A wind radii model using the deviation angle variance technique was applied to 523 tropical cyclones (TCs) in the Western North Pacific region from 1998 to 2018. The model, which uses high resolution satellite cloud images, was used to investigate the impact of the 34-, 50- and 64-kt winds from landfalling TCs. The wind impact analysis showed that the regions most exposed to TC tracks of Taiwan and the Philippines experienced 34-kt winds annually, and that these winds extended far into the mainland (up to approximately 500 km). The 50- and 64-kt winds occurred less frequently along the length of the Philippines but only extended down to the center of Vietnam and reached around 100 to 200 km inland

Significant research efforts have been devoted to understanding river-tide interactions in estuaries. However, studies on the impact of monsoon-driven fluctuations of river discharge are limited. Here, the role of varying river discharge on the tidal propagation and tidal limit along the Ganges-Brahmaputra-Meghna Delta (GBMD), a macrotidal estuary subject to seasonal and annual river discharge variations, is investigated. The Delft3D hydrodynamic model is validated and applied to an average flood year condition and nine idealized scenarios covering the typical hydrological conditions. Results reveal that the upper limit of the tidal propagation shifts 75 km upstream during the dry season. The residual water level slope and tidal damping rate increase with river discharge beyond 100 km from the estuary mouth. The balance between the generation and dissipation of quarterdiurnal tides shifts spatially as a result of changes in channel convergence and friction and temporally as a function of river discharge, which controls the total friction in the upper tidal river. The balance between tidal dissipation and generation depends on the residual velocity generated by river discharge and the velocity of the principal tides. The maximal generation of quarterdiurnal tides in the upper GBMD depends on the friction generated from the river-tide interaction. Critical river discharge thresholds produce an optimal condition of dissipation of semidiurnal tides and generation of quarterdiurnal tides through friction at the upper and middle estuary. River discharge above the critical river discharge amount more rapidly dissipates both semidiurnal and quarterdiurnal tides than generates quarterdiurnal tides from nonlinear interactions.

A multiple linear regression wind radii model developed in the North Atlantic basin based on the deviation angle variance technique was applied in the Australian region. The model was used to improve the historic database of 34-, 50- and 64-kt tropical cyclone (TC) wind radii estimates (R34, R50, and R64) for 374 TCs during the geostationary satellite era. Results during 2010-2016 produced quadrant mean absolute errors ranging between 49 and 61 km for the 34-kt radii, between 24 and 37 km for the 50-kt radii and between 20 and 25 km for the 64-kt radii.

The interannual variability of near-coastal eastern North Pacific tropical cyclones is described using a data set of cyclone tracks constructed from U.S. and Mexican oceanic and atmospheric reports for the period 1951-2006. Near-coastal cyclone counts are enumerated monthly, allowing us to distinguish interannual variability during different phases of the May-November tropical cyclone season. In these data more tropical cyclones affect the Pacific coast in May-July, the early months of the tropical cyclone season, during La Nina years, when equatorial Pacific sea surface temperatures are anomalously cool, than during El Nino years. The difference in early season cyclone counts between La Nina and El Nino years was particularly pronounced during the mid-twentieth century epoch when cool equatorial temperatures were enhanced as described by an index of the Pacific Decadal Oscillation. Composite maps from years with high and low near-coastal cyclone counts show that the atmospheric circulation anomalies associated with cool sea surface temperatures in the eastern equatorial Pacific are consistent with preferential steering of tropical cyclones northeastward toward the west coast of Mexico.

The second phase of the North American Monsoon Experiment (NAME) Model Assessment Project (NAMAP2) was carried out to provide a coordinated set of simulations from global and regional models of the 2004 warm season across the North American monsoon domain. This project follows an earlier assessment, called NAMAP, that preceded the 2004 field season of the North American Monsoon Experiment. Six global and four regional models are all forced with prescribed, time-varying ocean surface temperatures. Metrics for model simulation of warm season precipitation processes developed in NAMAP are examined that pertain to the seasonal progression and diurnal cycle of precipitation, monsoon onset, surface turbulent fluxes, and simulation of the low-level jet circulation over the Gulf of California. Assessment of the metrics is shown to be limited by continuing uncertainties in spatially averaged observations, demonstrating that modeling and observational analysis capabilities need to be developed concurrently. Simulations of the core subregion (CORE) of monsoonal precipitation in global models have improved since NAMAP, despite the lack of a proper low-level jet circulation in these simulations. Some regional models run at higher resolution still exhibit the tendency observed in NAMAP to overestimate precipitation in the CORE subregion; this is shown to involve both convective and resolved components of the total precipitation. The variability of precipitation in the Arizona/New Mexico (AZNM) subregion is simulated much better by the regional models compared with the global models, illustrating the importance of transient circulation anomalies (prescribed as lateral boundary conditions) for simulating precipitation in the northern part of the monsoon domain. This suggests that seasonal predictability derivable from lower boundary conditions may be limited in the AZNM subregion.

