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Xuyang Ge, Tim Li, and Melinda S. Peng

Abstract

The genesis of Typhoon Prapiroon (2000), in the western North Pacific, is simulated to understand the role of Rossby wave energy dispersion of a preexisting tropical cyclone (TC) in the subsequent genesis event. Two experiments are conducted. In the control experiment (CTL), the authors retain both the previous typhoon, Typhoon Bilis, and its wave train in the initial condition. In the sensitivity experiment (EXP), the circulation of Typhoon Bilis was removed based on a spatial filtering technique of Kurihara et al., while the wave train in the wake is kept. The comparison between these two numerical simulations demonstrates that the preexisting TC impacts the subsequent TC genesis through both a direct and an indirect process. The direct process is through the conventional barotropic Rossby wave energy dispersion, which enhances the low-level wave train, the boundary layer convergence, and the convection–circulation feedback. The indirect process is through the upper-level outflow jet. The asymmetric outflow jet induces a secondary circulation with a strong divergence tendency to the left-exit side of the outflow jet. The upper-level divergence boosts large-scale ascending motion and promotes favorable environmental conditions for a TC-scale vortex development. In addition, the outflow jet induces a well-organized cyclonic eddy angular momentum flux, which acts as a momentum forcing that enhances the upper-level outflow and low-level inflow and favors the growth of the new TC.

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Lei Shi, Ge Peng, and John J. Bates

Abstract

High-latitude ocean surface air temperature and humidity derived from intersatellite-calibrated High-Resolution Infrared Radiation Sounder (HIRS) measurements are examined. A neural network approach is used to develop retrieval algorithms. HIRS simultaneous nadir overpass observations from high latitudes are used to intercalibrate observations from different satellites. Investigation shows that if HIRS observations were not intercalibrated, then it could lead to intersatellite biases of 1°C in the air temperature and 1–2 g kg−1 in the specific humidity for high-latitude ocean surface retrievals. Using a full year of measurements from a high-latitude moored buoy site as ground truth, the instantaneous (matched within a half-hour) root-mean-square (RMS) errors of HIRS retrievals are 1.50°C for air temperature and 0.86 g kg−1 for specific humidity. Compared to a large set of operational moored and drifting buoys in both northern and southern oceans greater than 50° latitude, the retrieval instantaneous RMS errors are within 2.6°C for air temperature and 1.4 g kg−1 for specific humidity. Compared to 5 yr of International Maritime Meteorological Archive in situ data, the HIRS specific humidity retrievals show less than 0.5 g kg−1 of differences over the majority of northern high-latitude open oceans.

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Xuyang Ge, Tim Li, and Melinda Peng

Abstract

A set of idealized experiments using the Weather Research and Forecasting model (WRF) were designed to investigate the impacts of a midlevel dry air layer, vertical shear, and their combined effects on tropical cyclone (TC) development. Compared with previous studies that focused on the relative radial position of dry air with no mean flow, it is found that the combined effect of dry air and environmental vertical shear can greatly affect TC development. Moreover, this study indicates the importance of dry air and vertical shear orientations in determining the impact. The background vertical shear causes the tilting of an initially vertically aligned vortex. The shear forces a secondary circulation (FSC) with ascent (descent) in the downshear (upshear) flank. Hence, convection tends to be favored on the downshear side. The FSC reinforced by the convection may overcome the shear-induced drifting and “restore” the vertical alignment. When dry air is located in the downshear-right quadrant of the initial vortex, the dry advection by cyclonic circulation brings the dry air to the downshear side and suppresses moist convection therein. Such a process disrupts the “restoring” mechanism associated with the FSC and thus inhibits TC development. The sensitivity experiments show that, for a fixed dry air condition, a marked difference occurs in TC development between an easterly and a westerly shear background.

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George R. Halliwell Jr., Donald B. Olson, and Ge Peng

