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RISS 인기검색어
- 검색키워드
- 주제어 : tropical cyclone formation (검색결과 9 건)
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Russell L. Elsberry,William M. Clune,Grant Elliott,Patrick A. Harr 한국기상학회 2009 Asia-Pacific Journal of Atmospheric Sciences Vol.45 No.3
One of the objectives of the combined Tropical Cyclone Structure (TCS08) and THORPEX Pacific Asian Regional Campaign (T-PARC) field experiment during August and September 2008 was to improve understanding and prediction of western North Pacific tropical cyclone formation. One approach during TCS08 was to subjectively monitor the performance of the four global models as to whether any of the more than 50 deep convective regions labeled as TCSxxx were predicted to become a tropical depression. When a 850-mb vorticity maximum associated with the convective region could be followed in all four models, a consensus of the track forecasts to 72 h provided surprisingly good guidance for the pre-tropical cyclone seedlings and tropical depressions. An experienced analyst was able to recognize signatures in each of the models that indicate tropical cyclone formation was likely or unlikely. When all four global model forecasts were in agreement that formation was likely, high confidence could be given to the predicted scenario. This four-model consensus approach to predict tropical depression formation was most effective for the four pre-tropical cyclone seedlings that would later become typhoons, and did predict with less advance warning two seedlings that later became tropical storms. However, the global models did not consistently predict the four pre-tropical cyclone seedlings that only attained tropical depression status. The four-model consensus had a relatively small number of false alarms. Even though a three-model consensus had somewhat more false alarms, in most of the low latitude cases the lack of time continuity and a systematic decrease in consensus time to formation will make these cases relatively easy to recognize as false alarms.
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Hsiao-Chung Tsai,Russell L. Elsberry,Mary S. Jordan 한국기상학회 2013 Asia-Pacific Journal of Atmospheric Sciences Vol.49 No.4
An objective tropical cyclone (TC) track analog verification technique has been developed to select all ensemble storm tracks predicted by the ECMWF 32-day ensemble that match the overall Joint Typhoon Warning Center (JTWC) post-season best-tracks. Ensemble storms within specified time and space differences of each JTWC track are first extracted as potential analogs, and four metrics of shortest distance, average distance, distance at formation time, and distance at ending time are calculated. An objective quality measure that assesses the overall track similarity between the potential analogs and each JTWC track is calculated in terms of membership functions for the four track metrics. Weighting factors multiplying these membership functions are adjusted to match with the quality measures for the ECMWF ensemble storm forecasts in a previous subjective evaluation. Objective verifications for the 2009 and 2010seasons have been summarized in terms of Hits, Misses, False Alarms, and Correct Negatives that no TC would be present in the western North Pacific. The most important result is that the ECMWF ensemble was able to predict nearly all of the TCs in both seasons with only a small number of Misses that generally were short-lived tropical depressions. Good performance in terms of Correct Negatives was achieved during the 2010 season. False alarms are defined to be all ensemble storms that could not be matched any JTWC tracks within the specified thresholds. Evaluations of the characteristics of the false alarms indicate seasonal and geographic biases and that about 50% of the false alarm in the Week 1 forecasts originate from the initial the initial conditions in the model. A minimum of false alarms created in Week 2 forecasts is attributed to the decrease in horizontal resolution in the model that occurs at day 10. A steady and nearly uniform increase in false alarms in the Week 3 and Week 4forecasts may be attributed to net convective heating in response to persistent environmental forcing in the tropics.
