Summary of the UGROW subproject on tropospheric temperature bias during JJA over the Northern Hemisphere

UGROW is an ECMWF cross-departmental project focused on Understanding systematic error GROWth from hours to seasons ahead. In this sub-topic of the UGROW project we have investigated biases in the lower to mid-tropospheric temperature during the northern hemisphere summers. The bias was investigated across different-time scales and with a range of diagnostic tools. The bias peaks around 700 hPa and grows fastest during the first days of the forecast. The bias mainly appears over land masses in early forecast ranges and has a maximum over eastern Asia. Despite being robust both in terms of day-to-day and year-to-year variability, the investigations so far have not pointed to a clear model error source. The aim of this report is to document the findings about this specific bias during the UGROW project, which will serve as a starting point for future investigations.