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Michael J. Brennan and Sharanya J. Majumdar

Abstract

Sources of dynamical model track error for Hurricane Ike (2008) in the Gulf of Mexico are examined. Deterministic and ensemble model output are compared against National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analyses to identify potential critical features associated with the motion of Ike and its eventual landfall along the upper Texas coast. Several potential critical features were identified, including the subtropical ridge north of Ike and several synoptic-scale short-wave troughs and ridges over central and western North America, and Tropical Storm Lowell in the eastern North Pacific. Using the NCEP Gridpoint Statistical Interpolation (GSI) data assimilation scheme, the operational GSI analysis from the 0000 UTC 9 September 2008 cycle was modified by perturbing each of these features individually, and then integrating the GFS model using the perturbed initial state. The track of Ike from each of the perturbed runs was compared to the operational GFS and it was found that the greatest improvements to the track forecast were associated with weakening the subtropical ridge north of Ike and strengthening a midlevel short-wave trough over California. A GFS run beginning with an analysis where both of these features were perturbed produced a greater track improvement than either did individually. The results suggest that multiple sources of error exist in the initial states of the operational models, and that the correction of these errors in conjunction with reliable ensemble forecasts would lead to improved forecasts of tropical cyclone tracks and their accompanying uncertainty.

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Todd B. Kimberlain and Michael J. Brennan

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The 2009 eastern North Pacific hurricane season had near normal activity, with a total of 17 named storms, of which seven became hurricanes and four became major hurricanes. One hurricane and one tropical storm made landfall in Mexico, directly causing four deaths in that country along with moderate to severe property damage. Another cyclone that remained offshore caused an additional direct death in Mexico. On average, the National Hurricane Center track forecasts in the eastern North Pacific for 2009 were quite skillful.

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Michael J. Brennan and Gary M. Lackmann

Abstract

The role of a diabatically produced lower-tropospheric potential vorticity (PV) maximum in determining the precipitation distribution of the 24–25 January 2000 U.S. East Coast cyclone is investigated. Operational numerical weather prediction (NWP) models performed poorly with this storm, even within 24 h of the event, as they were unable to properly forecast the westward extent of heavy precipitation over the Carolinas and mid-Atlantic. The development of an area of incipient precipitation (IP) around 0600 UTC 24 January over the southeastern United States prior to rapid cyclogenesis was also poorly forecasted by the operational NWP models. It is hypothesized that the lower-tropospheric diabatic PV maximum initially produced by the IP was important to subsequent inland moisture transport over the Carolinas and mid-Atlantic. A PV budget confirms that latent heat release in the midtroposphere associated with the IP led to the initial formation of a PV maximum in the lower troposphere that propagated eastward in association with the IP to the Atlantic coast late on 24 January. The impact of this PV maximum on the westward moisture transport was quantified by piecewise Ertel PV inversion. Results from the inversion show that the balanced flow associated with this evolving cyclonic PV maximum contributed substantially to the onshore moisture flux into the Carolinas and Virginia. The balanced flow associated with the PV anomaly also contributed to quasigeostrophic forcing for ascent in the region. These findings suggest that accurate numerical prediction of the precipitation distribution in this event requires adequate representation of the IP and its associated impacts on the PV distribution.

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Michael J. Brennan and Gary M. Lackmann

Abstract

Previous research has shown that a lower-tropospheric diabatically generated potential vorticity (PV) maximum associated with an area of incipient precipitation (IP) was critical to the moisture transport north of the PV maximum into the Carolinas and Virginia during the 24–25 January 2000 East Coast cyclone. This feature was almost entirely absent in short-term (e.g., 6–12 h) forecasts from the 0000 UTC 24 January 2000 operational runs of the National Centers for Environmental Prediction (NCEP) North American Mesoscale (NAM, formerly Eta) and Global Forecast System (GFS, formerly AVN) models, even though it occurred over land within and downstream of a region of relatively high data density. Observations and model analyses are used to document the forcing for ascent, moisture, and instability (elevated gravitational and/or symmetric) associated with the IP, and the evolution of the IP formation is documented with radar and satellite imagery with the goal of understanding the fundamental nature of this precipitation feature and the models’ inability to predict it. Results show that the IP formed along a zone of lower-tropospheric frontogenesis in a region of strong synoptic-scale forcing for ascent downstream of an approaching upper trough and jet streak. The atmosphere above the frontal inversion was characterized by a mixture of gravitational conditional instability and conditional symmetric instability over a deep layer, and this instability was likely released when air parcels reached saturation as they ascended the frontal surface. The presence of elevated convection is suggested by numerous surface reports of thunder and the cellular nature of radar echoes in the region. Short-term forecasts from the Eta and AVN models failed to capture the magnitude of the frontogenesis, upper forcing, or elevated instability in the region of IP formation. These findings suggest that errors in the initial condition analyses, particularly in the water vapor field, in conjunction with the inability of model physics schemes to generate the precipitation feature, likely played a role in the operational forecast errors related to inland quantitative precipitation forecasts (QPFs) later in the event. A subsequent study will serve to clarify the role of initial conditions and model physics in the representation of the IP by NWP models.