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Russell L. Elsberry,Mary S. Jordan,Frederic Vitart 한국기상학회 2010 Asia-Pacific Journal of Atmospheric Sciences Vol.46 No.2
The objective of this study is to provide evidence of predictability on intraseasonal time scales (10-30 days) for western North Pacific tropical cyclone formation and subsequent tracks using the 51-member ECMWF 32-day forecasts made once a week from 5 June through 25 December 2008. Ensemble storms are defined by grouping ensemble member vortices whose positions are within a specified separation distance that is equal to 180 n mi at the initial forecast time t and increases linearly to 420 n mi at Day 14 and then is constant. The 12-h track segments are calculated with a Weighted- Mean Vector Motion technique in which the weighting factor is inversely proportional to the distance from the endpoint of the previous 12-h motion vector. Seventy-six percent of the ensemble storms had five or fewer member vortices. On average, the ensemble storms begin 2.5 days before the first entry of the Joint Typhoon Warning Center (JTWC) best-track file, tend to translate too slowly in the deep tropics, and persist for longer periods over land. A strict objective matching technique with the JTWC storms is combined with a second subjective procedure that is then applied to identify nearby ensemble storms that would indicate a greater likelihood of a tropical cyclone developing in that region with that track orientation. The ensemble storms identified in the ECMWF 32-day forecasts provided guidance on intraseasonal timescales of the formations and tracks of the three strongest typhoons and two other typhoons, but not for two early season typhoons and the late season Dolphin. Four strong tropical storms were predicted consistently over Week-1 through Week-4, as was one weak tropical storm. Two other weak tropical storms, three tropical cyclones that developed from precursor baroclinic systems, and three other tropical depressions were not predicted on intraseasonal timescales. At least for the strongest tropical cyclones during the peak season, the ECMWF 32-day ensemble provides guidance of formation and tracks on 10-30 day timescales.
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Russell L. Elsberry,Mary S. Jordan,Frederic Vitart 한국기상학회 2011 Asia-Pacific Journal of Atmospheric Sciences Vol.47 No.4
The performance of the ECMWF 32-day ensemble predictions of western North Pacific tropical cyclone events (formation plus track) made once a week during the 2009 season is evaluated with the same procedures as for the 2008 season. Seventeen of the 23tropical cyclones during the 2009 season occurred during multiple storm scenarios that are more difficult to predict, and many of the deficient track predictions involved unusual and rapidly changing tracks that typically involve interactions with adjacent synoptic circulations that are not predictable on intraseasonal timescales (10-30 days). Such incorrect predictions of the duration and tracks of these multiple cyclones were found to degrade the performance in predicting subsequent tropical cyclone formations and tracks during the 32-day integration. Predominantly northward tracks throughout the life cycle tended to be less predictable on intraseasonal timescales. Given these caveats, the overall performance of the ECMWF ensemble for the 12 typhoons was more successful than during the 2008 season. However, the performance for three tropical storms during the 2009 season was less successful due to the difficult track forecast scenarios. A surprisingly good performance was found in predicting the formation location and early track segments of eight minimal tropical storms or tropical depressions. The less satisfactory aspect for many of the late season tropical depressions was that the ECMWF ensemble continued to predict member vortices for extended periods after the system had actually dissipated.
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Western North Pacific Monsoon Depressions: Transitions to Pre-Tropical Cyclone Seedlings
Jodi C. Beattie,Russell L. Elsberry 한국기상학회 2016 Asia-Pacific Journal of Atmospheric Sciences Vol.52 No.5
The objective of this study is to describe how a monsoon depression in the western North Pacific, which typically has a diameter of 1000 km, may be transitioned into a tropical cyclone with an inner core of strong winds and deep convection on the order of 100 km. Our previous case study of the pre-Typhoon Man-Yi monsoon depression formation is extended to show that the same cross-equatorial airstream continued and led to enhanced equatorial westerlies on the equatorward side of the pre-Man-Yi circulation, and a surge in the trade easterlies was also present on the poleward side. As these surges in the near-equatorial flow are inertially unstable, inward-directed wave-activity fluxes then led to flux convergence over the eastern vorticity maximum of the monsoon depression, which resulted in a scale contraction to that of a pretropical cyclone seedling. Eight case studies of the transitions of monsoon depressions during 2009 are presented that document persistent inward-directed wave-activity fluxes over a vorticity maximum within the monsoon depression is a key feature of each transition. In some transitions, the same cross-equatorial airstream as led to the monsoon depression formation continues as the primary airstream, and in other transitions another airstream to the west or enhanced tropical easterlies become the primary airstream. Analysis of 10 non-transitioning monsoon depressions during 2009 indicated the airstream wave-activity flux did not persist after the formation of the monsoon depression. In another 11 non-transitioning monsoon depressions, the inward-directed wave-activity flux was small and no region of wave-activity flux convergence was associated with the vorticity maximum in the monsoon depression.
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