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148 Cutoff-Low Systems: Comparison NCEP versus RegCM3 Michelle Simões Reboita 1 , Raquel Nieto 2 , Luis Gimeno 2 , Rosmeri Porfírio da Rocha 1 and Tércio Ambrizzi 1 1 University of São Paulo, São Paulo, Brazil; 2 University of Vigo, Ourense, Spain; reboita@model.iag.usp.br 1. Introduction Cutoff low pressure systems (COLs) are usually closed circulations in the middle and upper troposphere developed from a deep trough in the westerlies (Palmén and Newton, 1969). These systems are different of other upper level lows because become completely detached from the main westerly current. COLs intensity is higher in the upper troposphere, decreasing downward. The troposphere below COLs is unstable and convective severe events can occur, depending on surface conditions. COLs have been extensively studied in the Northern Hemisphere (Nieto et al., 2008 and their references), but much less attention has been paid to those occurring in the Southern Hemisphere (SH). Fuenzalida et al. (2005) determined a climatology of COLs over the SH from 1969 to 1999. It was observed that the COLs tend to occur over three main continental areas: Africa, Australia and South America (SA). Marked seasonal cycles with winter maxima were found in SA and Africa but not over Australia. On the other hand, Campetella and Possia (2007) found higher COLs frequency in fall over SA from 1979 to 1988. Garreaud and Fuenzalida (2007) simulated a case of COL over the subtropical southeast Pacific during March 2005 (near SA western coast). The model used by these authors simulated well the COL and it allowed a detailed threedimensional structure description, as well as an evaluation of the vorticity and temperature budgets. Another experiment without Andes topography was done and the COL exhibited similar position, strength, and extension as in the control simulation. This indicates that the Andes has little influence on COL’s formation and intensification. Climatological simulations using regional climate models are rare. In this work we try to simulate COLs over SA and South Atlantic Ocean from 1990 to 1999 using a Regional Climate Model, RegCM3. 2. Methodology 2.1 RegCM3 and Simulations The RegCM3 is a primitive equation model, compressible, and in the sigma-pressure vertical coordinate (Pal et al. 2007). For RegCM3 applications, different physical parameterizations scheme options are available. In this study, the surface turbulent fluxes over the ocean were parameterized using the Zeng scheme (Zeng et al., 1998) and the moist processes were parameterized following the Grell convective scheme (Grell, 1993) with the Fritsch- Chappell closure (Pal et al. 2007). The simulation was carried out from September 1989 to the January 2000 in a domain between 84 o W-15 o E and 60 o S- 5 o S. The atmospheric initial and boundary conditions are from R2 NCEP reanalysis (Kanamitsu et al. 2002). Over the ocean, the sea surface temperature (SST) monthly mean has a horizontal resolution of 1.0 o x 1.0 o (Reynolds et al. 2002). The topography and land data are specified by using the 10 ’ horizontal resolution global archives from, respectively, United States Geological Survey (USGS) and Global Land Cover Characterization (GLCC), described by Loveland et al. (2000). The simulation used 60 km of horizontal resolution and 18 sigma-pressure vertical levels (model top at 80 hPa). 2.2 COLs Identification The COLs were identified using the objective method by Nieto et al. (2005) based on imposing the three main physical characteristics of the conceptual model of COLs (Winkler et al., 2001): the 200 hPa geopotential minimum, cutoff circulation, and the specific structure of both equivalent thickness and thermal front parameter fields. This first identification was done using R2 NCEP reanalysis with 2.5º x 2.5º resolution; the second one was made using RegCM3 data. Due the different data resolution, the RegCM3 data were interpolated to the same grid as R2 NCEP reanalysis to eliminate small scale noise and guarantee that we were seeking the same type of systems in both databases. 3. Results Figure 1a shows all grid points that fulfilled the exigencies of the COLs algorithm using R2 NCEP. Higher frequency of COLs is found over the continent between 25 o -40 o S and 70 o -50 o W. This spatial distribution is similar to Fuenzalida et al. (2005) and Campetella and Possia (2007) climatologies. Figure 1b shows RegCM3 simulation. This result points to that RegCM3 underestimates the COL number over the continental area between 25 o -40 o S and overestimates over the central part of South Atlantic Ocean (30 o -20 o W), and between 10 o -20 o S. The cause of differences between R2 NCEP and RegCM3 COLs climatologies was investigated computing the geopotential height and air temperature differences at 200 hPa during 1990. The difference field (Figure 2a) shows that the RegCM3 simulates higher (lower) geopotential height than R2 NCEP in the region where underestimates (overestimates) COLs. Since the COLs are identified using geopotential minimum, this can explain the absence of COLs in the RegCM3 simulations near Paraguay-Uruguay region and the excessive frequency of COLs at lower latitudes from 15 o S. Other fact is that the RegCM3 is colder poleward 40 o S and warmer equatorward 40 o S (Figure 2b) that implicates a more intense upper level jet (Reboita, 2008). This is unfavorable to COLs’ detection because these systems need weaker winds to detach from the main westerly current. 