See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/225858605 Bermuda Subtropical Storms Article in Meteorology and Atmospheric Physics · August 2007 DOI: 10.1007/s00703-006-0255-y CITATIONS READS 13 33 5 authors, including: Mark Guishard Elizabeth Harris Bermuda Institute of Ocean Sciences Ariel Re 18 PUBLICATIONS 174 CITATIONS 2 PUBLICATIONS 24 CITATIONS SEE PROFILE SEE PROFILE Jenni L Evans Pennsylvania State University 75 PUBLICATIONS 3,607 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Interaction of MCSs coupled to AEWs over North Africa View project Risk Prediction Initiative Report View project All content following this page was uploaded by Jenni L Evans on 10 July 2018. The user has requested enhancement of the downloaded file. Meteorol Atmos Phys (2007) DOI 10.1007/s00703-006-0255-y Printed in The Netherlands 1 Bermuda Weather Service, BAS-Serco Ltd., St. Georges, Bermuda 2 Department of Meteorology, The Pennsylvania State University, PA, USA 3 Department of Meteorology, Florida State University, FL, USA Bermuda subtropical storms M. P. Guishard1, E. A. Nelson1, J. L. Evans2, R. E. Hart3, and D. G. O’Connell1 With 14 Figures Received August 4, 2006; accepted September 19, 2006 Published online: March 14, 2007 # Springer-Verlag 2007 Summary customers (significant for an island with approx- 1 This investigation focuses on North Atlantic subtropical cy- imately 62,000 inhabitants ). This storm was clones which tracked within 100 nautical miles (185 km) of determined to have been a ‘‘subtropical storm’’ Bermuda from 1957 to 2005, identified through subtropi- by the US National Hurricane Center (NHC), cal structural characteristics distinguished using Cyclone the regional tropical cyclone authority under the Phase Space, from the European Centre for Medium-Range World Meteorological Organisation (WMO). The Weather Forecasts 45-year reanalyses. The study assesses the characteristics of these hybrid storms that affect the storm continued to evolve more tropical charac- Island, in order to aid the local forecaster. Reanalysis charts, teristics and, on October 12th, the storm was surface analyses, local observations, HURDAT tracks, and named Karen. Karen strengthened to a Category- satellite pictures, where available, were examined. This 2 hurricane, and went on to strike Nova Scotia data shows that subtropical cyclones affecting Bermuda as a tropical storm. During the approach and usually form in close proximity, to the south-southwest, rapid evolution of Karen over Bermuda, a Gale over water of an average of 26 C, under moderate vertical À1 wind shear, with an upper trough lying to the west-north- Warning (for winds of at least 17 m s affecting west. They then move in a north-northeastward direction, the local and marine area) was issued and later intensifying quickly, but not often reaching a peak intensity upgraded to a Storm Warning (25 m sÀ1)by of more than 26 m sÀ1. They generally have their begin- the Bermuda Weather Service (BWS). The gale- nings along old baroclinic zones. September is the peak to hurricane-force winds, warm lower core and month of occurrence. A direct hit by a severe subtropical cyclone, producing locally observed winds of over 26 m sÀ1, widespread (albeit asymmetric) convection asso- appears to be a rare event. However, these storms are cer- ciated with the system (Fig. 1) led forecasters to tainly a threat to the Island, particularly due to their lack of conclude that this system had at least partial trop- predictability, and conditions conducive to an incipient sub- ical characteristics (Williams, 2002). However, tropical cyclone with potential to affect the Island should the absence of advisories from the US National always be closely monitored. Hurricane Center (NHC) during the onset of gale force winds restricted local forecasters to the 1. Introduction recorded issuance of local marine wind warnings. The ensuing media coverage over the next two In October 2001, a rapidly developing ma- rine storm produced hurricane-force winds over 1 From Government of Bermuda Department of Statistics Census Bermuda, resulting in the loss of power to 23,000 2000. M. P. Guishard et al other than those already set forth in Guishard (2006); and (c) determine the conditions under which Bermuda STs occur, thus providing a basis for the successful forecasting of such systems. 