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Michael Fiorino
and
Russell L. Elsberry

Abstract

A two-dimensional Fourier decomposition procedure is used to isolate small (≤500 km), medium (500< λ ≤ 1500 km) and large (>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 direction of motion by influencing the rotation of the asymmetric gyres that are induced primarily by the largest scales. The most important implication of these tests for dynamical tropical cyclone forecast models is that the initial vortex specification will project energy onto longer wavelengths that significantly affect track prediction. The agreement between the scales in the initial structure of the vortex and in the environmental analysis needs to be carefully considered.

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Thomas D. Keenan
and
Michael Fiorino

Abstract

The Fleet Numerical Oceanography Center (FNOC) Numerical Variational Analysis (NVA) has been used to develop statistical–synoptic tropical cyclone forecast schemes for the Southern Hemisphere. Results, from a limited sample, showed that the best objective techniques were at least as good as the official Joint Typhoon Warning Center (JTWC) forecasts.

Development and testing of the aids also indicated that approaches used in the Northern Hemisphere did not necessarily lead to better forecasts. Geographically orientated, rather than track-orientated coordinate systems, were found to have lower errors, especially with operational data. Stratification of the datasets by geographical location did not result in any significant improvements in the forecasting accuracy. Similarly, use of prediction equations based on storm position relative to the subtropical ridge did not result in the improvement found for the Northern Hemisphere.

A comparison of techniques using height versus wind field data for the synoptic forcing component of the equations showed lowest errors with the mass field predictors. However, the differences were not statistically significant and this finding may have been a result of data quality rather than the field type.

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Edward J. Harrison Jr.
and
Michael Fiorino

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 Navy's 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.

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Michael Fiorino
and
Thomas T. Warner

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 September 1975, was considered. In procedure A, all data improvements were made and surface pressure was taken directly from a detailed analysis. Procedure B represented what might be done operationally—the only modification to the original NMC data was the insertion of a bogus storm based on composite data and the diagnosis of surface pressure from the 1000 mb heights and temperatures.

For each procedure, three model integrations were made to test the effect of DI by nudging on the forecast. Model results were evaluated in terms of track, the boundary-layer flow, surface pressure and rainfall rates. All forecasts with the improved data were much better than in the preliminary model experiments with the unmodified NMC analysis. Procedure B track predictions, which were based on initial conditions that contained the least amount of mesoscale information, were somewhat better than the others, with vector position errors of <80 km. Dynamic initialization had little effect on the path of the model storm. Intensity forecasts were best using procedure A, in which the greatest amount of hurricane scale information went into the initial conditions, and when DI was employed. However, large-scale mass-momentum adjustment and the proximity of the model storm to the lateral boundaries distorted the predictions of boundary-layer flow and rainfall rates.

A time composite of surface wind reports from land-based stations, buoys, and ships represented the type of data that might be available from future remote sensing satellites like Seasat-A. Because the data were valid at only one synoptic time, a DI could not be performed. The impact of the surface winds on the initialization could only be examined in terms of a 12 h forecast. Several methods of incorporating the surface wind observations into the initial conditions included direct insertion of the data into the NMC wind analysis and a diagnosis of surface pressure from the surface winds through a divergence equation. Although satellite winds improved the mesoscale realism of the initial boundary layer winds and the surface pressure, model forecasts were virtually unimproved. Forecast errors associated with the large-scale mass momentum adjustments, the limitations of the model physics, the data enhancement procedures, and the accuracy of the surface wind analysis, prevented our reaching any definite conclusion about the benefits of supplementary near-surface wind data.

A 12 h DI was performed in which the latent heat release due to convection was externally specified based upon satellite estimates of rainfall rate. A comparison of 12 h forecasts based on this DI and a static initialization showed that this type of DI produced forecasts of surface pressure and precipitation that were greatly improved and which were reflective of observed storm intensity. Track forecasts were not significantly changed.

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Michael Fiorino
and
Russell L. Elsberry

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 translation speed and direction are almost equal to the averaged of this ventilation flow over the area of significant cyclonic circulation in the vortex.

Analysis of the model streamfunction tendency equation demonstrates that the linear beta term is responsible for the initial formation of the asymmetric gyres. Nonlinear advection of the asymmetric circulation by the symmetric vortex flow twists the interior region between the gyres and orients the ventilation flow toward the northwest rather than toward the north. Because this term nearly balances the linear beta forcing, the stream-function time tendency (and storm motion) is predominantly due to the advection of the symmetric vortex by the ventilation flow between the gyres.

