engleski

Sensitivity of RegCM4.3 to the definition of PBL scheme in historical, RCP4.5 and RCP8.5 simulations

Turbulent eddies in the planetary boundary layer (PBL) govern heat, mass and momentum fluxes between the atmosphere and the land surface, but are unresolved and parameterized in the current generation of regional climate models (RCMs). In RCM RegCM4.3, two PBL parameterizations are implemented: the diagnostic Holtslag scheme and a prognostic, turbulent kinetic energy based, scheme (the so-called UW scheme). In this study, sensitivity of RegCM4.3 to both PBL schemes driven by different reference concentration pathways (RCP4.5 and RCP8.5) is tested and the relationship between the components of the land surface energy budget is analysed. In two experiments with different PBL schemes, RegCM4.3 at the 50-km horizontal resolution was forced by the HadGEM2-ES historical simulation from 1971 to November 2005 over the Mediterranean region. Next, RegCM4.3 was forced with HadGEM2-ES RCP4.5 and RCP8.5 boundary conditions from December 2005 to November 2099. The historical simulation shows that the Holtslag scheme (HOLT) is warmer than the UW scheme (UW), with differences of up to 1°C in the annual mean of near-surface temperature (T2m). For the land surface energy budget, the following relationships emerge: (1) the net surface shortwave radiation flux (SWR) is higher in HOLT than in UW up to 7 W/m2 (consistent with the lower total cloud cover) ; (2) energy loss via the net surface longwave radiation flux (LWR) is higher in HOLT (consistent with the higher T2m and higher energy gain via SWR) ; (3) evapotranspiration and associated latent heat flux (LHF) are higher in HOLT (again consistent with higher T2m) ; (4) energy loss via sensible heat flux (SHF) is larger in UW, implying a stronger vertical temperature gradient (consistent with less vertical mixing in UW). The annual mean SWR is stable in the late 20th and throughout the 21st centuries under both scenarios, regardless to the PBL scheme used, and an increasing trend in LWR (becoming less negative) is balanced by the decreasing trends in SHF and LHF (becoming more negative). Thus, in order to keep surface energy budget in balance, energy loss via turbulent eddies is becoming more important in scenario runs. Different PBL schemes have different impact on the RegCM4.3 mean climate, but interannual variability and trends are generally unaffected. The increase in the mean T2m during the 21st century depends on the choice of the RCP: 3°C increase in RCP4.5 and 6°C increase in RCP8.5 simulations. However, the temperature difference between different PBL schemes is maintained almost constant until the end of simulated period.

RegCM; regional climate; RCP scenarios

nije evidentirano