Traditional spatial decompositions such as application of Empirical Orthogonal Functions and Principal Components Analysis (PCA) of meteorological field data have received wide attention in numerous forecasting and analysis applications. The method has been used to identify the important spatial patterns in meteorological fields. Defining the variability between various cases of tropical cyclones is another application of the method. We are utilizing spatial and temporal data as a tool to differentiate the TCs that complete and fail to complete extratropical transition in the western North Pacific. It has been shown that EOF Analysis is a promising tool in the prediction of reintensification of TCs after ET

This chapter contains sections titled: Introduction Background Approach and Data Used Results and Discussion Concluding Remarks

Analyses and forecasts from a modern data assimilation and modeling system are used to evaluate the impact of a special rawinsonde dataset of 3-h soundings at seven sites interspersed with the seven regular sites along the West Coast (to form a so-called picket fence to intercept all transiting circulations) plus special 6-h rawin-sondes over the National Weather Service Western Region. Whereas four intensive observing periods (IOPs) are available, only two representative IOPs (IOP-3 and IOP-4) are described here. The special observations collected during each 12-h cycle are analyzed with the National Centers for Environmental Prediction (NCEP) Eta Data Assimilation System in a cold start from the NCEP-National Center for Atmospheric Research reanalyses as the initial condition. Forecasts up to 48 h with and without the special picket fence observations are generated by the 32-km horizontal resolution Eta Model with 45 vertical levels.The picket fence observations had little impact in some cases with smooth environmental flow. In other cases, relatively large initial increments were introduced offshore of the picket fence observations. However, these increments usually damped as they translated downstream. During IOP-3, the increments amplified east of the Rocky Mountains after only 24 h. Even though initially small, the increments in IOP-4 grew rapidly to 500-mb height increments similar to 20-25 m with accompanying meridional wind increments of 5-8 m s(-1) that contributed to maxima in shear vorticity. Many of the downstream amplifying circulations had associated precipitation increments similar to6 mm (6 h)(-1) between the control and experimental forecasts. The equitable threat scores against the cooperative station set for the first 24-h forecasts during IOP-3 had higher values at the 0.50- and 0.75-in-thresholds for the picket fence dataset. However, the overall four-IOP equitable threat scores were similar.Although the classical synoptic case was not achieved during the picket fence, these model forecasts suggest that such observations around the coast of the United States would impact the downstream forecasts when added in dynamically unstable regions. An ultimate picket fence of continuous remotely observing systems should be studied further.

The physical mechanisms associated with the transformation stage of the extratropical transition of a tropical cyclone are simulated with a mesoscale model using initial environmental conditions that approximate the mean circulations defined by Klein et al. The tropical cyclone structural changes simulated by the U.S. Navy Coupled Ocean-Atmosphere Model Prediction System mesoscale model during the three steps of transformation compare well with available observations. During step 1 of transformation when the tropical cyclone is just beginning to interact with the midlatitude baroclinic zone, the main environmental factor that affects the tropical cyclone structure appears to be the decreased sea surface temperature. The movement of the tropical cyclone over the lower sea surface temperatures results in reduced surface heat and moisture fluxes, which weakens the core convection and the intensity decreases. During step 2 of transformation, the low-level temperature gradient and vertical wind shear associated with the baroclinic zone begin to affect the tropical cyclone. Main structural changes include the development of cloud-free regions on the west side of the tropical cyclone, and an enhanced "delta'' rain region to the northwest of the tropical cyclone center. Gradual erosion of the clouds and deep convection in the west through south sectors of the tropical cyclone appear to be from mechanically forced subsidence due to the convergence between the midlatitude flow and the tropical cyclone circulation. Whereas the warm core aloft is advected downstream, the mid- to low-level warm core is enhanced by subsidence into the tropical cyclone center, which implies that the low-level cyclonic circulation may continue to be maintained.Step 3 of transformation is the logical conclusion of structural changes that were occurring during steps 1 and 2. Even though the tropical cyclone circulation aloft has dissipated, a broad cyclonic circulation is maintained below 500 mb. Although the low-level warm core is reduced from step 2, it is still significantly stronger than at step 1, and a second warm anomaly is simulated in a region of strong subsidence upshear of the tropical cyclone remnants. Whereas some precipitation is associated with the remnants of the northern eyewall and some cloudiness to the north-northeast, the southern semicircle is almost completely clear of clouds and precipitation.

