Tropical Cyclones in a regional climate change projections with RegCM4 over Central America CORDEX domain

Transcription

1 Tropical Cyclones in a regional climate change projections with RegCM4 over Central America CORDEX domain Gulilat Tefera Diro diro@sca.uqam.ca Centre ESCER, University of Quebec at Montreal (UQAM), Montreal, Canada Thanks to F. Giorgi, R. Fuentes-Franco, K. Walsh, G. Giuliani, E. Coppola May 214

2 Introduction Motivation Introduction Tropical Cyclones (TCs) are one of the high impact extreme events affecting many regions. Q. how do their characteristics (intensity, frequency, duration) change under greenhouse gas warming? Several studies with GCM w.r.t climate projections tend to agree on the characteristics of future TC activity on global scale. The change in specific ocean basins are however more uncertain, for example in Atlantic basin some found a decrease (e.g. Bengtsson et al., 27) and others found the opposite (e.g. Oouchi et al., 26), or found no significant change or the result depends on the model configuration (e.g., Murakami et al., 21) This implying the need for further regional investigations

3 Introduction Model set up RegCM (4.3) set up The model domain is following the CORDEX Central America specification(1w - 14W and 2S-42N) Horizontal resolution is km, and vertically 18 σ levels Perfect Boundary Condition integration : ERA-Interim for Initial and LBC BATS for land surface processes Convection:Emanuel over ocean and Grell over land The scenario integrations: 4 ensemble runs Two model configurations each driven by two CMIP GCMs The two configurations are (Grell-BATS) and (Eman-CLM) The two CMIP GCMs are HadGEM2-ES and MPI-ESM simulation period is with GHG forcing from the RCP8. pathway

4 Introduction Model set up Data and analysis period Observed TCs are obtained from the Unisys Web site ( The dataset consists of six hourly best-track data TCs with intensity corresponding surface wind of 17.ms 1 or greater are considered in the analysis Validation is performed for the ERA-Interim driven and for the GCM control (present day) runs. The analysis is carried out for 22 years of the present period ( ) The future period considered is the last 22 years of the 21 century

5 Tropical Cyclones in a regional climate change projections with RegCM4 over Central America CORDEX domain Introduction Model set up Some features of TCs

6 Introduction Model set up TC detection method The detection criteria are similar to Walsh et al., (24) and are as follows: The relative vorticity at 8hPa must be greater than 1. x1 s 1 There must be a closed pressure minimum within the radius of 1km. This minimum pressure is then defined as the center of the cyclone. The minimum surface pressure satisfying the above criteria is at least 2 hpa lower than the averaged surface pressure over the surrounding 2X2 grid boxes. The 1m surface wind speed must exceed 17.m s 1 The minimum pressure at the center of the cyclone at the genesis must be over ocean points where the SST is higher than 26 C.

7 Introduction Model set up TC tracking procedure Tracking is performed on those grid points satisfying the detection criteria. The following procedure is used to create a tracking database 1. For each detected cyclone point, a check is performed on each six hourly sample to see whether there are cyclones during the 24 hour period within a radius of 6 X 6 longitude-latitude grid boxes. 2. If there are some cyclones detected satisfying the previous condition, then the closest cyclone is chosen as belonging to the same track as the initial one. 3. To qualify as a track, there must be more than one detection within the next 24 hours and within a 6 X 6 longitude-latitude grid boxes.

8 present day TC characteristics from Perfect boundary run ERA-Interim forced runs

9 present day TC characteristics from Perfect boundary run Frequency and track JASO Cyclone tracks density from HURR dataset [1982:23] 4 JASO Cyclone tracks density from RegCM ERA Eman run [1982:23] The model underestimate the TC frequency over Pacific but overestimates over Atlantic 4 JASO Cyclone tracks from HURR dataset [1982:23] 4 JASO Cyclone tracks from ERA Interim runs [1982:23] HURR data RegCM-ERA Interim

10 present day TC characteristics from Perfect boundary run Inter-annual variability: JASO Atlantic cyclone inter annual variability 2. 2 r =.7 1. Standardized anomaly HURR RegCM ERA Year JASO East Pacific cyclone inter annual variability 2. 2 HURR RegCM ERA 1. Standardized anomaly r = Year RegCM4 realistically simulated the interannual variability TCs over North Atlantic

11 present day TC characteristics from Perfect boundary run Annual cycle TC frequency over Central America (14W 2W and N 4N) Number of TC grids/year Observed RegCM ERAint Month RegCM4 realistically simulated the annual cycle of TCs

12 present day TC characteristics from Perfect boundary run Intensity JASO Cyclone windspeed from HURR dataset [1982:23] Maximum wind speed from RegCM ERAInterim run [1982:23] km is still coarse to simulate intense cyclones

13 present day TC characteristics present day TC characteristics from GCMs Present day GCM forced runs

14 present day TC characteristics present day TC characteristics from GCMs Impact of model physics on TC climatology JASO Cyclone tracks density from RegCM HadGEM Eman run [1982:23] 4 3 JASO Cyclone tracks density from RegCM MPI Eman run [1982:23] Emanuel Emanuel Grell produces less TCs irrespective of the boundary forcing JASO Cyclone tracks density from RegCM HadGEM Grell run [1982:23] Grell JASO Cyclone tracks density from RegCM MPI Grell run [1982:23] Grell HadGEM forced run MPI forced run

