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Kun Gao and Isaac Ginis

Abstract

Horizontal roll vortices, or rolls, are frequently observed in the hurricane boundary layer (HBL). Previous studies suggest that these rolls can be generated by the inflection point instability of the HBL flow. In this study we investigate the formation of rolls due to this mechanism in the axisymmetric HBL using a numerical approach that explicitly resolves rolls. The effects of mean HBL wind and stratification distributions on rolls are evaluated. We identify two important factors of the mean HBL wind that affect the characteristics of rolls. The dynamical HBL height affects the wavelength of rolls, and the magnitude of the mean wind shear affects the growth rate of rolls. As a result, under neutrally stratified HBL, the wavelength of rolls increases with the radius (out of the radius of maximum wind), while the growth rate of rolls decreases. The stratification also plays an important role in the generation of rolls. The stable stratification suppresses the growth of rolls because of the negative work done by the buoyancy force. Nonuniform stratification with a mixed layer has less suppressing effect on rolls. Rolls can trigger internal waves in the stably stratified layer, which have both vertically propagating and decaying properties. We derive analytical solutions for the internal waves, which relate the properties of the internal waves to the boundary layer rolls. We find the properties of the internal waves are affected by the mixed-layer height.

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Kun Gao and Isaac Ginis

Abstract

In this study, the authors numerically simulate roll vortices (rolls) generated by the inflection-point instability in the hurricane boundary layer (HBL). The approach is based on embedding a two-dimensional high-resolution single-grid roll-resolving model (SRM) at selected horizontal grid points of an axisymmetric HBL model. The results from a set of idealized experiments indicate that the mixed-layer height is an important factor affecting the magnitude of the roll velocities and the structure of the internal waves triggered in the stably stratified layer above. This study reveals the important difference between the roll-induced cross-roll (nearly radial) and along-roll (nearly azimuthal) momentum fluxes: while the cross-roll momentum flux is well correlated to the cross-roll mean wind shear, the along-roll momentum flux is typically not correlated with the along-roll mean wind shear. Therefore, the commonly used K theory in the boundary layer parameterizations cannot reasonably capture the vertical distribution of the roll-induced along-roll momentum flux. Moreover, the authors find that the rolls induce more significant changes in the mean radial wind profile than in the mean azimuthal wind profile. Specifically, rolls reduce the inflow near surface, enhance the inflow at upper levels, and increase the inflow-layer height. Based on a linear dynamical HBL model, the authors find that the impact of rolls on the mean radial wind profile is essentially due to their redistribution effect on the mean azimuthal momentum in the HBL.

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Jordan G. Powers and Kun Gao

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A modeling investigation explores the impacts of the assimilation of satellite-retrieved soundings on forecast error in the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Simulations of the period of the U.S. Air Force’s Contrail Experiment (18–29 September 1995) vary the initialization method and datasets assimilated, the performance of first-guess reanalysis, the imposition of quality control (QC) on the satellite data, and the frequency of the model update cycle. MM5 experiments employing four-dimensional data assimilation (FDDA) are compared with a control experiment without FDDA. In the former, combinations of conventional surface and radiosonde observations and retrieved temperature and moisture soundings from the Defense Meteorological Satellite Program (DMSP) and Television and Infrared Observation Satellite Operational Vertical Sounder (TOVS) satellite instruments are assimilated. Forecast error statistics for the experiments are computed and analyzed. It is found that for retrieved temperatures the DMSP and TOVS sounding datasets used have similar, reasonable accuracy, but for retrieved dewpoints they display significant, and more differing, errors. Overall, the TOVS retrievals obtained are of poorer quality than are the DMSP retrievals. Sensitivity tests reveal that imposing a QC filter on the satellite data prior to assimilation does improve the resultant MM5 simulations. With such QC, it is found that assimilating DMSP and TOVS soundings with the methods used can significantly improve the forecasts of both temperature and moisture variables in the MM5. Model performance, however, can still reflect the relative quality of the satellite retrievals assimilated, with the lower-error DMSP data yielding better simulations than do the TOVS data. Tests exploring the reanalysis of first-guess fields obtained from FDDA show that it does benefit the short-term (0–12 h) forecast but that significant gains diminish thereafter.

