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Sim D. Aberson

Abstract

Since 1997, the Tropical Prediction Center and the Hurricane Research Division have conducted operational synoptic surveillance missions with a Gulfstream IV-SP jet aircraft to improve numerical forecast guidance. Due to limited aircraft resources, optimal observing strategies for these missions must be developed. In the current study, the most rapidly growing modes are represented by areas of large forecast spread in the NCEP bred-vector ensemble forecasting system. The sampling strategy requires sampling of the entire target region with regularly spaced dropwindsonde observations.

Three dynamical models were employed in testing the targeting and sampling strategies. With the assimilation into the numerical guidance of all the observations gathered during the surveillance missions, only the 12-h Geophysical Fluid Dynamics Laboratory Hurricane Model forecast showed statistically significant improvement. Assimilation of only the subset of data from the subjectively found fully sampled target regions produced a statistically significant reduction of the track forecast errors of up to 25% within the critical first 2 days of the forecast. This is comparable with the cumulative business-as-usual improvement expected over 18 yr.

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Sim D. Aberson

Abstract

Though operational tropical cyclone synoptic surveillance generally leads to smaller track forecast errors in the National Oceanic and Atmospheric Administration Global Forecasting System (GFS) than would occur otherwise, not every case is improved. Very large GFS forecast degradations due to surveillance are investigated. Small perturbations to model initial conditions may have a large impact locally or downstream in a short time. In these cases, the perturbations are due either to erroneous data assimilated into the models or to issues with the complex data assimilation system itself, and may have caused the forecast degradations. Investigation of forecast and observing system failures can lead to procedural changes that may eliminate some causes of future large forecast errors.

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Sim D. Aberson

Abstract

In 1997, the National Hurricane Center and the Hurricane Research Division began operational synoptic surveillance missions with the Gulfstream IV-SP jet aircraft to improve the numerical guidance for hurricanes that threaten the continental United States, Puerto Rico, the U.S. Virgin Islands, and Hawaii. During the first 10 yr, 176 such missions were conducted. Global Positioning System dropwindsondes were released from the aircraft at 150–200-km intervals along the flight track in the environment of each tropical cyclone to obtain wind, temperature, and humidity profiles from flight level (about 150 hPa) to the surface. The observations were processed and formatted aboard the aircraft and sent to the National Centers for Environmental Prediction and the Global Telecommunications System to be ingested into the Global Forecast System, which serves as initial and boundary conditions for regional numerical models that also forecast tropical cyclone track and intensity. The results of an observing system experiment using these data are presented.

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Sim D. Aberson

Abstract

Four aircraft released dropwindsondes in and around tropical cyclones in the west Pacific during The Observing System Research and Predictability Experiment (THORPEX) Pacific Area Regional Campaign (T-PARC) in 2008 and the Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR); multiple aircraft concurrently participated in similar missions in the Atlantic. Previous studies have treated each region separately and have focused on the tropical cyclones whose environments were sampled. The large number of missions and tropical cyclones in both regions, and additional tropical cyclones in the east Pacific and Indian Oceans, allows for the global impact of these observations on tropical cyclone track forecasts to be studied.

The study shows that there are unintended global consequences to local changes in initial conditions, in this case due to the assimilation of dropwindsonde data in tropical cyclone environments. These global impacts are mainly due to the spectral nature of the model system. These differences should be small and slightly positive, since improved local initial conditions should lead to small global forecast improvements. However, the impacts on tropical cyclones far removed from the data are shown to be as large and positive as those on the tropical cyclones specifically targeted for improved track forecasts. Causes of this unexpected result are hypothesized, potentially providing operational forecasters tools to identify when large remote impacts from surveillance missions might occur.

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Sim D. Aberson and Mark DeMaria

Abstract

A nested analysis and barotropic hurricane track forecast model (VICBAR) was run for tropical cyclone cases in the North Atlantic basin during the 1989–93 hurricane seasons. VICBAR is compared to the other operational hurricane track forecast models and is shown to perform as well as each of these. VICBAR forecasts are stratified by initial date, intensity, and location to assess the variability of model performance. VICBAR produces the best forecasts for hurricane case, for cases initiated earliest in the hurricane season, for cases moving the most slowly northward, and for those moving westward. The forecasts with the largest errors are examined to illustrate the limitations of the model and to determine whether these cases can be identified operationally.

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Sim D. Aberson and Jeffrey B. Halverson

Abstract

A photograph of vertically aligned Kelvin–Helmholtz billows in the eastern eyewall of Hurricane Erin on 10 September 2001 is presented. The vertical shear instability in the horizontal winds necessary to produce the billows is confirmed with a high-altitude dropwindsonde observation. This shear instability is not known to be common in tropical cyclone eyewalls and is likely only in cases with a very large eyewall tilt. However, research and reconnaissance aircraft pilots need to be aware of the possibility of their existence, along with other types of hazardous conditions, in such rare circumstances.

