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James D. Doyle

Abstract

The impact of ocean surface waves on the structure and intensity of three tropical cyclones and a topographically forced bora event is investigated using a coupled atmosphere–ocean wave modeling system. The coupled system is capable of representing surface momentum fluxes that are enhanced due to young ocean waves in fetch-limited conditions, which yield surface roughness lengths that significantly depart from the conventional Charnock-type formulation. In general, the impact of ocean-wave-induced stress on the tropical cyclone central pressure was quite variable with ocean wave feedback resulting in changes ranging from 8 hPa deeper to 3 hPa shallower. The increased low-level stress due to the ocean waves reduced the near-surface winds by 2–3 m s−1, with local differences in excess of 10 m s−1, which directly led to a 10% reduction in the significant wave height maxima. The reduced significant wave heights in the coupled model were in closer agreement with observations for Tropical Cyclone Bonnie than for the uncoupled model. The tropical cyclone tracks were generally insensitive to ocean wave feedback effects. The boundary layer structure was found to be generally insensitive to large roughness enhancements associated with coupled ocean wave feedbacks for topographically forced high wind phenomena, such as the bora.

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James D. Doyle

Abstract

The role of mesoscale orography along the central California coast in the development and evolution of a coastal jet and rainband is investigated using a high-resolution, triply nested, nonhydrostatic numerical model. Comparison of the model simulations, which use horizontal grid increments of 5 and 2 km on the inner computational meshes, with a coastal mesoscale observation network indicates that the finescale structure of the jet and rainband dynamics are adequately simulated, although phase and orientation errors occur. The observed and simulated near-surface winds have maximum speeds that exceed 22 m s−1 and a direction nearly parallel to the coastline and topography.

Force balance analysis indicates that blocking in the lowest 500 m and flow over the coastal range above this layer contribute to mesoscale pressure perturbations, including pressure ridging upstream of the coastal mountains, which forces the ageostrophic dynamics of the coastal jet. Pressure perturbations associated with the topographic flows induce a complex mesoscale response that adds rich mesoscale structure to the jet including a wake region that forms on the lee side of the coastal range that limits the horizontal scale of the jet. Sensitivity test results underscore the multiprocess character of the coastal dynamics and the importance of the coastal topography and differential frictional drag at the land–sea interface for the formation and amplification of the jet. The mesoscale response to steep coastal topography results in a 45% enhancement to the near-surface jet strength. The onshore movement of line convection at the leading edge of a weak front is impeded by steep coastal topography in both the radar observations and numerical simulations. Low-level blocking forces the rainband to emulate a wedge-shaped structure with a coastal jet that is dynamically trapped between the steep coastal topography and the front.

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James D. Doyle
and
Thomas T. Warner

Abstract

A field program in March 1982 obtained rewinsonde data over a mesoscale network that had resolution similar to that of the temperature and moisture data simultaneously obtained from VAS (Visible and infrared spin-span radiometer Atmospheric Sounder). This provides a unique opportunity to verify objective analysis procedures used to combine standard rawinsonde and VAS soundings of temperature and moisture.

In this study, various combinations of VAS data, conventional rawinsonde data, and gridded data from the National Weather Service's (NWS) global analysis, are used in successive-correction (SC) and variations objective analysis procedures. The analysis are objectively and subjectively compared with the AVE/VAS special-network rawinsonde data, where the major discernable mesoα-scale feature at this time was a cold-air pocket at 500 mb.

The objective three-dimensional verification statistics show that the use of VAS data to supplement the NWS rawinsonde data significantly decreased the mixing-ratio error, but also significantly increased the temperature error. The SC procedure used to analyse the VAS data reduced the mixing-ratio error more than did any of the variational procedure. Compared to the error associated with the basic NWS global analysis that has not been supplemented with rawinsonde or VAS data, the use of VAS temperature and mixing-ratio data had a positive impact when combined with these global fields. The positive impact on the moisture field was considerably greater however.