Abstract

Recent studies suggest that eddy Properties are significantly influenced by the mean current shear associated with the western Sargasso Sea subtropical frontal zone (SFZ). Between 19° and 34°N, the mean density structure is characterized by three layers separated by a climatologically permanent upper (“seasonal”) thermocline that shoals, and a lower (main) thermocline that deepens, toward the north; the thermoclines are separated by the wedge-shaped southern part of the subtropical mode water pool. The SFZ is evident as a zonal band between about 26° and 32°N where subtropical frontogenesis between the westerlies and trades enhances the slope of the mean seasonal thermocline. Classical linear as well as more recent nonlinear stability theories predict that the mean SFZ flow should be unstable. The linear eigenvalue problem suggests that the most unstable perturbations have wavelengths between 150 and 200 km. Analysis of a channel version of the Miami isopycnic-coordinate primitive equation numerical model verified these predictions and also characterized the further nonlinear evolution of the eddy field. As nonlinear effects become increasingly important, the eddies with wavelengths of 150-200 km predicted by linear theory that initially dominate the model fields continue to grow as the wavenumber spectrum becomes saturated. Following this, the eddies stop growing and energy shifts to longer wavelengths as predicted by geostrophic turbulence theory and observed in Geosat altimeter data. Zonal bands of mean flow also appear after the onset of the nonlinear energy cascade to larger scales as predicted by theory. Model results suggest baroclinic energy conversion and atmospheric forcing contribute roughly equally to eddy variability within the SFZ. Over three-fourths of the available potential energy released by the instability is extracted from the model seasonal thermocline. This agrees with the strong dependence of the strength of the instability on seasonal thermocline slope predicted by linear theory, and also agrees with the concentration of eddy potential energy within the seasonal thermocline revealed by analysis of historical XBT data. This may be one reason why clear evidence of baroclinic instability in the Sargasso Sea SFZ was not obtained from earlier moored measurements in this region (e.g., MODE); measurements in the main thermocline were emphasized.

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Xuyang Ge, Tim Li, Yuqing Wang, and Melinda S. Peng

Abstract

The three-dimensional (3D) Rossby wave energy dispersion of a tropical cyclone (TC) is studied using a baroclinic primitive equation model. The model is initialized with a symmetric vortex on a beta plane in an environment at rest. The vortex intensifies while becoming asymmetric and moving northwestward because of the beta effect. A synoptic-scale wave train forms in its wake a few days later. The energy-dispersion-induced Rossby wave train has a noticeable baroclinic structure with alternating cyclonic–anticyclonic–cyclonic (anticyclonic–cyclonic–anticyclonic) circulations in the lower (upper) troposphere.

A key feature associated with the 3D wave train development is a downward propagation of the relative vorticity and kinetic energy. Because of the vertical differential inertial stability, the upper-level wave train develops faster than the lower-level counterpart. The upper anticyclonic circulation rapidly induces an intense asymmetric outflow jet in the southeast quadrant, and then further influences the lower-level Rossby wave train. On one hand, the outflow jet exerts an indirect effect on the lower-level wave train strength through changing TC intensity and structure. On the other hand, it triggers downward energy propagation that further enhances the lower-level Rossby wave train. A sudden removal of the diabatic heating may initially accelerate the energy dispersion through the increase of the radius of maximum wind and the reduction of the lower-level inflow. The latter may modulate the group velocity of the Rossby wave train through the Doppler shift effect. The 3D numerical results illustrate more complicated Rossby wave energy dispersion characteristics than 2D barotropic dynamics.

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Eric A. Hendricks, Melinda S. Peng, Xuyang Ge, and Tim Li

Abstract

A dynamic initialization scheme for tropical cyclone structure and intensity in numerical prediction systems is described and tested. The procedure involves the removal of the analyzed vortex and, then, insertion of a new vortex that is dynamically initialized to the observed surface pressure into the numerical model initial conditions. This new vortex has the potential to be more balanced, and to have a more realistic boundary layer structure than by adding synthetic data in the data assimilation procedure to initialize the tropical cyclone in a model. The dynamic initialization scheme was tested on multiple tropical cyclones during 2008 and 2009 in the North Atlantic and western North Pacific Ocean basins using the Naval Research Laboratory’s tropical cyclone version of the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS-TC). The use of this initialization procedure yielded significant improvements in intensity forecasts, with no degradation in track performance. Mean absolute errors in the maximum sustained surface wind were reduced by approximately 5 kt for all lead times up to 72 h.

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Ge Peng, Christopher N. K. Mooers, and Hans C. Graber

Abstract

Thirteen-month records for the period of April 1994–April 1995 from eight (out of nine) Coastal-Marine Automatic Network (C-MAN) stations in south Florida are analyzed statistically to study alongshore variability of observed atmospheric variables. The surface variables largely are statistically homogeneous and coherent along the Straits of Florida. The maximum correlation for hourly wind components between adjacent stations (separated alongshore by 30–117 km) ranges from 0.9 to 0.75, respectively. However, there is a lack of coverage in the cross-shore direction; hence, a redistribution of C-MAN stations in the cross-shore direction should be considered to provide better spatial coverage of surface atmospheric variables in the south Florida region.

Surface winds from the National Centers for Environmental Prediction (NCEP) 80-km grid, η (Eta) Model analysis for the same period are compared statistically with observations from an air–sea interaction buoy and a C-MAN station in the south Florida coastal region. The η winds represent the low-frequency winds (periods between 3 days and 3 weeks) fairly well (e.g., the coherence exceeds 0.8 and the phase difference is less than 15°) but generally are weaker in magnitude than are the observed winds. The difference can be up to 2 m s−1 for the monthly mean and 1 m s−1 for the seasonal mean. The histogram of the η winds in winter has a single large peak instead of multiple peaks as occur in those of the observed winds. Southward bias in the η winds exists in summer.