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Da-Lin Zhang and Kun Gao

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An intense rear-inflow jet, surface pressure perturbations, and stratiform precipitation associated with a squall line during 10–11 June 1985 are examined using a three-dimensional mesoscale nested-grid model. It is found that the large-scale baroclinity provides favorable and deep rear-to-front flow within the upper half of the troposphere and the mesoscale response to convective forcing helps enhance the trailing extensive rear inflow. However, latent cooling and water loading are directly responsible for the generation of the descending portion of the rear inflow. The role of the rear inflow is generally to produce convergence ahead and divergence behind the system, and thus assist the rapid acceleration of the leading convection when the prestorm environment is convectively favorable and the rapid dissipation of the convection when encountering unfavorable conditions. In this case study, the rear-inflow jet appears to have caused the splitting of the surface wake low as well as the organized rainfall.

As considerable mass within the rear inflow subsides, an intense surface wake low is formed at the back edge of the squall system. This result confirms previous observations that the surface wake low develops hydrostatically as a consequence of adiabatic warming and drying by the descending rear inflow. The wake low is shown to be an end product of complicated reactions involving condensate production, fallout cooling and induced subsiding motion. It does not have any significant effects on the evolution of atmospheric features ahead but contributes to vertical destabilization over the wake region.

The simulation shows that the squall line initially leans downshear and later upshear as the low-level cold pool progressively builds up and the system moves into a convectively stable environment. During the mature stage, there are three distinct airflows associated with the squall system: a leading overturning updraft and an ascending front-to-rear (FTR) current that both are driven by high-θe, air from the boundary layer ahead of the line, and an overturning downdraft carrying low-θe, air from the rear. These features resemble previously published results using nonhydrostatic cloud models. Due to continuous deposit of FTR momentum at the upper levels, the FTR updraft is responsible for the rearward transport of high-θe, air mass for the generation of the trailing stratiform precipitation.

Several sensitivity experiments are conducted. The generation of the descending rear inflow, and the surface and midlevel pressure perturbations are found to be most sensitive to the parameterized moist downdrafts, hydrostatic water loading, evaporative cooling and ice ice microphysics, in that order. Without any one of these model processes, neither the rear inflow reaches the surface nor the surface mesohigh and wake low become well developed. The results illustrate that the descending rear inflow is a product of the dynamic response to the latent-cooling-induced circulation. Different roles of the parameterized versus grid-resolved downdrafts in the development of the descending rear inflow are also discussed.

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Kun Gao and Isaac Ginis

Abstract

Previous theoretical and numerical studies only focused on the formation of roll vortices (rolls) under a stationary and axisymmetric hurricane. The effect of the asymmetric wind structure induced by the storm movement on the roll characteristics remains unknown. In this study, we present the first attempt to investigate the characteristics of linear-phase rolls under a moving hurricane by embedding a linear two-dimensional (2D) roll-resolving model into a 3D hurricane boundary layer model. It is found that the roll horizontal wavelength under the moving hurricane is largely determined by the radial-shear-layer depth, defined as the thickness of the layer with positive radial wind shear. The horizontal distribution of the roll wavelength resembles the asymmetric pattern of the radial-shear-layer depth. Interestingly, the roll growth rate is not only affected by the radial wind shear magnitude alluded to in previous studies but also by the radial-shear-layer depth. A deeper (shallower) radial shear layer tends to decrease (increase) the roll growth rate. Such an effect is due to the presence of the bottom boundary. The bottom boundary constrains the lower-level roll streamlines and reduces the efficiency of rolls in extracting kinetic energy from the radial shear. This effect is more pronounced under a deeper shear layer, which favors the formation of larger-size rolls. This study improves the understanding of the main factors affecting the structure and growth of rolls and will provide guidance for interpreting the spatial distribution of rolls under realistic hurricanes in observations and high-resolution simulations.

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Philip L. Haagenson, Kun Gao, and Ying-Hwa Kuo

Abstract

Perfluorocarbon tracer data collected during the Across North America Tracer Experiment (ANATEX) are used to evaluate different meteorological analysis, simulations, and long-range transport calculations. Three basic types of meteorological analysis and simulation are evaluated: objective analysis of observed data, prognostic simulation with observed lateral boundary conditions, and four-dimensional data assimilation (FDDA). The evaluation is based on 1) the root-mean-square separation between two-dimensional meteorological trajectories (constructed from different analyses or simulations) and surface tracer trajectories and 2) the relationship between the upward displacement of three-dimensional trajectories and the maximum value of the surface tracer concentration. The root-mean-square data indicate that the optimum value of the Newtonian nudging coefficient for the FDDA wind field in the lower troposphere is 6 × 10−4 s−1, and the quality of the prognostic simulation is lower than that for FDDA or the objective analysis, particularly when surface fronts are present. These data also show that trajectory errors, with respect to transport distance, are larger in low-speed wind regimes than in medium- to high-speed regimes, and suggest that the rate of increase of trajectory error decreases with time, but the uncertainty of the rate of increase is quite large during the first 18 h of transit. The three-dimensional trajectories indicate that large-scale upward motion is a mechanism for removal of tracer from the boundary layer, and the strongest correlation between the upward displacement of the trajectory air parcels and the surface tracer concentration is obtained using the FDDA dataset. The overall results suggest that when both the vertical and horizontal components of the wind fields are considered, FDDA (using an appropriate value for the nudging coefficient) is better than the other methodologies.