15 present day TC characteristics present day TC characteristics from GCMs Why HadGEM produced less TCs over tropical Atlantic HadGEM mean JASO temperature [C] for [1982:23] MPI mean JASO temperature [C] for [1982:23] HadISST mean JASO temperature [C] for [1982:23] HadGEM SST is colder over the tropical Atlantic

16 Future changes: Frequency, Intensity and Tracks and Lifecycle Future Changes

17 Future changes: Frequency, Intensity and Tracks and Lifecycle Future Changes for JASO: TC Frequency Future (RCP8) Present for RegCM HadGEM Eman run 4 1 Future (RCP8) Present JASO from RegCM MPI Eman run A decrease in TC frequency over the tropical Atlantic Future Present HadGEM mean JASO wind shear Future Present MPI mean JASO wind shear These might be related to the increase in the wind shear over the tropical Atlantic 1

18 Future changes: Frequency, Intensity and Tracks and Lifecycle Frequency vs stability (Temperature profile) Pressure MPI mean JASO temperature [C] profile RCP8 Present MPI mean JASO temperature [C] profile RCP8 Present Atlantic Pressure East Pacific more stable atmosphere in the future climate

19 Future changes: Frequency, Intensity and Tracks and Lifecycle Future Changes: Tracks and life cycle JASO Cyclone tracks from MPI Eman runs [1982:23] JASO Cyclone tracks from MPI Eman RCP8 runs [278:299] Histogram of cyclones life cycle MPI Eman runs Present Future 8 Number of events Days The frequency of TCs particularly over tropical Atlantic decreases, however those will get reduced in number are the short lived ones

20 Future changes: Frequency, Intensity and Tracks and Lifecycle changes in Annual cycle Annual Cycle: Atlantic vs East Pacific TC frequency over North Atlantic TC frequency over East Pacific 16 Observed RegCM ERAint 16 Observed RegCM ERAint Number of TC grids/year MPI Eman HadGEM Eman MPI Eman RCP8 HadGEM Eman RCP8 Number of TC grids/year MPI Eman HadGEM Eman MPI Eman RCP8 HadGEM Eman RCP Month Month MPI run suggests an increase in the frequency over Pacific, whereas almost the opposite in HadGEM run Over the Atlantic basin, HadGEM2 shows a shift in the TCs season whereas MPI favours the reduction Number of TC grids/year TC frequency over Central America (14W 2W and N 4N) 18 Observed 16 RegCM ERAint MPI Eman HadGEM Eman MPI Eman RCP8 HadGEM Eman RCP Month

21 Future changes: Frequency, Intensity and Tracks and Lifecycle changes in Annual cycle Future Changes: Intensity JASO Future Present maximum wind speed of cyclones from RegCM HadGEM Eman run Future Present maximum wind speed from RegCM MPI Eman run East Pacific- the highest change in the wind intensity 3 2 Histogram of max. wind speed for HadGEM2 Eman Present Future 7 6 Histogram of max. wind speed MPI Eman Present Future Number of TCs Number of TCs Wind speed ms Wind speed ms 1 A decrease in the weak TCs but an increase in the frequency of the strongest TCs

22 Sensitivity to the removal of SST threshold from the detection criteria The impact of using SST threshold in the detection procedure on the differences in tropical cyclone formation between current and future climate JASO Cyclone tracks from MPI Eman runs [1982:23] JASO Cyclone tracks from MPI Eman RCP8 runs [278:299] JASO Cyclone tracks from MPI Eman WOSST WTMPF [1982:23] JASO Cyclone tracks from MPI Eman RCP8 WOSST WTMPF [278:299]

23 Sensitivity to the removal of SST threshold from the detection criteria Impact of SST threshold on Intensity 7 6 Histogram of max. wind speed MPI Eman Present Future Number of TCs Wind speed ms 1 Histogram of max. wind speed MPI Eman WOSST WTMPF 12 Present Future 1 Number of evnts Wind speed ms 1

24 Sensitivity to the removal of SST threshold from the detection criteria Impact of SST threshold on life cycle 1 8 Histogram of cyclones life cycle MPI Eman runs Present Future Number of events Days Cyclones life cycle MPI Eman WOSST WTMPF runs Present Future Number of events Days

25 Conclusions Conclusions TC climatology is found to be very sensitive to the convection scheme, and Grell scheme produces very few TC counts. HadGEM2-ES forced RegCM4 produces unrealistic TC counts over the tropical Atlantic because of a cold SST bias there. The MPI-ESM forced RegCM-Emanuel configuration reproduced well the TC climatology for the present climate and proven to be the best combination. Results under the RCP8. scenario suggest that the future frequency of the most intense TCs will increase but the number of week and short lived TCs will decrease. The overall frequency of TCs decreases in the tropical Atlantic and coastal regions of the Pacific but increases over Central Pacific and northern Atlantic. Removing the SST threshold from the detection criterion does not change the result substantially.