G4 flight in support of FASTEX IOP 7 (Prepared by Bob Gall) Date January 25, 1997 Takeoff Time (approx) : 5:00 pm Return Time (approx): 11;13 pm Flight Duration: 6 hr 13 min Purpose The meteorological situation consisted of a very strong (80 ms) jet core to the east and north of Iceland oriented northeast-southwest with a surface frontal system parallel to it and toward its east. The entrance region of the jet was to the west of Ireland and south of Iceland. Models were forecasting the possibility of weak frontal waves forming along this front east of Ireland and then traveling rapidly toward the northeast. A weak cutoff cyclone was situated toward the southeast of the frontal line and was forecast to remain in that region. MSA flights were scheduled to leave Shannon late in the day (after 9pm) to investigate any frontal wave systems that might be present along the front within range of Shannon. The G4 flight was to investigate the entrance region of the jet core and map the PV structure prior to the MSA flights. Flight patterns for the G4 were designed to first cut across the jet near its maximum and then proceed beyond to near the trough axis. Sondes were to be dropped at regular intervals across the jet. Once in the trough, a series of east-west traverses of the jet were planned (with a drops continuing at regular intervals) along lines progressively south of previous traverses. Each traverse would inspect increasingly weaker wind regions of the entrance to the jet. The sondes would be used to evaluate the secondary circulations in this region and its relation to frontal structures. The MSA flights were eventually flown for this case. Description of the Flight. Prior to takeoff, satellite pictures indicated a long band of clouds parallel to the upper level jet with a hint of weak frontal waves along it and to its east. These waves appeared to be east and northeast of Ireland. After takeoff we proceeded Northwest toward a point at 60N 33W that is slightly east of the southwest coast of Greenland. First sondes were released starting at 55.5N and 16W. Sondes were then released at 6 to 10 min intervals. We entered high cirrus that obscured the ground just prior to the first sonde release. The first three sondes indicated 10-20 ms southerly winds at the surface switching to northwest at the surface with the fourth sonde. Thus the surface front appeared to be near 57N 20 W. Flight level winds from the southwest began increasing once we entered the cirrus. Maximum winds on this leg were 75 ms. Once through the cirrus, which ended rather abruptly after we passed through the wind max, the sky was covered with stratocumulus clouds below the aircraft with tops at perhaps 500 mb. That is how the sky appeared until dark which occurred as we began the first southward leg. The air at flight level was very clear. After passing through the wind max, wind speeds dropped off rapidly and turned from southwest gradually slightly south of west. Some mild to moderate turbulence was encountered on both the cyclonic and anticyclonic sides of the jet. We reached the most northwestern point of the flight (point 3) at 7:08 or about two hours after takeoff. From point 3 we proceeded directly south for two degrees of latitude with conditions at flight level very uniform. Winds were nearly westerly at 20 ms. We turned southeast at point 4 on the second traverse of the jet to a point on 23 W. All sondes on this leg showed northwest surface winds so we did not get far enough east to see the surface front which we estimated to be at 20 w. We did pass through the jet again but the maximum was slightly over 60 ms. Wind direction began to change before the speed began to increase. The temperature began to rise as the direction began to change. At point 5 (at 8:00 pm) we proceeded south for one degree of latitude then turned at point 6 to a mostly westerly heading. This took us again through the jet core, with a maximum of 60 ms. To the west of the maximum the wind turned into the west at 20 ms. Point 7 at longitude 56 N 33 W was reached at 9 pm. Track was then south for two degrees to point 8 which was reached at 9:20 pm. On this leg the wind direction continued to veer into the northwest and decrease to about 15 ms. From point 8 the heading was mostly east or directly home. Within 8 min of the turn, wind backed rapidly to southwest and began to increase reaching a maximum of about 55 ms. The strongest turbulence of the flight was encountered on the cyclonic side of the jet on this leg. Wind and theta plots showed a strong correlation in this turbulent region. Turbulence was also encountered on the anticyclonic side of the jet and over the suspected position of the surface front. The last sonde that provided useful information while we were in flight indicated a very shallow layer of northerly winds at the surface. We had no sonde data on the aircraft that indicated we had passed over the front. The track from the last sonde was directly home with a landing at 11:13 pm. Flight level winds were fairly steady from the southwest during the remainder of the flight. Overall Assessment of the Flight The meteorological features encountered were as expected in the positions and strength expected. Model guidance on jet structure appeared to be very good. Satellite features were moving slowly eastward so we passed into and out of cloud features where we expected them from the satellite guidance. Flight level data provided very good descriptions of the jet structure. Sonde performance was disappointing. On many, the pressure failed on launch though the sonde continued to send wind, temperature and humidity data. It is expected that data from many of the sondes in which only the pressure failed can eventually be recovered. Thus the sonde data should provide a high resolution picture of the jet entrance region from the surface front to the 300 mb trough line. Sixteen of the sondes were entered onto the GTS in realtime. Overall the flight was considered to be successful.