Impact of Addition Arctic radiosonde observations on 5-day weather forecasts over Alaska during August 2015

Abstract

This study has investigated the impact of extra radiosonde observations from the 2015 Araon summer Arctic cruise over the Chukchi and East Siberian seas on 5-day forecasts over Alaska. The radiosonde sounding data produced every 12 hour from 00UTC 04 AUG to 12UTC 18 AUG are additionally assimilated to the conventional ALERA2 reanalysis. With the two reanalysis data sets as the initial conditions, two sets of 10-day ensemble forecasts are produced and compared (CTL without Araon vs. OSE_A with Araon). To verify the forecast performance, we compare the Z500 fields between CTL and OSE_A. Two forecasts are not quite different for the earlier period of the analysis-forecast cycle (from 04 AUG to 08 AUG), but the accumulated impact on the error reduction of OSE_A is shown for the later period from 12 AUG to 18 AUG. More detailed analyses are made for the two selected forecasts begun at 00UTC 12 AUG and 00UTC 14 AUG (Figure 1), in which the OSE_A forecasts are improved significantly after 4-6 days and 3-5 days, respectively. The evolution of the forecast errors over Alaska are similar for the first few days of the forecasts, but the forecast errors are reduced while the positive signals of extra sounding observations reach the downstrea Alaskan area after a couple of days. In those forecasts, the assimilation of extra Araon sounding data improved the geopotential height field by lowering abnormal high anomalies over Alaska.