Póster elaborado para la European Conference on Severe Storms celebrada en Palma de Mallorca del 3 al 7 de octubre de 2011Trabajo parcialmente patrocinado por: MEDICANES/CGL200801271 projec

Campins, Joan

Jansà Clar, Agustí

Picornell, María Ángeles

English

Agencia Estatal de Meteorología

AN ADAPTATION OF THE OBJECTIVE CYCLONEDETECTION ALGORITHM TO A MEDICANE EVENTM.A. Picornell∗, J. Campins and A. Jansa` D.T. Illes Balears, AEMET (Spain) *mpicornella@aemet.esAlthough tropical-like cyclones rarely affect the Mediterranean region, they can produce extremely strong winds. These warm-core cyclones, called MEDICANES (MEDIterraneanhurriCANES) are small size, tend to develop over the sea and are infrequent. For these reasons, the detection and forecast of medicanes is a difficult task and much efforts have beendevoted to identify them. In this sense five criteria had been established by some authors to define a medicane from the satellite images and a first data base of twelve medicaneshad been derived from the IR image data (1982-2005) of Meteosat satellite: cyclone eye clarity, symmetric shape, continuous cloud cover, diameter less than 300-km and lifetimegreater than 6 hours.The goal of this work is to establish a first step to assess the capability of the numerical models to simulate a medicane event and to develop some criteria to identify medicanesfrom numerical model outputs. To do that, a method for detection and tracking of the Mediterranean cyclones has been adapted to small-scale intense cyclonic perturbations.The procedure has been applied to a numerical simulation output of the tropical-like cyclone that affected the Balearic Sea on September 12th 1996, a medicane selected fromsatellite-derived data base. This simulation has been done with the ECMWF operational model (T1279L91Cy36r1, grid length 15km). The suitability of the presented objectivedetection of medicanes from numerical model outputs is evaluated.MEDICANE EVENTOn 12th of Setember 1996 a small quasi-tropical cyclone formed in the gulf of Valenciaand later affected Balearic Islands. This cyclone crossed the island of Mallorca with anintense fall of pressure, observed in the pressure recording of Palma, and strong windsrecorded on the island. From the radar images an eye of clear air sourrounded by a cir-cular wall of cumulonimbus. The cyclone moved to East - North-East, from Valencia toSardinia.This event has been simulated with the ECMWF operational model T1279L91Cy36r1,grid length 15km. A method for detection and tracking of the Mediterranean cycloneshas been adapted to small-scale intense cyclonic perturbations. First, the algorithm hasbeen modified to properly describe these small cyclones. Next, some parameters havebeen tuned to discriminate between medicanes and other small cyclones.The procedure has been applied to the numerical simulation output of this tropical-likecyclone,a medicane selected from satellite-derived database. The predicted cyclone andits evolution has been compared against some available observational data:Fig.1 VIS Meteosat image 12UTC, pressure recording in Palma, observed medicane track,radar image, echotop 0450UTC (from Gili et al.1997)MEDICANE DETECTIONThe minima of the mean sea level pressure (MSLP) fields areselected as possible medicanes if their pressure gradient over-comes a threshold value, ∇p ≥ 1,5hpa/100km.The cyclone domain is limited by zero-vorticity line. The vorti-city field is particularly sensitive to the presence of very smallstructures in the pressure field, because they are amplified toobtain the vorticity field and can mask the features of the cy-clone. The Cressman filter smoothing technique performs adistance-weighted average of mslp with all neighbors pointswithin a radius rcr and is used to eliminate these disturbances.A smoothing of 50 km radius (blue line in Fig.2) has been ap-plied, in an attempt to correctly describe small vortices. With amore