Subjective verification of the temperature fields at 500 mb produced additional insight. First, the VAS retrieval data were able to modify the very smooth global analysis to produce a fairly realistic temperature minimum in the verification-network region. Also, the variational procedures were able to successfully blend the rawinsonde and VAS data to that the bed subjective verification of the cold-pocket structure was produced when both data sources wore employed.

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James D. Doyle
and
Thomas T. Warner

Abstract

A nonhydrostatic version of the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, with a horizontal resolution of 5 km, is used with measurements taken during intensive observation period 2 of the Genesis of Atlantic Lows Experiment to study the offshore mesobeta-scale coastal front structure. Results from the 24-h model simulation and Doppler radar data indicate that precipitation bands, with embedded convective elements, are present along the coastal front in the vicinity of the Gulf Stream. As the frontogenesis evolves, the simulated surface frontal zone becomes fractured, and discontinuous lines of confluence and mesoscale ascent become apparent. A collapse of the cross-frontal thermal gradient is driven by intense gradients of the surface fluxes in the vicinity of the Gulf Stream.

A mesoscale wave train, consisting of a series of shallow, weak vortices with horizontal scales between 50 and 100 km, forms along the front in agreement with the Doppler radar data and surface observations. Diagnostic analysis of the model simulation and a series of model sensitivity experiments indicate that shearing instability along the frontal zone focuses the lower-tropospheric convergence. Subsequently, stretching of cyclonic vorticity, modulated by latent heating associated with the banded precipitation, leads to the generation of the mesobeta-scale vortices along the coastal front. The formation mechanisms of these vortices may have important implications for the genesis of coastal cyclones and polar lows along shallow baroclinic zones.

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James D. Doyle
and
Thomas T. Warner

Abstract

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the sensitivity of the structure and evolution of mesoscale coastal phenomena to the sea surface temperature (SST) distribution in the vicinity of the Gulf Stream during intensive observation period 2 (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE). Experiments are performed with three different SST analyses: A 1) a high-resolution 14-km analysis, 2) a medium-resolution 275-km analysis, and 3) a coarse-resolution 381-km analysis.

The results indicate that numerical simulations of mesoscale phenomena embedded in the marine atmospheric boundary layer (MABL) in the vicinity of the Gulf Stream are very sensitive to the SST distribution. The total (sensible and latent) average heat fluxes differ by less than 15% among the three experiments; however, the mesoscale distributions of the oceanic surface heat fluxes differ substantially. As a result of large differences in the lower-tropospheric diabatic heating, significant dissimilarities occur among the three experiments in terms of the intensity and movement of the north-wall MABL front, MABL structure, coastal front, cyclone, and precipitation. The maximum values of diagnosed quantities (e.g., vorticity, divergence, thermal gradients, and frontogenesis) in the vicinity of the Gulf Stream vary by as much as a factor of 8 among the three simulations. Also, the lower-tropospheric geostrophic forcing along the coast is relatively weak in the two simulations that used lower-resolution SST analyses. This weak geostrophic forcing in the lower-resolution SST experiments results in the development of a low-level jet that is weaker than observed and simulated in the experiment with the high-resolution analysis.

Among the three experiments, the high-resolution SST analysis simulation best captures the analyzed intensity, structure, and movement of the mesoscale coastal phenomena. Thus, the use of high-resolution SST analyses in research and operational mesoscale models may be essential in some cases for the accurate prediction of coastal cyclones and their associated mesoscale structures.

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James D. Doyle
and
Thomas T. Warner

Abstract

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the structure and dynamics of three low-level jets (LLJs) observed during the second intensive observation period of the Genesis of Atlantic Lows Experiment: 1) a Piedmont LLJ along the east slope of the Appalachians, 2) a coastal LLJ (the focus of this study) along the Carolina coastline, and 3) an LLJ to the rein of a cold-frontal system positioned over the Gulf Stream. Geostrophic forcing was important for the formation of the LLJs. Shallow local baroclinity near the top of the cold dome associated with the cold air dammed to the east of the Appalachian Mountains forced the Piedmont LLJ. An analysis of the model momentum tendencies reveals that the coastal LLJ developed and was maintained by strong geostrophic forcing associated with the coastal baroclinic zone, and its strength was modulated by strong, inertial accelerations. Significant horizontal structure in the coastal LLJ developed during the daytime as a result of the different vertical mixing properties associated with continental and maritime parcel source regions.