4. Conclusions The purpose of this work was evaluating the performance of the RegCM3 model to simulate the spatial distribution of COLs over SA and South Atlantic Ocean during 1990- 1999. The simulation was compared with R2-NCEP reanalysis. The results show that RegCM3 is not the best regional model to identify COL systems. The higher geopotencial height and warmer air at 200 hPa level reproduced by the model could explain the absence of COLs surrounding Paraguay-Uruguay region in the RegCM3 simulation. The opposite pattern could be
149 responsible for the excessive simulation of COLs between 10 o -15 o S. a) 5. Acknowledgments This work was funded by FAPESP process N o . 04/02446, CAPES process N o . BEX 0626/08-2, and CNPq process N o 476361/2006-0. The authors would also like to thank ICTP for provided the RegCM3 code and NCEP by reanalyzes data set. References Campetella, C. M.; N. E. Possia: Upper-level cut-off lows in southern South America. Meteorol. Atmos. Phys., 96, 181-191, 2007. Fuenzalida, H. A.; R. Sánchez; R. Garreaud: A Climatology of Cutoff Lows in the Southern Hemisphere., J. Geophys. Res., 110 (D18101), 1-10, 2005. b) Garreaud, R. D.; H. Fuenzalida: The Influence of the Andes on Cutoff Lows: A Modelling Study. Mon. Wea Rev., 135, 1596-1613, 2007. Grell, G. A.: Prognostic Evaluation of Assumptions used by Cumulus Parameterizations, Mon. Wea. Rev., 121, 764-787, 1993. Kanamitsu, M., and co-authors: NCEP-DOE AMIP-II Reanalysis (R-2). Bull. Am. Met. Soc., 83, 1631-1643, 2002. Figure 1. Number of COL points from 1990 to 1999: (a) R2 NCEP and (b) RegCM3. Loveland, T. R.; B. C Reed; J. F. Brown; D. O. Ohlen; J. Zhu; L. Yang; J. W. Merchant: Development of a global land cover characteristics database and IGBP DISCOVER from 1-km AVHRR Data. International Journal of Remote Sensing, 21, 1303-1330, 2000. -10 a) G (RegCM3-NCEP2) 20 Nieto, R., and co-authors: Climatological Features of Cutoff Low Systems in the Northern Hemisphere. J. Climate, 18, 3085-3103, 2005. -15 -20 -25 -30 -35 10 0 -10 Nieto, R.; M. Sprenger; H. Werli; R. M. Trigo; L. Gimeno: Identification and Climatology of Cut-off Lows neart the Tropopause. Annals of the New York Academy of Sciences, 256-289, 2008. -40 -45 -50 -55 -60 -80 -70 -60 -50 -40 -30 -20 -10 -10 -15 -20 -25 -30 -35 -40 -45 -50 -55 b) T (RegCM3-NCEP2) -60 -80 -70 -60 -50 -40 -30 -20 -10 Figure 2. RegCM3/R2-NCEP average difference of a) geopotential and b) air temperature at 200 hPa for 1990. -20 -30 -40 1.5 1 0.5 0 -0.5 Pal, J.S., and co-authors: The ITCP RegCM3 and RegCNET: Regional Climate Modeling for the Developing World. Bull. Amer. Meteor. Soc., 88 (9), 1395-1409, 2007. Palmén, E.; C. W. Newton: Atmospheric Circulation Systems: Their Structure and Physical Interpretation. New York: Academic Press, 603 p., 1969. Reboita, M. S.: Ciclones Extratropicais no Atlântico Sul: Simulação Climática e Experimentos de Sensibilidade. PhD. Thesis in Meteorology (in portuguese), IAG/USP, 294 p., 2008. Reynolds, R. W.; N. A. Rayner; T. M. Smith; D. C. Stokes; W. Wang: An improved in situ and satellite SST analysis for climate. J. Climate, 15, 1609-1625, 2002. Winkler, R.; V. Zwats-Meise: Manual of Synoptic satellite meteorology. Conceptual models, Vers. 6.0. (Available at Central Institute for Meteorology ans Geodynamics, Hohe Warte 38, 1190 Vienna, Austria). Zeng, X.; M. Zhao; R. E. Dickinson: Intercomparison of Bulk Aerodynamic Algorithms for the Computation of Sea Surface Fluxes Using TOGA COARE and TAO Data. J. Climate, 11, 2628-2644, 1998.
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2 nd International Lund RCM Worksho
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Associated Organisations ENSEMBLES
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Preface This Second Lund Regional-s
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II High-resolution dynamical downsc
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IV Investigation of added value at
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VI Soil organic layer: Implications
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VIII Session 4: Regional Observatio
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X Predicting water resources in Wes
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XII The impact of atmospheric therm
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XIV Observation and simulation with
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XVI Favot F .......................