intense smoothing (red, green and turquoise lines), whitha larger radius, the pressure away from the value of the originalfield and the domain assigned to the cyclone is larger.LongitudeCentral pressure (hPa)rcr= 50kmrcr=100kmrcr=150kmrcr=200km0 0.5E 1E 1.5E 2E 2.5 3E 3.5 4E100210041006LongitudeGeostrophic Vorticity0.5E 1.5E 2.5 3.0E 3.5 4.0E−510203040Fig2. Longitudinal section of pressure and geostrophic vorticitywith different filtersMSLP 1996091206 (H+18) 1996091212 (H+24) 1996091218 (H+30)The model is able to simulate an intense cyclone but never reached the pressure drop recorded in Palma.FEATURES AND TRACKINGGeostrophic Circulation (UC)1118 1200 1206 1212 1218 1300123450100150Mean Radius (km)The cyclone is small, with a mean radius between 90 - 140km. The maximum strength is arreached at 06UTC.From the comparison of the cyclone track against the trackfrom observations (fig1) it can be established that the me-dicane evolution is delayed 6 hours. At 12UTC the medi-cane reaches Mallorca but in the MSLP chart is located atWest of island.IDENTIFICATION OF WARM COREThe frontal nature of the cyclone (or lack thereof) and its sign are fundamental indictors of the type of cyclone and the stageof evolution. This frontal nature is defined by Hart as the storm-motion-relative 900-600 hPa thickness asymmetry across thecyclone within 500-km radius, measured by the parameter B:B = |(Z600−Z900)L−(Z600−Z900)R| L: left, R: right Thermal sym → non frontal cyclone → B < 10Thermal asym → frontal cyclone → B > 10In this work, a first step to identify a possible warm core structure within the small quasi-tropical cyclone has been to analyseits 700-925 thickness structure (equivalent to its thermal structure). A radius of 100-km is used in this case.96091206 B=14.5 96091212 B=11.5 96091218 B=3.3B [700−925 hPa Thickness Symetry]1118 1200 1206 1212 1218 1300051015In this simulation a warm core is observed at 12UTC, although the value of B is still more than 10. At 18UTC parameter valueis 3.3, low value according with the mature stage of the medicane.SUMMARYThe model is able to simulate the medi-cane, but not with strength enough andwith a time lag regarding to the observa-cions.Some criteria have been established todetect and characterize this medicane.However these thresholds must be vali-date with other medicane events. Proba-bly,with a more realistic simulation of themedicane event the criteria may be moreexigent and discriminant of other smallcyclones.Once the procedure is set to describethe medicanes features, it may contribu-te to obtaining a medicane databasefromevent simulations or from high resolutionoperational numerical models.ACKNOWLEDGEMENTSThe authors are grateful to J.A. Garcia-Moya (AEMET, Madrid) who performed theECMWFT1279L91Cy36r1 simulationsThis work has been partially suported by theMEDICANES/CGL200801271 project.References. Emanuel, K., 2005: Genesis and maintenance of ’Mediterraneanhurricanes’. Ad. Geo., 2, 217-220.. Gili, M.A., A. Jansa, J. Riesco J. and J.A. Garc´ı-Moya, 1997: Quasi-tropical cyclone on 12th September 1996 in the Balearics. INM-WMOInternational Symposium on Cyclones and Hazardous Weather in theMediterranean. INM, Palma de Mallorca, Spain, pp. 143-150.. Hart, R.E., 2003: A cyclone phase space derived from thermal windand thermal asymmetry. Mon. Wea. Rev., 131, 585-616.. Picornell, M.A., Jansa, A., Genoves, A., Campins, ’Automated da-tabase of mesocyclones from the HIRLAM(INM) 0.5 analyses in theWestern Mediterranean’, Int. J. Climatol. 21, 2001, pp. 335-354.. Tous, M. and R. Romero, 2011: MEDICANES: Criteris de catalo-gacio´ i exploracio´ dels ambients meteorolo`gics. Tethys, 8, 55.63.

An adaptation of the objective cyclone detection algorithm to a Medicane event

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An adaptation of the objective cyclone detection algorithm to a Medicane event

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