Model sensitivity experiments indicate that diabatic processes substantially influence the evolution of the coastal and cold-frontal LLJs, Latent heating associated with banded precipitation over the Gulf Strum to the rear of the front was the primary Forcing mechanism for the frontal LLJ. Sensible heating within the marine atmospheric boundary layer acted to enhance the coastal baroclinic zone and low-level geostrophic forcing, and to subsequently strengthen the coastal LLJ. Cold-air damming and strong lower-tropospheric sensible and latent heating in the vicinity of the Gulf Stream, which frequently occur during autumn and winter months along the East Coast, combine to produce a favorable mesoscale environment for LLJ formation with a wind direction parallel to the coastline.

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James D. Doyle
and
Thomas T. Warner

Abstract

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the structure and dynamics of coastal frontogenesis and mesoscale cyclogenesis observed during intensive observation period 2 (IOP 2) of the Genesis of Atlantic lows Experiment (GALE). The model accurately simulates many of the observed mesoscale Features including cold-air damming to the cast of the Appalachian Mountains, a coastal trough, coastal frontogenesis, and mesoscale cyclogenesis.

The coastal front becomes apparent approximately 6 h after the formation of a coastal trough in the vicinity of the Gulf Stream. An analysis of the model results indicates that both latent beating from banded precipitation over the Gulf Stream and surface sensible heating contribute to trough development. The deformation resulting from the isallobaric accelerations, associated with the pressure changes that occur as the coastal trough forms, initiates the coastal frontogenesis. Numerical sensitivity tests reveal that the diabatic processes dominate the coastal trough and front development. Initially, the frontogenetic effects of the deformation over the Gulf Stream are opposed by the frontolytic differential diabatic effects. The frontogenctic effects of differential diabatic heating at the coastline promote the westward movement of the northern portion of the front. With this westward movement of the coastal front, the deformation and diabatic effects act in concert to significantly strengthen the baroclinic zone.

A small-scale weak cyclone develops along the coastal front as a result of the strong low-level diabatic forcing associated with intense marine atmospheric boundary layer sensible heating and latent heating from copious precipitation. The mesoscale cyclone is characterized by a warm-core structure, with areas of ascent, cyclonic vorticity, and convergence confined to the lowest 3 km of the atmosphere. As the coastal cyclone moves northward along the coastal front, the baroclinic zone weakens substantially to its rear due to diabatic heating of the postfrontal air mass and strengthening westerlies to the rear of the cyclone.

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James D. Doyle
and
Thomas T. Warner

Abstract

During Intensive Observation Period 2 of the Genesis of Atlantic Lows Experiment, a number of mesoscale phenomena were observed with special and conventional observing systems over the land and coastal waters. This study involved analysis of these data for the period 24–26 January 1986 in order to define the structure and dynamics of three features: the coastal front; a shallow cyclone that propagated along the coastal front, modifying it as it moved northward; and a low-level jet that formed in the strong coastal pressure-gradient field.

The coastal front formed in an existing pressure trough over the Gulf Stream as a result of both ageostrophic deformation and differential diabatic heating. There existed considerable variability in the frontal strength and position on both the mesoalpha and mesobeta scales. The level of strongest frontogenesis was near the surface, with frontolysis calculated above 950 mb.