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2 5. Influence of SN on the Ensembl
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12 Sensitivity study of CRCM-simula
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18 The Asian summer monsoon in ERA4
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20 Added value of limited area mode
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22 Sensitivity of CRCM basin annual
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24 High-resolution dynamical downsc
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26 There is nevertheless a large ag
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28 technique, as it is based on the
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30 4. Heating mechanism In order to
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32 The geographical distribution of
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36 Performance of pattern scaling i
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38 Evaluation of the analyzed large
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42 5SL, the results suggest that 5S
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48 Figure 1. Precipitation rate (mm
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50 References Biau. G., E. Zorita,
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52 Investigation of regional climat
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7.5 8.5 9.5 10.5 60 For the climato
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64 3. Results Fig. 2 shows the anal
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72 Will, A., M. Baldauf, B. Rockel,
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74 ( ) with i = 1,K,n; j = 1,K,m;
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76 Investigation of precipitation o
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91 Examining the relative roles con
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93 Dynamical coupling of the HIRHAM
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95 A parameterization of aircraft i
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Page 149 and 150: 123 Temperature and precipitation s
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Page 187 and 188: 161 analyzed the surface wind behav
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199 Evaluation of European snow cov
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201 Projections of eastern Mediterr
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203 Atmospheric river induced heavy
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205 CLARIS project: Towards climate
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207 Influence of soil moisture init
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209 Projection of the Changes in th
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211 Regional climate model studies
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213 ENSEMBLES regional climate mode
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249 Regional climate change impact
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251 River runoff projection of futu
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259 Climate change impacts on the w
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261 Influence of soil moisture-near
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263 Forest damage in a changing cli
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265 Future challenges for regional
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267 Linking climate factors and ada
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269 GCMs. In the end of the 21 st c
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271 Climate change impacts on extre
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273 Winter storms with high loss po
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275 The impact of land use change a
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277 6. Analysis of Results (Rain) T
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279 (a) (b) Figure 3. Impact of veg
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281 that cold (Fig. 5). Winter temp
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283 Streamflow in the upper Mississ
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285 Dynamical downscaling of urban
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287 Souma, K., K. Tanaka, E. Nakaki
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289 In April 2000, the fire emissio
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291 Impacts of vegetation on global
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International BALTEX Secretariat Pu
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No. 34: BALTEX Phase II 2003 - 2012
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