Michael Fiorino

Some Aspects of Vortex Structure Related to Tropical Cyclone Motion

Abstract Some effect of tropical cyclone structure on the vortex motion are examined in a nondivergent, barotropic numerical model with no basic current. As suggested earlier by DeMaria, the initial maximum wind speed has little effect on the track. Vortex translation associated with the beta effect depends sensitively on the strength of the flow between 300 and 1000 km from the center. If the flow in this annulus is made more cyclonic, the track will turn cyclonically and move more toward the west in the Northern Hemisphere. The dynamics of this beta-drift is studied via a decomposition into symmetric and asymmetric circulations. The symmetric flow experiences a slight weakening of the maximum wind speed and an anticyclonic circulation is induced beyond 600 km. The asymmetric circulation is dominated by an azimuthal wavenumber one circulation with an anticyclonic gyre east of the center, a cyclonic gyre to the west and a nearly uniform, broad-scale ventilation flow between the gyres. The vortex translati...

Global Ensemble Predictions of 2009’s Tropical Cyclones Initialized with an Ensemble Kalman Filter

Abstract Verification was performed on ensemble forecasts of 2009 Northern Hemisphere summer tropical cyclones (TCs) from two experimental global numerical weather prediction ensemble prediction systems (EPSs). The first model was a high-resolution version (T382L64) of the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS). The second model was a 30-km version of the experimental NOAA/Earth System Research Laboratory’s Flow-following finite-volume Icosahedral Model (FIM). Both models were initialized with the first 20 members of a 60-member ensemble Kalman filter (EnKF) using the T382L64 GFS. The GFS–EnKF assimilated the full observational data stream that was normally assimilated into the NCEP operational Global Statistical Interpolation (GSI) data assimilation, plus human-synthesized “observations” of tropical cyclone central pressure and position produced at the National Hurricane Center and the Joint Typhoon Warning Center. The forecasts from the two experimental ensembl...

Multimodel Superensemble Forecasting of Tropical Cyclones in the Pacific

Using currently available operational forecast datasets on the tracks and intensities of tropical cyclones over the Pacific Ocean for the years 1998, 1999, and 2000 a multimodel superensemble has been constructed following the earlier work of the authors on the Atlantic hurricanes. The models included here include forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers for Environmental Prediction/Environmental Modeling Center [NCEP/EMC, the Aviation (AVN) and Medium-Range Forecast (MRF) Models], the U.S. Navy [Naval Operational Global Atmospheric Prediction System, (NOGAPS)], the U.K. Met Office (UKMO), and the Japan Meteorological Agency (JMA). The superensemble methodology includes a collective bias estimation from a training phase in which a multiple-regression-based least squares minimization principle for the model forecasts with respect to the observed measures is employed. This is quite different from a simple bias correction, whereby a mean value is simply shifted. These bias estimates are described by separate weights at every 12 h during the forecasts for each of the member models. Superensemble forecasts for track and intensity are then constructed up to 144 h into the future using these weights. Some 100 past forecasts of tropical cyclone days are used to define the training phase for each basin. The findings presented herein show a marked improvement for the tracks and intensities of forecasts from the proposed multimodel superensemble as compared to the forecasts from member models and the ensemble mean. This note includes detailed statistics on the Pacific Ocean tropical cyclone forecasts for the years 1998, 1999, and 2000.

Predictions of 2010’s Tropical Cyclones Using the GFS and Ensemble-Based Data Assimilation Methods

AbstractExperimental ensemble predictions of tropical cyclone (TC) tracks from the ensemble Kalman filter (EnKF) using the Global Forecast System (GFS) model were recently validated for the 2009 Northern Hemisphere hurricane season by Hamill et al. A similar suite of tests is described here for the 2010 season. Two major changes were made this season: 1) a reduction in the resolution of the GFS model, from 2009’s T384L64 (~31 km at 25°N) to 2010’s T254L64 (~47 km at 25°N), and some changes in model physics; and 2) the addition of a limited test of deterministic forecasts initialized from a hybrid three-dimensional variational data assimilation (3D-Var)/EnKF method.The GFS/EnKF ensembles continued to produce reduced track errors relative to operational ensemble forecasts created by the National Centers for Environmental Prediction (NCEP), the Met Office (UKMO), and the Canadian Meteorological Centre (CMC). The GFS/EnKF was not uniformly as skillful as the European Centre for Medium-Range Weather Forecasts ...

