April was predominately dominated by lows in the Bering Straights or over Alaska, with a high north of SHEBA ship (April 14 for example). This combination produced easterly surface winds most of the time with southerly or southeasterly winds aloft. However, wind directions changed to southwesterly when snow fell on the following days: April 6, 9, 10, and 13-17. Middle and lower clouds were present most of the time and upper level winds were usually southerly. Surface temperatures were from -12 to -17 C most of this period.
The first big low came into the Beaufort Sea southeast of SHEBA ship on April 18 and was northwest of the ship on April 21. It came out of Alaska across the Beaufort Sea east of SHEBA ship. This caused northerly to northeasterly surface winds for 3 days with snow and some low visibilities. Upper air winds remained southerly. Surface temperatures rose to -7 C in advance of the low.
Then a high came in from the northwest on April 24-25. This shifted both surface and upper winds to northwesterly for 3 days. Surface temperatures dropped to -25 C on April 25.
The rest of April returned to having lows in the Bering Straights and along Alaska's north shore (April 29 for example). Surface winds were from the northeast and snow occasionally feel.
The surface and 1.0 km trajectories show most air coming from the east. This occurred because of the lows in the Bering Straight and norhtern Alaska. The trajectories that traceback to the north occurred when a surface high was north west of SHEBA ship.
The higher trajectories, 3-9 km, mostly trace to the south or southwest. This relfects the dominance of a southerly upper air flow pattern. This air usually was ascending as it flowed north up the Bering Straight or acrossed Alaska. This is evident in a north-south cross-section of the trajectories. This ascension contributed to frequent cloud cover.
Ascending trajectories flowing north also can be seen in the 1.0 km and 1.5 km tractory cross- sections. Alaska spans 60 to 70 North on this cross- section. These trajectories show the low level air ascends it enters the Arctic Ocean from Alaska.
It should be noted that the height of the low level inversion was usually around 1 km. The 1.0 km trajectory was chosen to be in the warm part of the inversion while the 1.5 km trajectory was usually above it where the temperature began to decrease.