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Grant Thomas
,
Richard Cobb
,
Steven Fiorino
, and
Michael Hawks

Abstract

Daytime spectral sky radiance, or sky brightness, is deceptively complex to predict accurately. The Laser Environmental Effects Definition and Reference (LEEDR) first-principles atmospheric model propagates the spectral radiance of the sun to a sensor by modeling the scattering, absorption, and transmission of the radiated light through representative atmospheric layers. For this application, LEEDR was used to ingest numerical weather prediction (NWP) models, and scale the boundary layer and incorporate aerosol loading with ground-based measurements. This study compares LEEDR-derived spectral sky radiance simulations that include measured climatological, measured meteorological, and aerosol loading data to direct sky radiance measurements. Direct measurements of the daytime sky are accomplished with a 1-m-aperture telescope and simultaneous I-band and J-band camera observations (~0.8 and ~1.2 μm, respectively). LEEDR models of the daytime sky are compared to I-band and J-band radiances at multiple azimuths, elevations, and observation times. Residual error analysis is used to determine the accuracy of models including numerical weather prediction data, historical climatology, scaled aerosol loading via in situ particle count measurements, and meteorological updates. Key findings motivate the inclusion of real-time particle count measurements into future daytime sky radiance models for increased scattering accuracy via realistic atmospheric aerosol loading.

Open access
Arief Sudradjat
,
Ralph R. Ferraro
, and
Michael Fiorino

Abstract

This study compares monthly total precipitable water (TPW) from the National Aeronautics and Space Administration (NASA) Water Vapor Project (NVAP) and reanalyses of the National Centers for Environmental Prediction (NCEP) (R-1), NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) (R-2), and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) from January 1988 through December 1999. Based on the means, NVAP exhibits systematic wetter land regions relative to the other datasets reflecting differences in their analyses due to paucity in radiosonde observations. ERA-40 is wetter in the atmospheric convergence zones than the U.S. reanalyses and NVAP ranges in between. Differences in the annual cycle between the reanalyses (especially R-2) and NVAP are also noticeable over the tropical oceans. Analyses on the interannual variabilities show that the ENSO-related spatial pattern in ERA-40 follows more coherently that of NVAP than those of the U.S. reanalyses. The 1997/98 El Niño’s effect on TPW is shown to be strongest only in NVAP, R-1, and ERA-40 during the period of study. All the datasets show TPW decreases in the Tropics following the 1991 Mt. Pinatubo eruption. By subtracting SST-estimated TPW from the datasets, only NVAP and ERA-40 can well represent the spatial pattern of convergence and/or moist-air advection zones in the Tropics. Even though all the datasets are viable for water cycle and climate analyses with discrepancies (wetness and dryness) to be aware of, this study has found that NVAP and ERA-40 perform better than the U.S. reanalyses during the 12-yr period.

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Michael Fiorino
,
Edward J. Harrison Jr.
, and
Donald G. Marks

Abstract

This paper compares the performance of two multi-level high-resolution baroclinic tropical cyclone models which are currently in operational use. In the Atlantic, the National Hurricane Center utilizes the Movable Fine-mesh Model (MFM) for objective forecast guidance. In the western Pacific, the Navy's Nested Tropical Cyclone Model (NTCM) is employed by the Joint Typhoon Warning Center for this same purpose. However, the computer resource requirements, basic design and method of use are substantially different for the two models. Accordingly, two separate tests were conducted. In the first, or operational comparison, the NTCM was initialized with the Fleet Numerical Oceanography Center tropical analysis while the MFM was initialized with the National Meteorological Center (NMC) global analysis. Forecast errors for both models were very similar in this experiment. The other test isolated model differences by initializing both models with the NMC analysis only. Considerably degraded performance for the NTCM was noted. This result emphasizes the sensitivity of dynamic model performance to the large-scale analysis, and underlines the importance of analysis-prediction system co-development. Each model is briefly described and case-by-case performance evaluations are presented. The results indicate that dynamic models promise great potential for significant improvement in tropical cyclone movement forecasting, particularly for forecast intervals of greater than 36 h.

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Owen H. Shieh
,
Michael Fiorino
,
Matthew E. Kucas
, and
Bin Wang

Abstract

One 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-tropospheric (UT) “inverted” trough was a significant influence. An analysis of real-time numerical model guidance is provided and is discussed from an operational perspective, and high-resolution global model analyses are evaluated. Numerical model forecasts of the UT trough interaction with the TC circulation were determined to be a shortcoming that contributed to the intensity prediction errors for Vicente. This case study discusses the importance of considering UT features in TC intensity forecasting and establishes current modeling capabilities for future research.

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Thomas M. Hamill
,
Jeffrey S. Whitaker
,
Daryl T. Kleist
,
Michael Fiorino
, and
Stanley G. Benjamin

Abstract

Experimental 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 (ECMWF) ensemble prediction system. GFS/EnKF track forecasts had slightly higher error than ECMWF at longer leads, especially in the western North Pacific, and exhibited poorer calibration between spread and error than in 2009, perhaps in part because of lower model resolution. Deterministic forecasts from the hybrid were competitive with deterministic EnKF ensemble-mean forecasts and superior in track error to those initialized from the operational variational algorithm, the Gridpoint Statistical Interpolation (GSI). Pending further successful testing, the National Oceanic and Atmospheric Administration (NOAA) intends to implement the global hybrid system operationally for data assimilation.

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