Mark DeMaria

The Effect of Vertical Shear on Tropical Cyclone Intensity Change

Abstract The effect of vertical shear on tropical cyclone intensity change is usually explained in terms of “ventilation” where heat and moisture at upper levels are advected away from the low-level circulation, which inhibits development. A simple two-layer diagnostic balance model is used to provide an alternate explanation of the effect of shear. When the upper-layer wind in the vortex environment differs from that in the lower layer, the potential vorticity (PV) pattern associated with the vortex circulation becomes tilted in the vertical. The balanced mass field associated with the tilted PV pattern requires an increased midlevel temperature perturbation near the vortex center. It is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development. Previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation. These studies have also shown that there is an adiabatic process that acts to redu...

Further Improvements to the Statistical Hurricane Intensity Prediction Scheme (SHIPS)

Modifications to the Atlantic and east Pacific versions of the operational Statistical Hurricane Intensity Prediction Scheme (SHIPS) for each year from 1997 to 2003 are described. Major changes include the addition of a method to account for the storm decay over land in 2000, the extension of the forecasts from 3 to 5 days in 2001, and the use of an operational global model for the evaluation of the atmospheric predictors instead of a simple dry-adiabatic model beginning in 2001. A verification of the SHIPS operational intensity forecasts is presented. Results show that the 1997–2003 SHIPS forecasts had statistically significant skill (relative to climatology and persistence) out to 72 h in the Atlantic, and at 48 and 72 h in the east Pacific. The inclusion of the land effects reduced the intensity errors by up to 15% in the Atlantic, and up to 3% in the east Pacific, primarily for the shorter-range forecasts. The inclusion of land effects did not significantly degrade the forecasts at any time period. Results also showed that the 4–5-day forecasts that began in 2001 did not have skill in the Atlantic, but had some skill in the east Pacific. An experimental version of SHIPS that included satellite observations was tested during the 2002 and 2003 seasons. New predictors included brightness temperature information from Geostationary Operational Environmental Satellite (GOES) channel 4 (10.7 m) imagery, and oceanic heat content (OHC) estimates inferred from satellite altimetry observations. The OHC estimates were only available for the Atlantic basin. The GOES data significantly improved the east Pacific forecasts by up to 7% at 12–72 h. The combination of GOES and satellite altimetry improved the Atlantic forecasts by up to 3.5% through 72 h for those storms west of 50°W.

Large-Scale Characteristics of Rapidly Intensifying Tropical Cyclones in the North Atlantic Basin

The National Hurricane Center (NHC) and Statistical Hurricane Intensity Prediction Scheme (SHIPS) databases are employed to examine the large-scale characteristics of rapidly intensifying Atlantic basin tropical cyclones. In this study, rapid intensification (RI) is defined as approximately the 95th percentile of over-water 24-h intensity changes of Atlantic basin tropical cyclones that developed from 1989 to 2000. This equates to a maximum sustained surface wind speed increase of 15.4 m s 21 (30 kt) over a 24-h period. It is shown that 31% of all tropical cyclones, 60% of all hurricanes, 83% of all major hurricanes, and all category 4 and 5 hurricanes underwent RI at least once during their lifetimes. The mean initial (t 5 0 h) conditions of cases that undergo RI are compared to those of the non-RI cases. These comparisons show that the RI cases form farther south and west and have a more westward component of motion than the non-RI cases. In addition, the RI cases are typically intensifying at a faster rate during the previous 12 h than the non-RI cases. The statistical analysis also shows that the RI cases are further from their maximum potential intensity and form in regions with warmer SSTs and higher lower-tropospheric relative humidity than the non-RI cases. The RI cases are also embedded in regions where the upper-level flow is more easterly and the vertical shear and upper-level forcing from troughs or cold lows is weaker than is observed for the non-RI cases. Finally, the RI cases tend to move with the flow within a higher layer of the atmosphere than the non-RI cases. A simple technique for estimating the probability of RI is described. Estimates of the probability of RI are determined using the predictors for which statistically significant differences are found between the RI and nonRI cases. Estimates of the probability of RI are also determined by combining the five predictors that had the highest individual probabilities of RI. The probability of RI increases from 1% to 41% when the total number of thresholds satisfied increases from zero to five. This simple technique was used in real time for the first time during the 2001 Atlantic hurricane season as part of the Joint Hurricane Testbed (JHT).

