Summer Large-Scale Surface Air Temperature Bias Pattern over the CONUS in UFS Prototype 8

This study examined the subseasonal variability (weeks 2-5) of surface air temperature bias over the Contiguous United States (CONUS) in the latest prototype of the National Oceanic and Atmospheric Administration, Unified Forecast System (UFS; prototype 8, P8). The mean bias of temperature is slightly improved or similar to previous prototypes. However, it is noticeable that the variability of bias in each initialization is much larger than the mean bias.

Using empirical orthogonal function (EOF) analysis, a large-scale surface air temperature bias pattern is separated in the summer months of June through September (JJAS). The bias pattern seen in the 1st week seems to be related to initialization. After the 2nd week, the large-scale bias pattern is robust and consists of an east-west dipole over the CONUS. This bias pattern is not robustly shown in winter, presumably due to more complex temperature variability.

The east-west dipole bias pattern significantly correlates with the upper-level atmospheric circulations forced from the tropical Central Pacific. First, the bias in convective activity over central Tropical Pacific in UFS P8 can enhance the dipole surface air temperature bias pattern over the CONUS. Second, the upper-level Rossby waves excited by the convective activity in the central Tropical Pacific are not well reproduced in the UFS. This also has an impact on the S2S forecast skill of temperature over the CONUS. Therefore, the forecast skill in central Pacific convection and the tropics to mid-latitude teleconnection need to be improved for a successful temperature forecast over the CONUS.

Results of this study suggest that the UFS P8 inability to forecast the tropics to mid-latitude teleconnections in summer could be related to the biases in the subtropical jet over the North Pacific.