G4 Flight in support of IOP 11 (prepared by Bob Gall) Date: February 5-6, 1997 Takeoff Time: 22:32 Z (Shannon) Return: 04:32 Z Flight Duration: 6 hr Purpose A rapidly developing low pressure system was expected in the northwest edge of the MSA around 0000 Z on February 6. This low was referred to as low 30. The mission described here was a part of a four aircraft IOP on this system. The UK C130 took off first around 1800 Z followed about one hour later by the the Electra. These two aircraft worked together in a warm frontal region northeast of Ireland until the Electra had to return. The P3 took off about 2200 and joined the C130 in the vicinity of the low center which at this time was forecast to be an elongated region oriented north-east south-west out beyond 20 W. Finally, the G4 took off at 22:30 Z to provide a survey through the suspected center of the low, then west to just beyond the position of the 300 mb trough. The track was then southeast to beyond the surface front, turning north to the northern edge of the cloud shield then return to base. Together these four aircraft with the radar observations coupled with over 100 dropsondes would provide one of the most comprehensive coverage of a developing oceanic storm system ever. Description of the Flight The entire flight was at night so there were no observations of the clouds over which we were flying. We were hoping that during the first leg of the flight, which would be generally westward from Shannon, we would be able to drop sondes from 45,000 ft. Unfortunately we were not cleared to that altitude and so all drops were from 26,000 ft (358 mb). The initial leg was toward 55 N 32 W with the first sonde release at 54 N 14 W and the next two were separated by a distance equivalent to 1.5 degrees of latitude. After 18 W the separation of the sonde drops was decreased by half and remained at that spacing until half way through the southeast leg. After that the spacing was again equivalent to 1.5 degree of latitude until the last sonde just before our return to Shannon. Early in the first leg flight level winds were from the southwest at speeds near 50 ms. The first sondes indicated surface winds above 25 ms also from the southwest and surface temperatures near 10C. Subsequent sondes showed surface temperatures increasing to 11.7 degrees and then decreasing slightly as the surface front was approached. Up to the surface front, surface winds remained from the southwest at 20 to 25 ms. Flight level winds increased slowly and backed slightly. Sondes indicated a strong low level jet with speeds of 30 ms and strong reverse shear near 850 mb slightly on the warm side of the front. We passed over the surface front near 22 W where winds at the surface veered rapidly to due north with speeds near 15 ms. This wind shift occurred between adjacent sondes, about 1 of longitude degree apart. From there westward, surface temperature decreased gradually to near 0 C and winds continued out of the north to north-northwest at 15 ms. At flight level, the wind maximum was passed near 27 W. As soon as the wind speed began to roll off, turbulence as indicated by WI time series, rather suddenly increased. Some bumps were felt and they became very strong near 28 W. At 29 W the dewpoint at flight level dropped suddenly indicating we had passed into stratospheric air. Winds backed only slightly indicating that we never reached the trough axis at flight level. Theta dropped 13 C from the wind max to the turning point. We reached the turn point at 55 N 32 W at 1245 Z. From there the course was toward the southeast to a point 50 N 24 W. The surface front as indicated by the dropsondes was located near 51.5 N 25 W where the winds switched from northwest to southwest with speeds on both sides near 20 ms. The wind on the warm side of the front was slightly more westerly than it was further to the north. Temperature on the warm side was about the same as further north, 11C. The wind maximum at flight level was near 53.5 N 29 W. On the anticyclonic side of the jet, shear was very weak with speeds near 60 ms at flight level. The turning point was reached at 0149 Z. The track was then to the north-northwest passing over the first leg near 55 N 25 W and toward a point 59 N 26 W. Two sondes were launched on the two legs at nearly the same point (55 N 25 W) but 2 hr 25 min apart. The soundings were similar but it was clear from the cooling indicated in the second sounding that the front had progressed significantly toward the east bu the time of the second sonde at that point. These sondes were both on the cold side of the front. The surface front was passed near 52 N 24 W where surface winds again switched to the northwest on the cold side. Temperature at the surface fell rapidly within a degree of latitude across the front from above 11 C to below 6 C. Further to the north, temperature gradually fell to below 2 C and winds backed slightly but remained near 30 ms up to the turning point. At flight level winds were fairly constant from the southwest at 60 ms until approximately 58 N and then dropped off first rapidly then gradually until the turn point. The turning point at 59 N 26 W was reached at 0250 Z. The route was then directly home. During this leg we reached the wind maximum at about 20 W and found the surface front with the sondes to be near 57 N 18 W. Conditions on either side of the front were similar to those found on the earlier legs. Wind speeds at flight level gradually fell off and the wind veered slightly. The last sonde was released near 54 N 12 W at 0408 Z. Overall assessment of the Flight. We saw no evidence of a closed low system in the sonde data on any of the legs. In this respect the data suggest that the forecasts by the Hirlam model which showed an elongated shear region rather than a well defined low center was correct. We passed the front at almost precisely the position predicted by the Hirlam and model predictions of temperature and pressure were very close. Sonde success rate, at least as displayed in realtime in flight was disappointing. Failure rate, especially on the last half of the flight appeared to approach 50%. It is not known at the time of this writing, how much of the data form the sondes that appeared to fail in the realtime data could in fact be recovered during post flight analysis and editing. Even if only two thirds of the sondes could eventually provide useful data, it was felt that the flight was highly successful.