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Sim D. Aberson and Charles R. Sampson

Abstract

A new Northwest Pacific climatology and persistence (CLIPER) model is derived with historical tropical cyclone tracks during the satellite and aircraft reconnaissance era (1970–95). The new CLIPER extends the forecasts from 3 to 5 days and exhibits smaller forecast biases than the previous CLIPER, although forecast errors are comparable. The new model is based on more accurate historical tropical cyclone track data, and a simpler derivation of the regression equations, than is the old model. Nonlinear systems analysis shows that the predictability timescale in which the average errors increase by a factor e is just over 15 h, which is about the same as that calculated by similar methods near Australia and in the North Atlantic. This suggests that 5-day tropical cyclone track forecasts may be beneficial, assuming small initial errors; therefore, a CLIPER model extended to 5 days is needed as a baseline to measure the forecast skill.

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Daniel P. Stern, George H. Bryan, and Sim D. Aberson

Abstract

Previous studies have found surprisingly strong vertical motions in low levels of some tropical cyclones. In this study, all available dropsondes (12 000) within tropical cyclones during 1997–2013 are examined, in order to create a dataset of the most extreme updrafts (10 m s−1; 169 sondes) and wind speeds (90 m s−1; 64 sondes). It is shown that extreme low-level (0–3 km) updrafts are ubiquitous within intense (category 4 and 5) tropical cyclones, and that few such updrafts have been observed within weaker storms. These extreme updrafts, which are almost exclusively found within the eyewall just inward of the radius of maximum winds, sometimes occur in close association with extreme horizontal wind speeds. Consistent with previous studies, it is suggested that both the extremes in vertical velocity and wind speed are associated with small-scale (1 km) vortices that exist along the eye–eyewall interface. As a substantial number of updrafts are found within a kilometer of the surface, it can be shown that it is implausible for buoyancy to be the primary mechanism for vertical acceleration. Additionally, the azimuthal distribution of both the extreme updrafts and wind speeds is strongly associated with the orientation of the environmental vertical wind shear.

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Sim D. Aberson, Jun A. Zhang, and Kelly Nuñez Ocasio

Abstract

During a routine penetration into Hurricane Felix late on 2 September 2007, NOAA42 encountered extreme turbulence and graupel, flight-level horizontal wind gusts of over 83 m s−1, and vertical wind speeds varying from 10 m s−1 downward to 31 m s−1 upward and back to nearly 7 m s−1 downward within 1 min. This led the plane to rise nearly 300 m and then return to its original level within that time. Though a dropwindsonde was released during this event, the radars and data systems on board the aircraft were rendered inoperable, limiting the amount of data obtained.

The feature observed during the flight is shown to be similar to that encountered during flights into Hurricanes Hugo (1989) and Patricia (2015), and by a dropwindsonde released into a misovortex in Hurricane Isabel (2003). This paper describes a unique dataset of a small-scale feature that appears to be prevalent in very intense tropical cyclones, providing new evidence for eye–eyewall mixing processes that may be related to intensity change.

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Tomislava Vukicevic, Altuğ Aksoy, Paul Reasor, Sim D. Aberson, Kathryn J. Sellwood, and Frank Marks

Abstract

In this study the properties and causes of systematic errors in high-resolution data assimilation of inner-core tropical cyclone (TC) observations were investigated using the Hurricane Weather Research and Forecasting (HWRF) Ensemble Data Assimilation System (HEDAS). Although a recent study by Aksoy et al. demonstrated overall good performance of HEDAS for 83 cases from 2008 to 2011 using airborne observations from research and operational aircraft, some systematic errors were identified in the analyses with respect to independent observation-based estimates. The axisymmetric primary circulation intensity was underestimated for hurricane cases and the secondary circulation was systematically weaker for all cases. The diagnostic analysis in this study shows that the underestimate of primary circulation was caused by the systematic spindown of the vortex core in the short-term forecasts during the cycling with observations. This tendency bias was associated with the systematic errors in the secondary circulation, temperature, and humidity. The biases were reoccurring in each cycle during the assimilation because of the inconsistency between the strength of primary and secondary circulation during the short-term forecasts, the impact of model error in planetary boundary layer dynamics, and the effect of forecast tendency bias on the background error correlations. Although limited to the current analysis the findings in this study point to a generic problem of mutual dependence of short-term forecast tendency and state estimate errors in the data assimilation of TC core observations. The results indicate that such coupling of errors in the assimilation would also lead to short-term intensity forecast bias after the assimilation for the same reasons.

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