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Sharanya J. Majumdar, Michael J. Brennan, and Kate Howard

Abstract

Because of the threat that Hurricane Irene (2011) posed to the United States, supplemental observations were collected for assimilation into operational numerical models in the hope of improving forecasts of the storm. Synoptic surveillance aircraft equipped with dropwindsondes were deployed twice daily over a 5-day period, and supplemental rawinsondes were launched from all upper-air sites in the continental United States east of the Rocky Mountains at 0600 and 1800 UTC, marking an unprecedented magnitude of coverage of special rawinsondes at the time. The impact of assimilating the supplemental observations on National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) model track forecasts of Irene was evaluated over the period that these observations were collected. The GFS track forecasts possessed small errors even in the absence of the supplemental observations, providing little room for improvement on average. The assimilation of the combined dropwindsonde and supplemental rawinsonde data provided small but statistically significant improvements in the 42–60-h range for GFS forecasts initialized at 0600 and 1800 UTC. The primary improvement from the dropwindsonde data was also within this time range, with an average improvement of 20% for 48-h forecasts. The rawinsonde data mostly improved the forecasts beyond 3 days by modest amounts. Both sets of observations provided small, additive improvements to the average cross-track errors. Investigations of individual forecasts identified corrections to the model analyses of the Atlantic subtropical ridge and an upstream midlatitude short-wave trough over the contiguous United States due to the assimilation of the extra data.

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Michael J. Brennan, Hugh D. Cobb III, and Richard D. Knabb

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A climatology of gale- and storm-force gap wind events in the Gulf of Tehuantepec is constructed for the first time using 10 yr of ocean surface vector wind data from the SeaWinds scatterometer on board NASA’s Quick Scatterometer (QuikSCAT) satellite. These wind events are among the most severe that occur within the National Hurricane Center’s (NHC) area of marine forecasting responsibility outside of tropical cyclones. The 10-yr climatology indicates that on average 11.9 gale-force events and 6.4 storm-force events occur in the Gulf of Tehuantepec each cold season. About 84% of these events occur between November and March, with the largest number of gale-force events occurring in December. Storm-force events are most frequent in January.

Operational numerical weather prediction model forecasts of these events from the NCEP Global Forecast System (GFS) and North American Mesoscale (NAM) models were evaluated during the 2006/07 cold season. Results show that neither model is able to consistently forecast storm-force Tehuantepec wind events; however, the models do have some ability to forecast gale-force events. The NAM model showed a significant increase in probability of detection over the GFS, possibly due to increased horizontal and vertical resolutions as well as differences in boundary layer mixing and surface flux schemes.

Finally, the prospects of observing these gap wind events in the post-QuikSCAT era will be discussed.

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David R. Novak, David R. Bright, and Michael J. Brennan

Abstract

Key results of a comprehensive survey of U.S. National Weather Service operational forecast managers concerning the assessment and communication of forecast uncertainty are presented and discussed. The survey results revealed that forecasters are using uncertainty guidance to assess uncertainty, but that limited data access and ensemble underdispersion and biases are barriers to more effective use. Some respondents expressed skepticism as to the added value of formal ensemble guidance relative to simpler approaches of estimating uncertainty, and related the desire for feature-specific ensemble verification to address this skepticism. Respondents reported receiving requests for uncertainty information primarily from sophisticated users such as emergency managers, and most often during high-impact events. The largest request for additional training material called for simulator-based case studies that demonstrate how uncertainty information should be interpreted and communicated.

Respondents were in consensus that forecasters should be significantly involved in the communication of uncertainty forecasts; however, there was disagreement regarding if and how forecasters should adjust objective ensemble guidance. It is contended that whether forecasters directly modify objective ensemble guidance will ultimately depend on how the weather enterprise views ensemble output (as the final forecast or as a guidance supporting conceptual understanding), the enterprise’s commitment to provide the necessary supporting forecast infrastructure, and how rapidly ensemble weaknesses such as underdispersion, biases, and resolution are addressed.

The survey results illustrate that forecasters’ operational uncertainty needs are intimately tied to the end products and services they produce. Thus, it is critical that the process to develop uncertainty information in existing or new products or services be a sustained collaborative effort between ensemble developers, forecasters, academic partners, and users. As the weather enterprise strives to provide uncertainty information to users, it is asserted that addressing the forecaster needs identified in this survey will be a prerequisite to achieve this goal.

