Arctic weather during the FIRE/ACE flights in 1998

The weather systems, cyclones and anticyclones, along with air trajectories and cloud forms, are compared to past studies of the Arctic to assess compatibility of the month study of FIRE/ACE with past climatologies. The frequency and movement of cyclones (lows) and anticyclones (highs) followed the general eastward and northeastward directions indicated by past studies. Most cyclones (lows) came from eastern Siberia and the Bering Sea to the south and moved north across the Bering Straight or Alaska into the Arctic Ocean. They generally weakened in central pressure as they moved poleward. Anticyclones (highs) were most common in the eastern Beaufort Sea near Canada in June and July, as predicted from previous studies. However, many cyclones and anticyclones moved in westward directions, which is rare in other latitudes. Erratic changes in shape and intensity on a daily basis also were observed. The NOAA National Center for Environmental Prediction (NCEP) analysis generally reflected the SHEBA Ship WMO observations which it used. However, NCEP temperatures were biased warm by 1.0 degrees to 1.5 degreesC in April and early May. In July, when the surface temperatures were at the freezing/thawing point, the NCEP analysis changed to a cold bias of -1.0 degreesC. Dew points had smaller biases except for July where they were biased cold by -1.4 degreesC. Wind speeds had a -2 m/s low bias for the six windiest days. Surface barometric pressures had consistently low biases from-1.2 to -2.8 hPa in all four months. Air parcel historical trajectories were mainly from the south or from local anticyclonic gyres in the Beaufort Sea. Most air came to the SHEBA ship from the North Pacific Ocean or from Alaska and Canada and occasionally from eastern Siberia. Very few trajectories traced back across the pole to Europe and central Asia. Cloud cover was high, as expected, from 69 to 86% of the time. Satellite data also indicate frequent stratus, altostratus, and cirrus clouds (occurring 61 % of the time) above the expected boundary layer fog and Arctic stratus clouds.