A series of numerical simulations of tropical cyclones in idealized large-scale environments is performed to examine the effects of vertical wind shear on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic Pennsylvania State University-National Center for Atmospheric Research fifth-generation Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes.When large-scale vertical shears are applied to mature tropical cyclones, the storms quickly develop wavenumber one asymmetries with upward motion and rainfall concentrated on the left side of the shear vector looking downshear, in agreement with earlier studies. The asymmetries develop due to the storm's response to imbalances caused by the shear. The storms in shear weaken with time and eventually reach an approximate steady-state intensity that is well below their theoretical maximum potential intensity. As expected, the magnitude of the weakening increases with increasing shear. All of the storms experience time lags between the imposition of the large-scale shear and the resulting rise in the minimum central pressure. While the lag is at most a few hours when the storm is placed in very strong (15 m s(-1)) shear, storms in weaker shears experience much longer lag times, with the 5 m s(-1) shear case showing no signs of weakening until more than 36 h after the shear is applied. These lags suggest that the storm intensity is to some degree predictable from observations of large-scale shear changes. In all cases both the development of the asymmetries in core structure and the subsequent weakening of the storm occur before any resolvable tilt of the storm's vertical axis occurs.It is hypothesized that the weakening of the storm occurs via the following sequence of events: First, the shear causes the structure of the eyewall region to become highly asymmetric throughout the depth of the storm. Second, the asymmetries in the upper troposphere, where the storm circulation is weaker, become sufficiently strong that air with high values of potential vorticity and equivalent potential temperature are mixed outward rather than into the eye. This allows the shear to ventilate the eye resulting in a loss of the warm core at upper levels, which causes the central pressure to rise, weakening the entire storm. The maximum potential vorticity becomes concentrated in saturated portions of the eyewall cloud aloft rather than in the eye. Third, the asymmetric features at upper levels are advected by the shear, causing the upper portions of the vortex to tilt approximately downshear. The storm weakens from the top down, reaching an approximate steady-state intensity when the ventilated layer can descend no farther due to the increasing strength and stability of the vortex at lower levels.

The contribution that a mesoscale convective vortex that developed within the circulation of Typhoon Robyn (1993) may have had to a significant tropical cyclone track change is simulated with a mesoscale model using initial conditions that approximate the circulations measured by aircraft during the Tropical Cyclone Motion (TCM-93) field experiment. A dry version of the PSU-NCAR Mesoscare Model is used to first investigate the dynamic aspects of the interaction. A deceleration of about 2 m s(-1) and then more northward movement, similar to that observed for Typhoon Robyn, could have been produced by an interaction with the mesoscale convective vortex of the type modeled in the control run. Sensitivity of the simulated track change is tested for various aspects of the mesoscale vortex and tropical cyclone. It is found that track deflections between 67 and 130 km in 18-24 h could be produced under a variety of realistic scenarios.As the mesoscale vortex is advected around the tropical cyclone by the cyclonic winds, it is also being filamented by the horizontal shear of the tropical cyclone outer winds. Whereas the rate at which the vortex is advected about the tropical cyclone is critical to the amount of curvature of the tropical cyclone track deflection, the timescale of the filamentation of the mesoscale vortex is critical to the longevity of the track deflection, and thus maintenance of the vortex is a crucial factor. For the same tropical cyclone and separation distance, the track deflections are greater for a larger, deeper, and more intense mesoscale vortex. Far the same mesoscale vortex, a variety of track deflections is possible, depending on the outer wind structure of the tropical cyclone due to both the advective effect and to the change in the gradient of vorticity. For a large tropical cyclone, positive vorticity extends farther from the core region, and the curvature of the track deflection is greater than for a smaller tropical cyclone where the vorticity becomes anticyclonic at smaller radii. Although a large initial effect on the tropical cyclone path occurs for small separation distances, the mesoscale vortex is rapidly filamented so that effects on the tropical cyclone track are negligible by 12 h. For larger separation distances, the mesoscale vortex does not filament as rapidly, and a smaller but longer lasting track deflection is simulated.When the control simulation is extended to a beta plane, it is found that the primary contribution to the tropical cyclone track change is still due to the interaction with the mesoscale convective vortex. However, a secondary effect due to a nonlinear interaction between the beta gyres and the mesoscale convective vortex adds a small component of propagation to the tropical cyclone that becomes significant after about 15 h of simulation.