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Da-Lin Zhang, Kun Gao, and David B. Parsons

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A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-divergence flow structure; 6) the evolution of a traveling 700 mb shortwave; 7) the development of a rear-inflow jet; 8) the leading convective rainfall followed by a transition zone and trailing stratiform precipitation; 9) the observed configuration of front-to-rear relative flow at both upper and lower levels separated by the rear-to-front flow at midlevels; 10) the simulation of “onion-shaped” soundings; 11) the splitting of the wake low; 12) the maintenance and intensification of a mesovortex; 13) the distribution and magnitude of convective and stratiform rainfall; and 14) the diurnal cycle of the planetary boundary layer.

One of the encouraging results is that the model accurately simulates the rear-inflow jet as verified against Doppler windprofiler data after the 18-h integration from essentially synoptic-scale initial conditions. The results confirm the previously proposed hypothesis that the wake low develops hydrostatically as a consequence of adiabatic warming by descending flow entering the squall line within the rear-inflow jet The observed “onion-shaped” soundings are a manifestation of the warming and drying of air within the descending rear inflow jet. It is found that the present wake low is not a transient meso-β scale phenomenon, but has a time scale of more than 50% of the squall line lifetime. Another finding is that the present mesovortex is not produced by latent heat release associated with the squall line but was in existence prior to the model initialization time. The vortex appears to have a significant effect on the distribution of the rainfall associated with the squall line and on the intensity of the rear-inflow jet. Other mesoscale circulation features are also documented in this paper.

This study, along with previous investigations using the model, indicates that the meso-β scale structure and evolution of MCSs under certain synoptic-scale environmental conditions can be well simulated using the standard network observations if compatible grid resolution, reasonable model physics and initial conditions are utilized.

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Kun Gao, Isaac Ginis, James D. Doyle, and Yi Jin

Abstract

In this study, the authors numerically investigate the response of an axisymmetric tropical cyclone (TC) vortex to the vertical fluxes of momentum, heat, and moisture induced by roll vortices (rolls) in the boundary layer. To represent the vertical fluxes induced by rolls, a two-dimensional high-resolution Single-Grid Roll-Resolving Model (SRM) is embedded at multiple horizontal grid points in the mesoscale COAMPS for Tropical Cyclones (COAMPS-TC) model domain. Idealized experiments are conducted with the SRM embedded within 3 times the radius of maximum wind of an axisymmetric TC. The results indicate that the rolls induce changes in the boundary layer wind distribution and cause a moderate (approximately 15%) increase in the TC intensification rate by increasing the boundary layer convergence in the eyewall region and induce more active eyewall convection. The numerical experiments also suggest that the roll-induced tangential momentum flux is most important in contributing to the TC intensification process, and the rolls generated at different radii (within the range considered in this study) all have positive contributions. The results are not qualitatively impacted by the initial TC vortex or the setup of the vertical diffusivity in COAMPS-TC.

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Kun Gao, Lucas Harris, Linjiong Zhou, Morris Bender, and Matthew Morin

Abstract

We investigate the sensitivity of hurricane intensity and structure to the horizontal tracer advection in the Geophysical Fluid Dynamics Laboratory (GFDL) Finite-Volume Cubed-Sphere Dynamical Core (FV3). We compare two schemes, a monotonic scheme and a less diffusive positive-definite scheme. The positive-definite scheme leads to significant improvement in the intensity prediction relative to the monotonic scheme in a suite of 5-day forecasts that mostly consist of rapidly intensifying hurricanes. Notable storm structural differences are present: the radius of maximum wind (RMW) is smaller and eyewall convection occurs farther inside the RMW when the positive-definite scheme is used. Moreover, we find that the horizontal tracer advection scheme affects the eyewall convection location by affecting the moisture distribution in the inner-core region. This study highlights the importance of dynamical core algorithms in hurricane intensity prediction.

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Da-Lin Zhang, John S. Kain, J. Michael Fritsch, and Kun Gao

Abstract

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