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Michael J. Brennan, Gary M. Lackmann, and Kelly M. Mahoney

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The use of the potential vorticity (PV) framework by operational forecasters is advocated through case examples that demonstrate its utility for interpreting and evaluating numerical weather prediction (NWP) model output for weather systems characterized by strong latent heat release (LHR). The interpretation of the dynamical influence of LHR is straightforward in the PV framework; LHR can lead to the generation of lower-tropospheric cyclonic PV anomalies. These anomalies can be related to meteorological phenomena including extratropical cyclones and low-level jets (LLJs), which can impact lower-tropospheric moisture transport.

The nonconservation of PV in the presence of LHR results in a modification of the PV distribution that can be identified in NWP model output and evaluated through a comparison with observations and high-frequency gridded analyses. This methodology, along with the application of PV-based interpretation, can help forecasters identify aspects of NWP model solutions that are driven by LHR; such features are often characterized by increased uncertainty due to difficulties in model representation of precipitation amount and latent heating distributions, particularly for convective systems.

Misrepresentation of the intensity and/or distribution of LHR in NWP model forecasts can generate errors that propagate through the model solution with time, potentially degrading the representation of cyclones and LLJs in the model forecast. The PV framework provides human forecasters with a means to evaluate NWP model forecasts in a way that facilitates recognition of when and how value may be added by modifying NWP guidance. This utility is demonstrated in case examples of coastal extratropical cyclogenesis and LLJ enhancement. Information is provided regarding tools developed for applying PV-based techniques in an operational setting.

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Michael J. Brennan, Gary M. Lackmann, and Steven E. Koch

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Appalachian cold-air damming (CAD) is characterized by the development of a cool, stable air mass that is advected southwestward along the eastern slopes of the Appalachian Mountains by low-level ageostrophic flow. Operational forecasters have identified the demise of CAD as a major forecasting challenge, in part because numerical weather prediction models have a tendency to erode the cold air too quickly. Previous studies have considered the role of clouds and precipitation in the initiation and maintenance of CAD; generally, precipitation is thought to reinforce CAD due to the cooling and stabilization resulting from evaporation. Here, the impact of precipitation on CAD during a situation where the lower-tropospheric air mass was near saturation prior to the arrival of precipitation is considered.

Previous studies have indicated that the passage of a cold front can bring about CAD demise, as the synoptic-scale flow becomes northwesterly behind the front and low-level stable air is scoured. Additional complexity is evident in the case of split cold fronts (or cold fronts aloft). In these situations, precipitation bands are found well to the east of the surface cold front and may be accompanied by severe weather. Here, the impact of a split-front rainband on a mature CAD event from 14 February 2000 is investigated.

The coastal front, marking the eastern boundary of the CAD region, made significant inland progress as the split-front rainband passed. Computations from Eta Model forecast fields revealed substantial latent heat release above the cold dome during the passage of the rainband. The CAD cold dome persisted longer in an MM5 model numerical simulation in which the effects of latent heat were withheld relative to both a full-physics control run and to observations. A third model simulation where the low levels of the cold dome were initially dried showed that once saturation occurred, the cold dome began to erode. Analysis of model output and observations suggests that, in this case, precipitation contributed to the retreat of the cold dome through lower-tropospheric pressure falls, an isallobaric wind response, and a resultant inland jump of the coastal front.

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Michael J. Brennan, Christopher C. Hennon, and Richard D. Knabb

Abstract

The utility and shortcomings of near-real-time ocean surface vector wind retrievals from the NASA Quick Scatterometer (QuikSCAT) in operational forecast and analysis activities at the National Hurricane Center (NHC) are described. The use of QuikSCAT data in tropical cyclone (TC) analysis and forecasting for center location/identification, intensity (maximum sustained wind) estimation, and analysis of outer wind radii is presented, along with shortcomings of the data due to the effects of rain contamination and wind direction uncertainties. Automated QuikSCAT solutions in TCs often fail to show a closed circulation, and those that do are often biased to the southwest of the NHC best-track position. QuikSCAT winds show the greatest skill in TC intensity estimation in moderate to strong tropical storms. In tropical depressions, a positive bias in QuikSCAT winds is seen due to enhanced backscatter by rain, while in major hurricanes rain attenuation, resolution, and signal saturation result in a large negative bias in QuikSCAT intensity estimates.

QuikSCAT wind data help overcome the large surface data void in the analysis and forecast area of NHC’s Tropical Analysis and Forecast Branch (TAFB). These data have resulted in improved analyses of surface features, better definition of high wind areas, and improved forecasts of high-wind events. The development of a climatology of gap wind events in the Gulf of Tehuantepec has been possible due to QuikSCAT wind data in a largely data-void region.

The shortcomings of ocean surface vector winds from QuikSCAT in the operational environment at NHC are described, along with requirements for future ocean surface vector wind missions. These include improvements in the timeliness and quality of the data, increasing the wind speed range over which the data are reliable, and decreasing the impact of rain to allow for accurate retrievals in all-weather conditions.

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