The η Model simulates well the flow patterns of a tropical cyclone and an extratropical cyclone on the regional scale but lacks local spatial variability. As demonstrated, local spatial variability can be represented better by a blend of model and observed winds than by either the model-based or observed local surface winds alone.

These issues need to be reexamined periodically with upgraded versions of NCEP’s operational models.

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Xuyang Ge, Ziyu Yan, Melinda Peng, Mingyu Bi, and Tim Li

Abstract

The impact of different vertical structures of a nearby monsoon gyre (MG) on a tropical cyclone (TC) track is investigated using idealized numerical simulations. In the experiment with a relatively deeper MG, the TC experiences a sharp northward turn at a critical point when its zonal westward-moving speed slows down to zero. At the same time, the total vorticity tendency for the TC wavenumber-1 component nearly vanishes as the vorticity advection by the MG cancels the vorticity advection by the TC. At this point, the TC motion is dominated by the beta effect, as in a no-mean-flow environment, and takes a sharp northward turn. In contrast, the TC does not exhibit a sharp northward turn with a shallower MG nearby. In the case with a deeper MG, a greater relative vorticity gradient of the MG promotes a quicker attraction between the TC and MG through the vorticity segregation process. In addition, a larger outer size of the TC also favors a faster westward propagation from its initial position, thus having more potential to collocate with the MG. Once the coalescence is in place, the Rossby wave energy dispersion associated with the TC and MG together is enhanced and rapidly strengthens the southwesterly flow on the eastern flank of both systems. The steering flow from both the beta gyre and the Rossby wave dispersion leads the TC to take a sharp northward track when the total vorticity tendency is at its minimum. This study indicates the importance of good representations of the TC structure and its nearby environmental flows in order to accurately predict TC motions.

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Shengjun Zhang, Tim Li, Xuyang Ge, Melinda Peng, and Ning Pan

Abstract

A combined tropical cyclone dynamic initialization–three-dimensional variational data assimilation scheme (TCDI–3DVAR) is proposed. The specific procedure for the new initialization scheme is described as follows. First, a first-guess vortex field derived from a global analysis will be spun up in a full-physics mesoscale regional model in a quiescent environment. During the spinup period, the weak vortex is forced toward the observed central minimum sea level pressure (MSLP). The so-generated balanced TC vortex with realistic MSLP and a warm core is then merged into the environmental field and used in the subsequent 3DVAR data assimilation. The observation system simulation experiments (OSSEs) demonstrate that this new TC initialization scheme leads to much improved initial MSLP, warm core, and asymmetric temperature patterns compared to those from the conventional 3DVAR scheme. Forecasts of TC intensity with the new initialization scheme are made, and the results show that the new scheme is able to predict the “observed” TC intensity change, compared to runs with the conventional 3DVAR scheme or the TCDI-only scheme. Sensitivity experiments further show that the intensity forecasts with knowledge of the initial MSLP and wind fields appear more skillful than do the cases where the initial MSLP, temperature, and humidity fields are known. The numerical experiments above demonstrate the potential usefulness of the proposed new initialization scheme in operational applications. A preliminary test of this scheme with a navy operational model shows encouraging results.

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Ziyu Yan, Xuyang Ge, Zhuo Wang, Chun-Chieh Wu, and Melinda Peng

Abstract

Typhoon Jongdari (2018) had an unusual looping path before making landfall in Japan, which posed a forecasting challenge for operational numerical models. The impacts of an upper-tropospheric cold low (UTCL) on the track and intensity of Jongdari are investigated using numerical simulations. The storm track and intensity are well simulated in the control experiment using the GFS analysis as the initial and boundary conditions. In the sensitivity experiment (RCL), the UTCL is removed from the initial-condition fields using the piecewise potential vorticity inversion (PPVI), and both the track and intensity of Jongdari change substantially. The diagnosis of potential vorticity tendency suggests that horizontal advection is the primary contributor for storm motion. Flow decomposition using the PPVI further demonstrates that the steering flow is strongly affected by the UTCL, and the looping path of Jongdari results from the Fujiwhara interaction between the typhoon and UTCL. Jongdari first intensifies and then weakens in the control experiment, consistent with the observation. In contrast, it undergoes a gradual intensification in the RCL experiment. The UTCL contributes to the intensification of Jongdari at the early stage by enhancing the eddy flux convergence of angular momentum and reducing inertial stability, and it contributes to the storm weakening via enhanced vertical wind shear at the later stage when moving closer to Jongdari. Different sea surface temperatures and other environmental conditions along the different storm tracks also contribute to the intensity differences between the control and the RCL experiments, indicating the indirect impacts of the UTCL on the typhoon intensity.

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