Prediction of Landfalling Hurricanes with the Advanced Hurricane WRF Model

Abstract Real-time forecasts of five landfalling Atlantic hurricanes during 2005 using the Advanced Research Weather Research and Forecasting (WRF) (ARW) Model at grid spacings of 12 and 4 km revealed performance generally competitive with, and occasionally superior to, other operational forecasts for storm position and intensity. Recurring errors include 1) excessive intensification prior to landfall, 2) insufficient momentum exchange with the surface, and 3) inability to capture rapid intensification when observed. To address these errors several augmentations of the basic community model have been designed and tested as part of what is termed the Advanced Hurricane WRF (AHW) model. Based on sensitivity simulations of Katrina, the inner-core structure, particularly the size of the eye, was found to be sensitive to model resolution and surface momentum exchange. The forecast of rapid intensification and the structure of convective bands in Katrina were not significantly improved until the grid spacing ap...

A Statistical Hurricane Intensity Prediction Scheme (SHIPS) for the Atlantic Basin

Abstract A statistical model for predicting intensity changes of Atlantic tropical cyclones at 12, 24, 36, 48, and 72 h is described. The model was developed using a standard multiple regression technique with climatological, persistence, and synoptic predictors. The model developmental sample includes all of the named Atlantic tropical cyclones from 1989 to 1992, with a few additional cases from 1982 to 1988. The sample includes only the times when the storms were over the ocean. The four primary predictors are 1) the difference between the current storm intensity and an estimate of the maximum possible intensity determined from the sea surface temperature, 2) the vertical shear of the horizontal wind, 3) persistence, and 4) the flux convergence of eddy angular momentum evaluated at 200 mb. The sea surface temperature and vertical shear variables are averaged along the track of the storm during the forecast period. The sea surface temperatures along the storm track are determined from monthly climatologi...

Sea Surface Temperature and the Maximum Intensity of Atlantic Tropical Cyclones

Abstract An empirical relationship between climatological sea surface temperature (SST) and the maximum intensity of tropical cyclones in the North Atlantic basin is developed from a 31-year sample (1962–1992). This relationship is compared with the theoretical results described by Emanuel. The theoretical results are in agreement with the observations over a wide range of SST, provided that the tropopause temperature is assumed to be a function of SST. Each storm is examined to determine how close the observed intensity comes to the maximum possible intensity (MPI). Results show that only about 20% of Atlantic tropical cyclones reach 80% or more of their MPI at the time when they are the most intense. On average, storms reach about 55% of their MPI. Storms that are farther west and farther north tend to reach a larger fraction of their MPI. Storms are also more likely to reach a larger fraction of their MPI in August–November than in June–July. There is considerable interannual variability in the yearly ...

A Revised Tropical Cyclone Rapid Intensification Index for the Atlantic and Eastern North Pacific Basins

Abstract A revised rapid intensity index (RII) is developed for the Atlantic and eastern North Pacific basins. The RII uses large-scale predictors from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) to estimate the probability of rapid intensification (RI) over the succeeding 24 h utilizing linear discriminant analysis. Separate versions of the RII are developed for the 25-, 30-, and 35-kt RI thresholds, which represent the 90th (88th), 94th (92nd), and 97th (94th) percentiles of 24-h overwater intensity changes of tropical and subtropical cyclones in the Atlantic (eastern North Pacific) basins from 1989 to 2006, respectively. The revised RII became operational at the NHC prior to the 2008 hurricane season. The relative importance of the individual RI predictors is shown to differ between the two basins. Specifically, the previous 12-h intensity change, upper-level divergence, and vertical shear have the highest weights for the Atlantic basin, while the previous 12-h intensity change, symme...