2. Methods ST characteristics determined using the Cyclone Phase Space (CPS; Hart, 2003) diagnostics of ERA-40 and GFS analyses are a lower-tropo- spheric warm core and an upper-tropospheric cold core (Fig. 2). The initiation time of a ST is defined as the onset of gales, since weaker sys- tems do not generate the societal peril and pub- Fig. 1. GOES-8 visible imagery and satellite-derived winds lic awareness that initially motivated this study. valid for 12:45 UTC October 12th 2001. Image courtesy of In this section, the datasets and model fields used Univ. Wisconsin CIMSS days confirmed that the Storm Warning issued did not capture the attention of the public as well as a Tropical Storm Warning likely would have. In the historical record of warm season storms affecting Bermuda, it is apparent that the nearby development of Karen was not a unique oc- currence, and that it would be to the benefit of Bermudian forecasters to have a record of similar events. The purpose of this study is to examine all such ‘‘Bermuda storms’’ with subtropical char- acteristics in the last 49 years (for which there is reliable local observational data). Such subtropi- cal cyclones (STs) may have been mistaken for tropical or extratropical systems, or poorly fore- cast. Subtropical storms are hybrid cyclones with cold upper and warm lower cyclonic components, spawned as baroclinic developments in the pres- ence of positive low-level vorticity over relatively warm sea surface temperatures (SST). Guishard (2006) presents an Atlantic basin climatology of warm season cyclones with subtropical storm characteristics. This climatology was compiled through the initial detection and tracking of the cyclones in the 45-year ECMWF Reanalyses (ERA-40; Uppala et al, 2004) and corroborated with other evidence, where available. Fig. 2. Lifecycle evolution in the CPS of (a) a typical ST Using the methods described in Guishard which transitions into a TC and then undergoes extratropi- (2006), the characteristics of STs which have im- cal transition; and (b) an extratropical system which devel- pacted Bermuda are documented in this study. The ops tropical characteristics (tropical transition) and then objectives of this project are to (a) catalogue sub- undergoes extratropical transition. Note that while the posi- tion within the phase space is similar for each evolution, tropical systems which affected Bermuda with the direction of the path determines what the phase of the direct hits or ‘‘near misses’’; (b) elucidate any cyclone. A snapshot of the cyclone’s structure is not suffi- common characteristics in origin and evolution, cient to discern its subsequent evolution Bermuda subtropical storms to generate these analyses are reviewed and the must pass within 100 nm (185 km) of Bermuda. process of identification of STs from the general This distance corresponds to the operational list of warm season cyclones in the vicinity of ‘‘threat’’ criterion for the Bermuda marine area. Bermuda is elucidated. It is reasonable to suppose that a system within 100 nm could conceivably affect Bermuda with damaging winds and seas, change direction or in- 2.1 Datasets tensify quickly ‘catching people off guard’ (e.g., The compilation of storms for this study utilizes hurricane Emily in 1987; Gerrish, 1987). a range of datasets. Guishard (2006) describes an Gale force winds must be associated with Atlantic basin climatology based on 45 years of each candidate storm at some time during the the ERA-40 reanalyses (Uppala et al, 2004) and subtropical part of its lifecycle. Determination a set of rules developed to distinguish ST from of the presence or absence of gales is based on other warm season storm types. This coarser res- Bermuda and satellite observations of winds and olution study was complimented by a survey of pressure, and ERA-40=GFS gale algorithms). The the GFS operational analyses (also described in occurrence of gale force winds is operationally Guishard, 2006) for a five year period. The agree- considered the threshold for societal impacts due ment between these two methods of STs detec- to loss of life, injury, or property damage (e.g., tion in the overlapping time period, as well as Higgs, 2005). their concurrence with the recent NHC records There is disagreement between meteorologists (e.g., Stewart, 2000; 2001), provides confidence on the nature of ST genesis. Even hurricane spe- that the techniques are robust. These ERA-40 and cialists within NHC exhibit different operational GFS surveys are drawn upon for this study. opinions; one specialist’s ST storm is another’s Corroborating evidence of significant storm im- non-tropical low or extratropical system (personal pacts (if any) is then sought from historical ob- communications, Beven, Franklin, Roth, Stewart, servations over Bermuda, using the occurrence Landsea, Fogarty). For the purposes of this study, of gale force winds as a discriminator. Further, subtropical structural characteristics are distin- the Global ISCCP B1 Browse System (GIBBS) guished using CPS diagnostics of the global satellite dataset, available from the US National model analyses and satellite signatures. Satellite Climatic Data Center (Knapp, 2004), is used to signatures are determined using the methodology refine the determination of the overall synoptic of Hebert and Poteat (1975), who provide a ST characteristics (e.g., frontal, tropical, subtropi- analog to Dvorak (1975). Any period of purely cal). Storms which are ambiguous in structure extratropical or tropical structural characteristics (i.e., not clearly ST) and could be frontal in prior to the onset of gales for a candidate storm is nature have been discarded from the catalogue.