Numerical simulations of tropical-cyclone-like vortices are performed to analyze the effects of unidirectional vertical wind shear and translational flow upon the organization of convection within a hurricane's core region and upon the intensity of the storm. A series of dry and moist simulations is performed using the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model version 5 (MM5) with idealized initial conditions. The dry simulations are designed to determine the patterns of forced ascent that occur as the vortex responds to imposed vertical wind shear and translational flow, and the mechanisms that modulate the vertical velocity field are explored. The moist simulations are initialized with the same initial conditions as the dry runs but with a cumulus parameterization and explicit moisture scheme activated. The moist simulations are compared to the dry runs in order to test the hypothesis that the forced vertical circulation modes modulate the convection and hence latent heat release in the hurricane core, as well as to evaluate the net effect of the imposed environmental flow on the storm intensity and structure.The results indicate that the pattern of convection in the storm's core is strongly influenced by vertical wind shear, and to comparable degree by boundary layer friction. In the early stages of moist simulations, typical of the tropical depression stage, the regions of forced ascent and the mechanisms that cause them ale similar to those in the dry runs. However, once the moist storm runs deepen enough to develop saturation in part of the eyewall, the patterns of vertical motion and associated rainfall differ between the paired dry and moist runs with identical initial conditions. The dry runs tend to produce a strong, deep region of ascent in the sector of the storm that lies downshear right of the center. The moist runs begin similarly, but as the storms intensify they strongly favor upward motion and rainfall downshear left of the center.It appears that the vertical motion patterns in the dry and moist simulations are dominated by similar adiabatic lifting mechanisms prior to the development of partial eyewall saturation. Once the moist runs reach saturation, this adiabatic lifting mechanism no longer occurs due to the latent heat release within the ascending air. Hence, the patterns of forced ascent in the dry runs should be relevant for understanding patterns of convection in loosely organized systems such as tropical depressions, but not in mature hurricanes, The rainfall patterns produced by the moist simulations are in good agreement with recent observational analyses of the relationships between rainfall distribution and vertical wind shear in Atlantic hurricanes.

Five characteristic, low-level, large-scale dynamical patterns associated with tropical cyclogenesis in the western North Pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis. An 8-yr analysis set for the region is used to identify, and create composites for, the five characteristic patterns of monsoon shear line, monsoon confluence region, monsoon gyre, easterly waves, and Rossby energy dispersion. This brings out the common processes that contribute to tropical cyclogenesis within that pattern, which are described in detail.A 3-yr set of satellite data is then used to analyze the mesoscale convective system activity for ail cases of genesis in that period and to stratify based on the above large-scale patterns. It is found that mesoscale convective systems develop in ail cases of genesis except one. Seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44% of cases developed multiple mesoscale convective systems at a single time. Stratification by pattern type indicates some differentiation in mesoscale convective activity and it is inferred that this is due to the large-scale processes. Two of the five patterns, the monsoon shear line and the monsoon confluence region, had more than the average amount of mesoscale convective activity during the genesis period. These patterns also account for 70% of the total genesis events in the 8-yr period. The analysis for the other three patterns exhibit less mesoscale convective system activity during genesis. This may indicate either that genesis processes for these patterns are not as dominated by mesoscale convective system activity, or that genesis occurs more rapidly in these cases.

The development of Typhoon Irving is investigated using a variety of data, including special research aircraft data from the Tropical Cyclone Motion (TCM-92) experiment, objective analyses, satellite data, and traditional surface and sounding data. The development process is treated as a dry-adiabatic vortex dynamics problem, and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation. Synoptic-scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development, Mesoscale interactions with the low-level, large-scale circulations and with other midlevel, mesoscale features result in development of vorticity in the midlevels and enhancement of the low-level vorticity associated with the developing surface cyclone.Multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low-level confluence and a tendency toward recurrent development of associated mesoscale convective vortices. This is observed in both aircraft data and satellite imagery where subsequent interactions, including mergers with the low-level, tropical depression vortex, are observed. A contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices. Results from a simple baroclinic experiment show that the midlevel vortices affect the large-scale, low-level circulation in two ways: 1) initially, interactions between midlevel vortices produce a combined vortex of greater depth; 2) interaction between midlevel vortices and the low-level circulation produces a development downward of the midlevel vorticity, This strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere.