represent a less favorable interaction or are simply trapped ageostrophic flow due to the synoptic scale without a Kelvin wave response. These dynamic differences imply different stratus behavior, which is important to forecasters.

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Wendell A. Nuss

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Three numerical simulations of cyclogenesis in a baroclinic channel model with realistic physical parameterizations are compared to examine the influence of surface heat and moisture fluxes on the structure and development mechanisms of an idealized midlatitude cyclone. Identical atmospheric initial conditions are used in the three numerical simulations, which differ only in their surface flux conditions. The impact of the surface heat and moisture fluxes on the thermal structure and vertical circulation in the model cyclone is examined by comparing the geopotential height, temperature, static stability, surface convergence and wind stress as well as the surface heat and moisture flux distributions in the three simulations. Boundary layer stratification effects on convergence are examined and the associated changes in the boundary layer-free atmosphere interaction are compared.

Results indicate that the direct influence of the surface beat and moisture fluxes is limited to the boundary layer structure of the developing cyclone except when horizontal variations in the surface wind stress act to increase the boundary layer convergence. Upward surface heat fluxes to the northeast of the surface low and warm front produce unstable boundary layer stratification in this region, which increases the downward momentum fluxes and associated ageostrophic flow toward the surface low and warm front. Increased convergence results and increases the vertical circulation and the release of latent heat above the boundary layer due to this increased Ekman convergence. This boundary layer-free atmosphere interaction offsets the strong damping effect of the surface heating to lessen the baroclinity from which the cyclone derives its energy.

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Wendell A. Nuss

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Eric Metzger
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Wendell A. Nuss

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Total lightning detection systems have been in development since the mid-1980s and deployed in several areas around the world. Previous studies on total lightning found intra- and intercloud lightning (IC) activity tends to fluctuate significantly during the lifetime of thunderstorms and have indicated that lightning jumps or rapid changes in lightning flash rates are closely linked to changes in the vertical integrated liquid (VIL) reading on the National Weather Service’s Weather Surveillance Radar-1988 Doppler (WSR-88D) systems. This study examines the total lightning and its relationship to WSR-88D signatures used operationally to determine thunderstorm severity to highlight the potential benefit of a combined forecast approach. Lightning and thunderstorm data from the Dallas–Fort Worth, Texas, and Tucson, Arizona, areas from 2006 to 2009, were used to relate total lightning behavior and radar interrogation techniques. The results indicate that lightning jumps can be classified into severe wind, hail, or mixed-type jumps based on the behavior of various radar-based parameters. In 25 of 34 hail-type jumps and in 18 of 20 wind-type jumps, a characteristic change in cloud-to-ground (CG) versus IC lightning flash rates occurred prior to the report of severe weather. For hail-type jumps, IC flash rates increased, while CG flash rates were steady or decreased. For wind-type jumps, CG flash rates increased, while IC flash rates either increased (12 of 18) or were steady or decreased (6 of 18). Although not every lightning jump resulted in a severe weather report, the characteristic behavior in flash rates adds information to radar-based approaches for nowcasting the severe weather type.

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Wendell A. Nuss
and
Richard A. Anthes

Abstract

Several physical processes and properties of the initial state that affect marine cyclogenesis are examined using a mesoscale numerical model. The sensitivity of an idealized cyclone to the effects of latent heat release, surface heat and moisture fluxes as well as the initial meridional temperature gradient and static stability is examined by comparing various numerical stimulations of cyclogenesis in a baroclinic channel-flow model. Idealized initial conditions are derived analytically and are characterized by strong low-level baroclinity and a very weak upper-level trough. These initial conditions are used to examine which factors in baroclinic cyclogenesis are most important for rapid development (1 m h−1 for 24 h or more) and how diabatic processes modify the development rate.

A strong low-level meridional gradient (40°C/2000 km) and low static stability (a mean lapse rate of 6.0°C km−1) resulted in rapid development of the model cyclone. The model cyclogenesis is more sensitive to small changes in the initial baroclinity than to physical processes during the development, which suggests that sustained rapid development requires substantial baroclinic instability. Inclusion of latent heat release during the development resulted in only a 10% increase in the average deepening rate. This effect of latent heating depended crucially upon the moisture distribution and is more representative of large-scale stable condensation than strong convection. Modification of the model cyclogenesis by various surface heat and moisture flux distributions indicated that the phase and magnitude of these fluxes relative to the low-level atmospheric baroclinity is important. A distribution of surface heating that enhanced the low-level baroclinity resulted in a 15% increase in growth rare, suggesting an important interaction during certain periods of development. Surface heating distributions that reduced the low-level baroclinity by counteracting thermal advection damped the development of the model cyclone as suggested by previous studies.

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Wendell A. Nuss
and
David W. Titley

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The method of multiquadric interpolation is described and compared to the Barnes and Cressman methods of meteorological objective analysis. The method of multiquadric interpolation uses hyperboloid radial basis functions to fit scattered data to a uniform grid. Results for an analytical function indicate that the method is more accurate than the Barnes or Cressman methods. Application to actual meteorological data indicates that multiquadric interpolation produces excellent analyses that retain small-scale features resolved by the observations in any subregion of the analysis.

