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Mengyuan Ma
,
Melinda S. Peng
,
Tim Li
, and
Lijuan Wang

Abstract

The unusual movement of Typhoon Lionrock (2016) that posed great challenges for operational numerical predictions was investigated. Analysis of the steering flow at different levels shows that Lionrock’s southwestward motion before 25 August was mainly controlled by the upper-level steering, and the dominant steering shifted to lower levels as the storm turned northeastward abruptly afterward. To examine the influence of the environmental flow on this major turning of Lionrock, three numerical simulations are conducted using the Weather Research and Forecasting (WRF) Model with different starting times. The study indicates that the initial southwestward movement of Lionrock is attributed to the westward extension of the mid- to upper-level subtropical high, and the later turning to northeast is caused by the low-level southwesterly flow associated with the monsoon gyre northeast of Lionrock. In an experiment in which the monsoon gyre is removed from the initial and boundary fields, Lionrock continues its southwestward movement without turning northeastward. This result suggests that the transition of the steering from high to low levels plays a crucial role in the major turning of Lionrock. More sensitivity experiments with modifications of the initial and/or the boundary conditions indicate a low predictability of Lionrock’s major turning.

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R. T. Williams
,
Melinda S. Peng
, and
D. A. Zankofski

Abstract

The hydrostatic Boussinesq equations are used to simulate the passage of fronts over a two-dimensional mountain in a cyclic domain. The fronts are forced by a confluent, periodic deformation field that moves with the uniform mean flow over the mountain. The initial conditions are selected to give a cold front confined to the lower part of the domain. Fourth-order diffusion terms are included in the numerical model to control energy cascade to the grid size scale. A numerical frontogenesis experiment with no topography produces a realistic surface front in about two days. Numerical solutions for flow over the mountain with no front are found by integrating the equations from the initial conditions, which are semigeostrophic steady-state solutions. Various mountains are considered that have the same height but different widths. The numerical solutions for wide mountains remain close to the semigeostrophic initial conditions, while for narrower mountains vertically propagating waves and a hydraulic jump develop on the lee side of the mountain. The frontal solution and the mountain solution are combined to produce the initial conditions for the basic experiments. The numerical solutions show reduced frontogenesis on the upwind slope and increased frontogenesis on the lee slope. This behavior is caused by the mountain-forced divergence on the upwind side and convergence on the lee side in agreement with the semigeostrophic solution of Zehnder and Bannon. Further experiments with no deformation forcing are carried out to correspond to the semigeostrophic passive scalar studies of Blumen and Gross. A passive scalar that represents the perturbation potential temperature is advected with the mountain solution. The frontal scale, based on the tracer field, increases on the upwind side until it reaches a maximum at the top and then decreases on the lee side, back to its original value as the front moves away from the mountain. The numerical solutions for the interactive potential temperature field have a similar behavior, although some additional blocking effects are present. For the narrower mountains the frontal structure is distorted by the gravity waves on the lee side of the mountain. These solutions resemble those of Schumann for smaller-scale mountains.

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Shang-Wu Li
,
Melinda S. Peng
, and
R. T. Williams

Abstract

The objective of this study is to investigate mountain effects on a frontal system in three dimensions. The frontal system is developed from the most unstable Eady wave in a baroclinic state without a mountain. The developed frontal system is then introduced into a new model domain that contains mountains with different sizes, shapes, and orientations. In general, it is found that the cold front experiences a weakening on the upwind slope and strengthening on the downwind slope of a mountain. The locations of these upwind and downwind sides are determined by the horizontal winds associated with the front. Before the front reaches a mountain, the prevailing wind impinging on the mountain is the prefrontal southwesterly. After the front reaches the top of the mountain, the impinging wind shifts to be the postfrontal northwesterly. Therefore, mountain-induced fronto-genetic forcing by these winds varies spatially as the front passes the mountain. When the front moves down the slope, it speeds up and the frontal deformation is then caused by the strong advection over the northern part of the mountain. After the front has moved away from the mountain, its original horizontal structure and location are restored. The frontogenetic forcing is dominated mainly by the convergence-divergence associated with the flow over the mountain. The front experiences major intensification when it is in the leeside convergence zone. As the front moves farther downstream, it enters the divergence zone and its intensity is reduced. When the front has moved away from the influence of the mountain, its intensity returns approximately to its original level irrespective of the mountain's size and shape. The postfrontal winds contribute to the strong convergence, which causes enhanced lee frontogenesis. For an east-west oriented elliptic mountain that resembles the Alps, the Ieeside downslope wind induced by the postfrontal flow is toward the south instead of toward the east as in the other cases. Therefore, the front moves with an average speed that is the same as the front with no mountain. In this case, the front also has a net increase in its intensity for the same period of integration. Simulations with this mountain profile compare favorably with many observed phenomena near the Alps. Overall, the most important factor that determines the net effect of the mountain on a front is its orientation relative, to the front.

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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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Melinda S. Peng
,
Bao-Fong Jeng
, and
R. T. Williams

Abstract

The effect of planetary vorticity gradient (beta) and the presence of a uniform mean flow on the intensification of tropical cyclones are studied using a limited-area primitive equation model. The most intense storm evolves on a constant-f plane with zero-mean flow and its structure is symmetric with respect to the vortex center. The presence of an environmental flow induces an asymmetry in a vortex due to surface friction. When f varies the vortex is distorted by the beta gyres. Fourier analysis of the wind field shows that a deepening cyclone is associated with a small asymmetry in the low-level wavenumber-one wind field. A small degree of asymmetry in the wind field allows a more symmetric distribution of the surface fluxes and low-level moisture convergence. On the other hand, a weakening or nonintensifying cyclone is associated with a larger asymmetry in its wavenumber-one wind field. This flow pattern generates asymmetric moisture convergence and surface fluxes and a phase shift may exist between their maxima. The separation of the surface flux maximum and the lateral moisture convergence reduces precipitation and inhibits the development of the tropical cyclone. Since the orientation of the asymmetric circulation induced by beta is in the southeast to northwest direction, the asymmetry induced by a westerly flow partially cancels the beta effect asymmetry while that of an easterly flow enhances it. Therefore, in a variable-f environment, westerly flows are more favorable for tropical cyclone intensification than easterly flows of the same speed.

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Gan Zhang
,
Zhuo Wang
,
Melinda S. Peng
, and
Gudrun Magnusdottir

Abstract

This study investigates the characteristics of extratropical Rossby wave breaking (RWB) during the Atlantic hurricane season and its impacts on Atlantic tropical cyclone (TC) activity. It was found that RWB perturbs the wind and moisture fields throughout the troposphere in the vicinity of a breaking wave. When RWB occurs more frequently over the North Atlantic, the Atlantic main development region (MDR) is subject to stronger vertical wind shear and reduced tropospheric moisture; the basinwide TC counts are reduced, and TCs are generally less intense, have a shorter lifetime, and are less likely to make landfalls. A significant negative correlation was found between Atlantic TC activity and RWB occurrence during 1979–2013. The correlation is comparable to that with the MDR SST index and stronger than that with the Niño-3.4 index. Further analyses suggest that the variability of RWB occurrence in the western Atlantic is largely independent of that in the eastern Atlantic. The RWB occurrence in the western basin is more closely tied to the environmental variability of the tropical North Atlantic and is more likely to hinder TC intensification or reduce the TC lifetime because of its proximity to the central portion of TC tracks. Consequently, the basinwide TC counts and the accumulated cyclone energy have a strong correlation with western-basin RWB occurrence but only a moderate correlation with eastern-basin RWB occurrence. The results highlight the extratropical impacts on Atlantic TC activity and regional climate via RWB and provide new insights into the variability and predictability of TC activity.

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Eric A. Hendricks
,
Melinda S. Peng
,
Bing Fu
, and
Tim Li

Abstract

Composite analysis is used to examine environmental and climatology and persistence characteristics of tropical cyclones (TCs) undergoing different intensity changes in the western North Pacific (WPAC) and North Atlantic (ATL) ocean basins. Using the cumulative distribution functions of 24-h intensity changes from the 2003–08 best-track data, four intensity change bins are defined: rapidly intensifying (RI), intensifying, neutral, and weakening. The Navy Operational Global Atmospheric Prediction System daily 0000 and 1200 UTC global analysis and Tropical Rainfall Measuring Mission Microwave Imager data are then used as proxies for the real atmosphere, and composites of various environmental fields believed relevant to TC intensity change are made in the vicinity of the TCs. These composites give the average characteristics near the TC, prior to undergoing a given intensity change episode.

For the environmental variables, statistically significant differences are examined between RI storms and the other groups. While some environmental differences were found between RI and weakening/neutral TCs in both basins, an interesting result from this study is that the environment of RI TCs and intensifying TCs is quite similar. This indicates that the rate of intensification is only weakly dependent on the environmental conditions, on average, provided the environment is favorable. Notable exceptions were that in the WPAC, RI events occurred in environments with significantly larger conditional instability than intensifying events. In the ATL, RI events occurred in environments with weaker deep-layer shear than intensifying events. An important finding of this work is that SSTs are similar between intensifying and rapidly intensifying TCs, indicating that the rate of intensification is not critically dependent on SST.

The TCs in both basins were more intense prior to undergoing an RI episode than an intensifying or neutral episode. In the WPAC, the three groups had similar translational speeds and headings, and average initial position. In the ATL, RI storms were located farther south than intensifying and neutral storms, and had a larger translational speed and a more westward component to the heading.

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Melinda S. Peng
,
Bing Fu
,
Tim Li
, and
Duane E. Stevens

Abstract

This study investigates the characteristic differences of tropical disturbances that eventually develop into tropical cyclones (TCs) versus those that did not, using global daily analysis fields of the Navy Operational Global Atmospheric Prediction System (NOGAPS) from the years 2003 to 2008. Time filtering is applied to the data to extract tropical waves with different frequencies. Waves with a 3–8-day period represent the synoptic-scale disturbances that are representatives as precursors of TCs, and waves with periods greater than 20 days represent the large-scale background environmental flow. Composites are made for the developing and nondeveloping synoptic-scale disturbances in a Lagrangian frame following the disturbances. Similarities and differences between them are analyzed to understand the dynamics and thermodynamics of TC genesis. Part I of this study focuses on events in the North Atlantic, while Part II focuses on the western North Pacific.

A box difference index (BDI), accounting for both the mean and variability of the individual sample, is introduced to subjectively and quantitatively identify controlling parameters measuring the differences between developing and nondeveloping disturbances. Larger amplitude of the BDI implies a greater possibility to differentiate the difference between two groups. Based on their BDI values, the following parameters are identified as the best predictors for cyclogenesis in the North Atlantic, in the order of importance: 1) water vapor content within 925 and 400 hPa, 2) rain rate, 3) sea surface temperature (SST), 4) 700-hPa maximum relative vorticity, 5) 1000–600-hPa vertical shear, 6) translational speed, and 7) vertically averaged horizontal shear. This list identifies thermodynamic variables as more important controlling parameters than dynamic variables for TC genesis in the North Atlantic. When the east and west (separated by 40°W) Atlantic are examined separately, the 925–400-hPa water vapor content remains as the most important parameter for both regions. The SST and maximum vorticity at 700 hPa have higher importance in the east Atlantic, while SST becomes less important and the vertically averaged horizontal shear and horizontal divergence become more important in the west Atlantic.

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Bing Fu
,
Melinda S. Peng
,
Tim Li
, and
Duane E. Stevens

Abstract

Global daily reanalysis fields from the Navy Operational Global Atmospheric Prediction System (NOGAPS) are used to analyze Northern Hemisphere summertime (June–September) developing and nondeveloping disturbances for tropical cyclone (TC) formation from 2003 to 2008. This is Part II of the study focusing on the western North Pacific (WNP), following Part I for the North Atlantic (NATL) basin. Tropical cyclone genesis in the WNP shows different characteristics from that in the NATL in both large-scale environmental conditions and prestorm disturbances.

A box difference index (BDI) is used to identify parameters in differentiating between the developing and nondeveloping disturbances. In order of importance, they are 1) 800-hPa maximum relative vorticity, 2) rain rate, 3) vertically averaged horizontal shear, 4) vertically averaged divergence, 5) 925–400-hPa water vapor content, 6) SST, and 7) translational speed. The study indicates that dynamic variables are more important in TC genesis in the WNP, while in Part I of the study the thermodynamic variables are identified as more important in the NATL. The characteristic differences between the WNP and the NATL are compared.

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

Abstract

Three different dynamic initialization schemes for tropical cyclone (TC) prediction in numerical prediction systems are described and evaluated. The first scheme involves the removal of the analyzed vortex, followed by the insertion of a dynamically initialized vortex into the model analyses. This scheme is referred to as the tropical cyclone dynamic initialization scheme (TCDI) because the TC component is nudged to the observed surface pressure in an independent three-dimensional primitive equation model prior to insertion. The second scheme is a 12-h relaxation to the analyses' horizontal momentum before the forecast integration begins, and is called the dynamic initialization (DI) scheme. The third scheme is a combination of the previous two schemes, and is called the two-stage dynamic initialization scheme (TCDI/DI). In the first stage, TCDI is implemented in order to improve the representation of the TC vortex. In the second stage, DI is invoked in order to improve the balance between the inserted TC vortex and its environment. All three dynamic initialization schemes are compared with a control (CNTL) scheme, which creates the initial vortex using synthetic TC observations that match the observed intensity and structure in a three-dimensional variational data assimilation (3DVAR) system. The four schemes are tested on 120 cases in the North Atlantic and western North Pacific basins during 2010 and 2011 using the Naval Research Laboratory's TC prediction model: Coupled Ocean–Atmosphere Mesoscale Prediction System-Tropical Cyclones (COAMPS-TC). It is demonstrated that TCDI/DI performed the best overall with regard to intensity forecasts, reducing the average minimum central pressure error for all lead times by 24.4% compared to the CNTL scheme.

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