An Evaluation of the Real-Time Tropical Cyclone Forecast Skill of the Navy Operational Global Atmospheric Prediction System in the Western North Pacific

Abstract The meteorological quality and operational utility of the Navy Operational Global Atmospheric Prediction System (NOGAPS) in forecasting tropical cyclones is evaluated and it is shown that the model can provide useful predictions of motion and formation on a real-time basis in the western North Pacific. The evaluation was conducted during the 1990 operational testing of a procedure to improve the initial analysis or specification of tropical cyclones (TCs) in NOGAPS by the U.S. Navy Fleet Numerical Oceanography Center (FNOC). The NOGAPS TC analysis procedure generates synthetic TC observations based on operational vortex data (e.g., location and maximum surface wind speed) and then adds the observations to the observational data base with flags to force their assimilation. Results from the first year of testing were favorable, despite intermittent application of the procedure. The meteorological characteristics of the NOGAPS tropical cyclone predictions were evaluated by examining the formation of...

Incorporating Surface-Winds and Rainfall Rates into the Initialization of a Mesoscale Hurricane Model

Abstract The initialization of a three-dimensional model with operational data for Hurricane Eloise (1975) was studied to assess the impact of using bogus storm data, surface winds, rainfall rates, and a high-resolution surface pressure analysis in the initialization of forecasts of hurricane track and intensity. Because the track and intensity forecasts based on the unaugmented NMC analyses were unsatisfactory, various data improvement procedures were used. Boundary-layer flow was diagnosed from the surface pressure with a primitive equation PBL model, a climatological hurricane circulation was inserted into the NMC wind analysis above the boundary layer, and the three-dimensional moisture field was defined with the aid of visible-image satellite photographs. Model simulations with this improved data set were designed to test the effectiveness of dynamic initialization (DI) and the data enhancement procedures in improving the numerical hurricane forecasts. A 24 h time period, starting at 0000 GMT 21 Sept...

Contributions to Tropical Cyclone Motion by Small, Medium and Large Scales in the Initial Vortex

Abstract A two-dimensional Fourier decomposition procedure is used to isolate small (≤500 km), medium (500 1500 km) scale components of some typical tangential wind profiles used in theoretical studies of tropical cyclone motion. The contribution of these scales to the vortex motion is studied in a nondivergent barotropic model with no initial basic flow by selectively retaining or deleting different scales. Transfer of energy between wave groups due to nonlinear scale interaction occurs slowly in this model so that a scale group that is removed in the initial conditions is not restored by 72 h. The largest scales, which account for a significant fraction of the vortex structure, primarily determine the speed of motion. That is, the speed of motion is proportional to the percentage of the total vortex that projects onto the largest scales. The medium and small males that contain less energy (because of the assumed vortex structure parameters) have a significant effect on the dire...

Extreme Rapid Intensification of Typhoon Vicente (2012) in the South China Sea

AbstractOne of the primary challenges for both tropical cyclone (TC) research and forecasting is the problem of intensity change. Accurately forecasting TC rapid intensification (RI) is particularly important to interests along coastlines and shipping routes, which are vulnerable to storm surge and heavy seas induced by intense tropical cyclones. One particular RI event in the western North Pacific Ocean with important scientific implications is the explosive deepening of Typhoon Vicente (2012). Vicente underwent extreme RI in the northern South China Sea just prior to landfall west of Hong Kong, China, with maximum sustained winds increasing from 50 kt (1 kt = 0.51 m s−1) at 0000 UTC 23 July to 115 kt at 1500 UTC 23 July. This increase of 65 kt in 15 h far exceeds established thresholds for TC RI. Just prior to this RI episode, Vicente exhibited a near-90° poleward track shift. The relationship between the track and intensity change is described, and the authors speculate that the passage of an upper-tro...

A Vertically Flow-Following Icosahedral Grid Model for Medium-Range and Seasonal Prediction. Part I: Model Description

AbstractA hydrostatic global weather prediction model based on an icosahedral horizontal grid and a hybrid terrain-following/isentropic vertical coordinate is described. The model is an extension to three spatial dimensions of a previously developed, icosahedral, shallow-water model featuring user-selectable horizontal resolution and employing indirect addressing techniques. The vertical grid is adaptive to maximize the portion of the atmosphere mapped into the isentropic coordinate subdomain. The model, best described as a stacked shallow-water model, is being tested extensively on real-time medium-range forecasts to ready it for possible inclusion in operational multimodel ensembles for medium-range to seasonal prediction.

A Comprehensive Test of the Navy Nested Tropical Cyclone Model

Abstract The tropical cyclone forecasting skill level of the Joint Typhoon Warning Center (JTWC) has reached a plateau and has shown little or no lasting improvement during the last 10 years. Because JTWC relies on an array of statistical and climatological forecast aids for objective guidance, it seems reasonable to conclude that the skill of these aids has also reached an upper limit. We compare the forecast skill of the Navys new “two-way interactive” nested model with the official forecast of JTWC for a large number (220) of tropical storm and typhoon cases over the period 1975–80. Even with this relatively simple model, significant potential for improved operational forecast accuracy is apparent, especially in the longer range (48–72 h) forecasts. Skill at still longer periods is also demonstrated in the results of 50 five-day forecasts. We conclude that dynamic models offer great promise for more, accurate forecasts.