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Robert G. Fleagle
and
Wendell A. Nuss

Abstract

Surface meteorological observations have been used to calculate the distributions of surface fluxes of momentum, sensible heat, and latent heat and the distributions of surface divergence and curl of surface stress which are characteristic of ocean storms in the Gulf of Alaska. Flux calculations were based on aerodynamic equations which include the effects of high wind speed and stratification of the surface layer. Distributions of divergence determined directly from surface winds were compared with those calculated using the curl of the surface stress.

Results indicate that heat fluxes were weak or oven negative for the most intense storms in a band about 300 km wide immediately east of strong cold or occluded fronts. Maximum upward latent heat fluxes occurred to the west of the front, with secondary maxima along the eastern edge of the storm, beyond the 300 km band of weak flux.

Surface convergence occurred to the east and divergence to the west of cold or occluded fronts. Vertical velocities calculated from the curl of the surface stress and including other important terms in the vorticity equation correspond in distribution and magnitude with measured divergences. Based on these independent calculations of vertical velocity and divergence, it can be inferred that the vertical gradient of stress was a maximum at the surface and decreased with height. The results provide a basis for assessing numerical models of ocean storms and for further developing and verifying parameterizations of surface fluxes.

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Wendell A. Nuss
and
Shun I. Kamikawa

Abstract

An explosive and nonexplosive cyclone that developed during March 1986 along the coast of Japan are analyzed to illustrate the possible interaction of moist baroclinic dynamics, boundary layer structure and surface heat and moisture fluxes in their development. The moist symmetric stability is examined in the warm frontal ascent regions to investigate the contribution of surface energy fluxes to the moist baroclinic dynamics. Results indicate that moist symmetric neutrality occurs in the updraft regions of both cyclones and suggest a direct coupling between surface energy fluxes and the moist baroclinic processes.

Surface flux distributions for the two cyclones show that large positive surface heat and moisture fluxes to the northeast of the low persisted in the explosive cyclone but were quite transient in the nonexplosive cyclone. Boundary layer equivalent potential temperature (θe) budgets reveal that approximately half of the local surface θe increase in the warm frontal region throughout the development of the explosive cyclone is due to surface fluxes with the remainder due to horizontal advection. This contrasts sharply with the nonexplosive cyclone where there was a nearly equal initial contribution to local surface θe increases by the surface fluxes in the warm frontal region that diminished rapidly during the development.

Maintenance of the strong surface fluxes in the updraft region of the explosive cyclone is attributed to the interaction of the warm frontal dynamics with a downstream upper-level jet streak. Strong ageostrophic advection of cold air in the boundary layer resulted from the thermally direct vertical circulation of the warm front and jet entrance region. The nonexplosive cyclone lacked this strong thermally direct vertical circulation. Consequently, the large surface energy flux contributions in the explosive cyclone depended significantly on the baroclinic processes of the cyclone. This relationship was not present in the weaker cyclone.

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Hung-chi Kuo
and
Wendell A. Nuss

Abstract

The quasigeostrophic geopotential tendency equation is derived using P vectors in both pressure and entropy coordinates. This vector form of the geostrophic forcing in the geopotential tendency equation is similar to the Q-vector form of the ω equation. It is shown that the horizontal components of P are the advection of geostrophic momentum and the vertical component of P is the horizontal temperature advection. The P vectors are shown to be related to Q and C vectors, as well as the ageostrophic circulation. The three-dimensional pseudocurl of the P vector gives the C vector that equals the ageostrophic pseudovorticity in the quasigeostrophic model. The horizontal components of the pseudocurl of P are perpendicular and proportional to the Q. The horizontal divergence of the P vector is the geostrophic absolute vorticity advection while the three-dimensional divergence of the P vector is the geostrophic quasigeostrophic potential vorticity advection. The ageostrophic wind can be partitioned into the P vector (geostrophic advective) and isallobaric wind contributions.

A numerical simulation of an idealized cyclone is used to characterize the distribution of the P vectors and P-vector diagnostics in order to demonstrate their potential application to the diagnosis of synoptic-scale circulations. The distribution of the P vectors clearly indicates the advections of geostrophic momentum and temperature that characterize cyclogenesis. An examination of the P vectors and the isallobaric wind demonstrates that the P vectors provide insight into the ageostrophic circulation of the cyclone. Diagnoses of the three-dimensional P-vector divergence and curl are shown to produce useful depictions of cyclonic vortex spinup and the propagation of both the large- and smaller-scale features of the system. These diagnostics can be interpreted from a variety of perspectives, including the height tendency and the advection of quasigeostrophic potential vorticity. The use of P vectors to diagnose synoptic-scale circulations appears to provide potentially useful insights into the dynamics of synoptic-scale disturbances not readily obtained from other diagnoses.

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