A stratocumulus thermodynamic analysis: July 5 case study

On July 5 (NCAR Electra flight 4, Mission 186-G) the Electra flew a single aircraft mission which consisted of cross and along-wind legs at 6 different altitudes between 10:43 to 16:00 PDT (17:43 to 23:00 GMT). The leg length was kept short (8 to 10 minutes) to permit maximum vertical resolution, and there were 8 soundings. Observer notes report a thin, solid stratocumulus cloud deck which gradually became more broken in the afternoon. Winds were from the north at 10 to 13/ms throughout the night. Sea surface temperature measurements and conservative variable analyses for several of the July 5 legs are presented. These results are preliminary to a study of the thermodynamic budget on July 5; they indicate that: the sea surface temperature dropped more than 1 K (from 17.3 to 15.9 C) over the course of the flight (18:01 and at 21:51 GMT); mixing lines for each of the horizontal subcloud legs show the effect of a strong north-south gradient in SST; and there is a clear demarcation over a transition of 5 to 10 km between air to the south and cooler, moister air to the north. The FSSP measurements indicate there are small clouds/scud 250 m below cloud base on the cold northern side of this transition. The transition is seen in the saturation point diagrams at 984, 959, and 946 mb. There is no corresponding change in the horizontal wind across the transition regions.