The marine atmospheric boundary layer (MABL) over the Gulf Stream was conducive to cyclone formation. Latent and sensible heat fluxes of up to 800 W m−2 and 400 W m−2 respectively, were calculated early in the study period, and a deep, moist conditionally unstable boundary layer was present. Calculation of the vorticity tendency associated with the sensible heating yielded a narrow band of positive values to the east of the coastline. As a weak midtropospheric wave reached this favorable region to the cut of Florida, a shallow cyclone formed along the coastal front. As the cyclone tracked northeastward along the front, geostrophic deformation ahead of it strengthened the front while strong cold-air advection to its rear displaced the coastal front to the east, leaving behind a dry, stable MABL with low-level, cold-air advection and weak descent. As the cyclone moved northward along the front, conditionally unstable, moist, low-level air ahead was forced by the southeasterly flow to rise along the coastal front and its extension over the cold air near the coastline, causing enhanced precipitation.

A low-level northeasterly jet was also observed over the Carolinas, and formed as a result of the strong low- level pressure gradient created by the proximity of the cold continental air over land and the warm air of the Gulf Stream MABL near the coast. This jet, with a maximum near 960 mb, showed a diurnal variation of up to 20 m s−1 which likely resulted from day/night variations in mixing at jet level, an inertial oscillation with the frictional decoupling of the low-level flow at sunset, and isallobaric accelerations.

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James D. Doyle
and
Thomas T. Warner

Abstract

During the Intensive Observation Period 2 of the Genesis of Atlantic Lows Experiment a persistent, diurnally varying, northeasterly low-level jet (LLJ) was observed along the Carolina coastal plain. Nocturnal maxima of over 20 m s−1 were observed near 960 mb. The daytime speed reduction varied considerably from a 2–5 m s−1 decrease in extreme eastern North Carolina to a 10–16 m s−1 decrease at locations to the west along the coastal plain. An intense coastal baroclinic zone, associated with cold air dammed to the east of the Appalachian Mountains and the warm marine-atmospheric boundary layer over the Gulf Stream, resulted in a northeasterly low-level geostrophic wind maximum near the surface almost parallel to the coast.

A simulation of the LLJ evolution using a one-dimensional planetary boundary layer model (Zhang and Anthes 1982) indicates that the initial acceleration of the LLJ was controlled by the increasing low-level geostrophic wind speed. The large daytime speed reduction resulted from a rapid increase in the frictional stress at the jet level. The LLJ was reestablished the following evening when the nocturnal inversion developed and the ageostrophic component, which increased substantially during the daytime, rotated to a direction nearly parallel to the jet.

Sensitivity experiments indicate that a specific geostrophic wind velocity profile was necessary to produce many of the observed Carolina LLJ characteristics. The LLJ was insensitive to separate reasonable changes in the roughness length, moisture availability, albedo, and thermal inertia, however, when the surface parameters were simultaneously changed to correspond to a surface covered by snow, a temporally continuous LLJ structure resulted. A maritime source region influenced the boundary layer in extreme eastern North Carolina; however, locations to the west had more of a continental source region. Thus, the observed spatial variations in the daytime Carolina LLJ structure may have been a result of upwind differences in the roughness length and diurnal mixing effects.

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Qingfang Jiang
and
James D. Doyle

Abstract

The impact of moist processes on mountain waves over Sierra Nevada Mountain Range is investigated in this study. Aircraft measurements over Owens Valley obtained during the Terrain-induced Rotor Experiment (T-REX) indicate that mountain waves were generally weaker when the relative humidity maximum near the mountaintop level was above 70%. Four moist cases with a RH maximum near the mountaintop level greater than 90% have been further examined using a mesoscale model and a linear wave model. Two competing mechanisms governing the influence of moisture on mountain waves have been identified. The first mechanism involves low-level moisture that enhances flow–terrain interaction by reducing windward flow blocking. In the second mechanism, the moist airflow tends to damp mountain waves through destratifying the airflow and reducing the buoyancy frequency. The second mechanism dominates in the presence of a deep moist layer in the lower to middle troposphere, and the wave amplitude is significantly reduced associated with a smaller moist buoyancy frequency. With a shallow moist layer and strong low-level flow, the two mechanisms can become comparable in magnitude and largely offset each other.

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