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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

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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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Qingfang Jiang and James D. Doyle

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Two topographically generated cirrus plume events have been examined through satellite observations and real-data simulations. On 30 October 2002, an approximately 70-km-wide cirrus plume, revealed by a high-resolution Moderate Resolution Imaging Spectroradiometer (MODIS) image and a series of Geostationary Operational Environmental Satellite (GOES) images, originated from the Sierra Nevada ridge and extended northeastward for more than 400 km. On 5 December 2000, an approximately 400-km-wide cloud plume originated from the Southern Rocky Mountain massif and extended eastward for more than 500 km, the development of which was captured by a series of GOES images. The real-data simulations of the two cirrus plume events successfully capture the presence of these plumes and show reasonable agreement with the MODIS and GOES images in terms of the timing, location, orientation, length, and altitude of these cloud plumes. The synoptic and mesoscale aspects of the plume events, and the dynamics and microphysics relevant to the plume formation, have been discussed. Two common ingredients relevant to the cirrus plume formation have been identified, namely, a relatively deep moist layer aloft with high relative humidity and low temperature (≤−40°C near the cloud top), and strong updrafts over high terrain and slow descent downstream in the upper troposphere associated with terrain-induced inertia–gravity waves. The rapid increase of the relative humidity associated with strong updrafts creates a high number concentration of small ice crystals through homogeneous nucleation. The overpopulated ice crystals decrease the relative humidity, which, in return, inhibits small crystals from growing into large crystals. The small crystals with slow terminal velocities (<0.2 m s−1) can be advected hundreds of kilometers before falling out of the moist layer.

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Qingfang Jiang and James D. Doyle

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The impact of diurnal forcing on a downslope wind event that occurred in Owens Valley in California during the Sierra Rotors Project (SRP) in the spring of 2004 has been examined based on observational analysis and diagnosis of numerical simulations. The observations indicate that while the upstream flow was characterized by persistent westerlies at and above the mountaintop level the cross-valley winds in Owens Valley exhibited strong diurnal variation. The numerical simulations using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) capture many of the observed salient features and indicate that the in-valley flow evolved among three states during a diurnal cycle. Before sunrise, moderate downslope winds were confined to the western slope of Owens Valley (shallow penetration state). Surface heating after sunrise weakened the downslope winds and mountain waves and eventually led to the decoupling of the well-mixed valley air from the westerlies aloft around local noon (decoupled state). The westerlies plunged into the valley in the afternoon and propagated across the valley floor (in-valley westerly state). After sunset, the westerlies within the valley retreated toward the western slope, where the downslope winds persisted throughout the night.

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Qingfang Jiang and James D. Doyle

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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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Qingfang Jiang and James D. Doyle

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The diurnal variation of mountain waves and wave drag associated with flow past mesoscale ridges has been examined using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) and an analytical boundary layer (BL) model. The wave drag exhibits substantial diurnal variation in response to the change in the atmospheric BL characteristics, such as the BL depth, shape factor, and stability. During daytime, a convective BL develops, characterized by a shallow shear layer near the surface and a deep well-mixed layer aloft, both of which tend to decrease the wave drag. As a result, the convective BL could significantly weaken mountain waves and reduce the momentum flux by up to 90%. Near the surface, the flow pattern resembles a potential flow with a surface wind maximum located near the ridge crest. During nighttime, a shallow stable BL develops, and the modulation of wave drag by the stable nocturnal BL is governed by the BL Froude number (Fr). If the BL flow is supercritical, the drag increases as Fr decreases toward unity and reaches the maximum around Fr = 1, where the drag could be several times larger than the corresponding free-slip hydrostatic wave drag. If the BL flow is subcritical because of excessive cooling, the drag decreases with decreasing Froude number and the flow pattern near the surface resembles a typical subcritical solution with the wind maximum located near the ridge crest.

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Qingfang Jiang and James D. Doyle

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The characteristics of gravity waves excited by the complex terrain of the central Alps during the intensive observational period (IOP) 8 of the Mesoscale Alpine Programme (MAP) is studied through the analysis of aircraft in situ measurements, GPS dropsondes, radiosondes, airborne lidar data, and numerical simulations.

Mountain wave breaking occurred over the central Alps on 21 October 1999, associated with wind shear, wind turning, and a critical level with Richardson number less than unity just above the flight level (∼5.7 km) of the research aircraft NCAR Electra. The Electra flew two repeated transverses across the Ötztaler Alpen, during which localized turbulence was sampled. The observed maximum vertical motion was 9 m s−1, corresponding to a turbulent kinetic energy (TKE) maximum of 10.5 m2 s−2. Spectrum analysis indicates an inertia subrange up to 5-km wavelength and multiple energy-containing spikes corresponding to a wide range of wavelengths.

Manual analysis of GPS dropsonde data indicates the presence of strong flow descent and a downslope windstorm over the lee slope of the Ötztaler Alpen. Farther downstream, a transition occurs across a deep hydraulic jump associated with the ascent of isentropes and local wind reversal. During the first transverse, the turbulent region is convectively unstable as indicated by a positive sensible heat flux within the turbulent portion of the segment. The TKE derived from the flight-level data indicates multiple narrow spikes, which match the patterns shown in the diagnosed buoyancy production rate of TKE. The turbulence is nonisotropic with the major TKE contribution from the υ-wind component. The convectively unstable zone is advected downstream during the second transverse and the turbulence becomes much stronger and more isotropic.

The downslope windstorm, flow descent, and transition to turbulence through a hydraulic jump are captured by a real-data Coupled Ocean–Atmosphere Mesoscale Predition System (COAMPS) simulation. Several idealized simulations are performed motivated by the observations of multiscale waves forced by the complex terrain underneath. The simulations indicate that multiscale terrain promotes wave breaking, increases mountain drag, and enhances the downslope winds and TKE generation.

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James D. Doyle and Thomas T. Warner

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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

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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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Martin Weissmann, Andreas Dörnbrack, and James D. Doyle

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A method is presented to compute the spanwise vorticity in polar coordinates from 2D vertical cross sections of high-resolution line-of-sight Doppler wind lidar observations. The method uses the continuity equation to derive the velocity component perpendicular to the observed line-of-sight velocity, which then yields the spanwise vorticity component. The results of the method are tested using a ground-based Doppler lidar, which was deployed during the Terrain-Induced Rotor Experiment (T-REX). The resulting fields can be used to identify and quantify the strength and size of vortices, such as those associated with atmospheric rotors. Furthermore, they may serve to investigate the dynamics and evolution of vortices and to evaluate numerical simulations. A demonstration of the method and comparison with high-resolution numerical simulations reveals that the derived vorticity can explain 66% of the mean-square vorticity fluctuations, has a reasonably skillful magnitude, exhibits no significant bias, and is in qualitative agreement with model-derived vorticity.

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