Ability of CCSM4 to simulate California extreme heat conditions from evaluating simulations of the associated large scale upper air pattern

This study assesses how well the Community Climate System Model version 4 (CCSM4) simulates the large scale conditions needed for extreme hot surface temperatures in the California Central Valley (CV). Extreme hot summer days in the CV are associated with a large scale meteorological pattern (LSMP) described in Grotjahn and Faure (2008). The strength and sign of that pattern are assessed using a circulation index developed in Grotjahn (2011). The circulation index is strongly linked to daily maximum surface temperature normalized anomalies at stations spanning the CV. (Extreme heat events in the CV also affect a wider area of California and United States west coast). This study makes two primary points. First, the approach used in Grotjahn (2011) can be applied as a novel tool to evaluate how skillfully a model simulates conditions present during CV hot spells by evaluating how well the LSMP is simulated. The circulation index is calculated from historical simulations by CCSM4 and its distribution compared with that of the observed circulation index. Second, values of the CCSM4 based circulation index have smaller standard deviation than observed in reanalysis data. CCSM4 can generate a few comparably large circulation index values (implying high surface temperature anomalies) but not as often as in reanalysis data. Correct simulation of this large scale pattern is a necessary condition for successful simulation of California extreme hot days by a regional climate model. Also, the CCSM4 topography does not have a CV, but a broad topographic slope instead. Various choices of CCSM4 grid points were tested and none satisfactorily represented the CV maximum temperatures. These results should discourage use of CCSM4 surface data directly but encourage use of a regional climate model driven by the CCSM4 to capture hot spells in the CV.