Landfalling Tropical Cyclones: Forecast Problems and Associated Research Opportunities

Abstract The Fifth Prospectus Development Team of the U.S. Weather Research Program was charged to identify and delineate emerging research opportunities relevant to the prediction of local weather, flooding, and coastal ocean currentsassociated with landfalling U.S. hurricanes specifically, and tropical cyclones in general. Central to this theme are basicand applied research topics, including rapid intensity change, initialization of and parameterization in dynamical models, coupling of atmospheric and oceanic models, quantitative use of satellite information, and mobile observing strategies to acquire observations to evaluate and validate predictive models. To improve the necessary understanding ofphysical processes and provide the initial conditions for realistic predictions, a focused, comprehensive mobile observing system in a translating storm-coordinate system is required. Given the development of proven instrumentation andimprovement of existing systems, three-dimensional atmospheric and oceanic d...

A Tropical Cyclone Genesis Parameter for the Tropical Atlantic

Abstract A parameter to evaluate the potential for tropical cyclone formation (genesis) in the North Atlantic between Africa and the Caribbean islands is developed. Climatologically, this region is the source of about 40% of the Atlantic basin tropical cyclones but roughly 60% of the major hurricanes. The genesis parameter is the product of appropriately scaled 5-day running mean vertical shear, vertical instability, and midlevel moisture variables. The instability and shear variables are calculated from operational NCEP analyses, and the midlevel moisture variable is determined from cloud-cleared GOES water vapor imagery. The average shear and instability variables from 1991 to 1999 and moisture variable from 1995 to 1999 indicate that tropical cyclone formation in the early part of the season is limited by the vertical instability and midlevel moisture. Formation at the end of the season is limited by the vertical shear. On average, there is only a short period from mid-July to mid-October when all thre...

Application of Oceanic Heat Content Estimation to Operational Forecasting of Recent Atlantic Category 5 Hurricanes

Research investigating the importance of the subsurface ocean structure on tropical cyclone intensity change has been ongoing for several decades. While the emergence of altimetry-derived sea height observations from satellites dates back to the 1980s, it was difficult and uncertain as to how to utilize these measurements in operations as a result of the limited coverage. As the in situ measurement coverage expanded, it became possible to estimate the upper oceanic heat content (OHC) over most ocean regions. Beginning in 2002, daily OHC analyses have been generated at the National Hurricane Center (NHC). These analyses are used qualitatively for the official NHC intensity forecast, and quantitatively to adjust the Statistical Hurricane Intensity Prediction Scheme (SHIPS) forecasts. The primary purpose of this paper is to describe how upper-ocean structure information was transitioned from research to operations, and how it is being used to generate NHC’s hurricane intensity forecasts. Examples of the utility of this information for recent category 5 hurricanes (Isabel, Ivan, Emily, Katrina, Rita, and Wilma from the 2003–05 hurricane seasons) are also presented. Results show that for a large sample of Atlantic storms, the OHC variations have a small but positive impact on the intensity forecasts. However, for intense storms, the effect of the OHC is much more significant, suggestive of its importance on rapid intensification. The OHC input improved the average intensity errors of the SHIPS forecasts by up to 5% for all cases from the category 5 storms, and up to 20% for individual storms, with the maximum improvement for the 72–96-h forecasts. The qualitative use of the OHC information on the NHC intensity forecasts is also described. These results show that knowledge of the upper-ocean thermal structure is fundamental to accurately forecasting intensity changes of tropical cyclones, and that this knowledge is making its way into operations. The statistical results obtained here indicate that the OHC only becomes important when it has values much larger than that required to support a tropical cyclone. This result suggests that the OHC is providing a measure of the upper ocean’s influence on the storm and improving the forecast.