After the first mission the PIs gathered in Augusta for a concluding science meeting (including NWS Science Officer from Wichita) to outline our near future and on-going plans for analysis of what we think is a very good data set.

The second mission was carried out as the showery weather (light rain) dissipated and fair weather with high broken cloud and scattered cumulus regained a hold on the watershed. The showery weather produced patchy soil moisture distribution with the western part of the watershed more moist than the eastern.

The second mission commenced from Augusta and a "V" pattern flown (instead of the triangle) between the three profilers. Soundings were taken at two of the three profilers. Following that a short study was conducted to ascertain the influence of Butler Lake on its surroundings.

Folliwng that a five level stack was performed at the southern edge of the watershed. We then moved northward and performed another five level stack finishing with a 60 nm alongwind run to obtain gradient information south of the watershed. This mission profile will produce budgets as well as a last check of the profilers. Since there was no CLASS sounding support this mission will also be more like future missions which will rely upon the remotely sensed profiles rather than ballon profiles; a fitting end to CASES97.

Friday we will rerun the flux computations from all 7 IOPs as a first quick look data set (currently only available to the PIs). Three IOPs have been run, using the same software for the Twin Otter as well.

1. Two aircraft, two missions.
2. Morning transition: 2-aircraft Stack + Cat's Ears. KA and TO switch roles, with KA doing Cat's Ears.
3. Evening Transition: Cat's Ears + Stack. Both aircraft land at Ponca City between flights.

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Monday will have N to NE winds throughout the day. Winds will change to easterly overnight, with a chance of scattered showers. Tuesday will see winds from E to SE to S (changing in the afternoon). Another chance of precip.

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This IOP afforded the opportunity to document boundary-layer growth over patchy surface moisture. This patchiness arose from an uneven distribution of precip through the night of May 13, not from an uneven distribution of dry-down: the eastern stations received over one inch of preceip, the western stations received much less, and the southern station received no precip at all. Strong horizontal advection by a low-level jet centered above the boundary layer during the morning implied that

1. the boundary layer was growing into warming air with stable stratification.
2. the boundary layer was growing into moister air (rather than drier air, as is normally the case).

The result was an extended 'morning' (quotes appropriate for this case) transition, with considerable variation of PBL height on the mesoscale through 1630 CDT. This variation made it difficult to determine the heights for aircraft flux profiles, though it appeared, after the fact, that useful choices were made.

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With the air calm at sunrise, the surface inversion layer was quite shallow and surmounted with a near-neutral layer. Once the potential temperature of the surface layer matched that of the near-neutral layer (around 11 a.m. CDT (10 a.m. CST)), boundary-layer growth was explosive. This explosive growth transpired around 11 a.m. CDT (10 a.m. CST). At this time, the radar scientists noted considerable variation in their measurements of the depth of the mixed layer from clear-air echo; their estimates varied from ~500 m to ~1 km. This explosive growth could explain why the aircraft scientists were confused about boundary-layer depth at this time: one aircraft reported a BL depth of 2500 ft msl (0.76 km msl) and the other subsequently reported a depth of 6000 ft msl (1.83 km msl or 1.4 km agl, which is about the depth the BL reached).

The radiosondes and profilers document divergent histories of the PBL after 2-3 p.m. CDT (1-2 p.m. CST). The profilers show the boundary layer depth decreasing at a rate of about 2 cm/sec, while the sondes show the depth of the PBL to be roughly constant. This inconsistency could be related to our ignorance of how to interpret profiler signals: when we think they are seeing the top of the boundary layer, the profiler might actually be seeing part of the stable inversion above the boundary layer.

Nevertheless, we find ample evidence for subsidence of the very dry air layer observed above the mixed layer from

1. tracking of the height of the extreme humidity value, from the sondes (and assuming no horizontal advection effects), yielded a subsidence of 1.7 cm/sec.
2. the boundary layer depth decrease, as revealed from signal-to- noise ratio at the Beaumont and Whitewater profilers, was about 2 cm/sec.
3. the top of the 'fossil' boundary layer from the previous day, as revealed by the signal-to-noise ratio, appeared to subside at a rate of about 1.6 cm/sec.

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A comparison of extreme humidity values and surface humidity values suggests some systematic biases between the stations. Careful comparisons to surface towers (Argonne and NCAR/CLASS), plus intercomparison with a standard, should mitigate these problems.

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We did some comparisons of radar and sounding winds; comparisons were very good. However, radar estimates of inversion heights varied significantly from morning sonde observations, possibly because of the elevated mixed layer or large groups of migrating insects. After discussion, we agreed to follow the afternoon scanning pattern for nighttime, in order to isolate the effects of migrating birds. We anticipate comparing the night-time winds from the sondes, profilers, and radar.

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During this period, radiosondes will be launched from Beaumont, Oxford, and Whitewater each hour and a half, meaning balloons will be released at 1630, 1800, 1930, 2100, 2230, 2400 (April 28, 1997), 0130, 0300, 0430, 0600, 0730, 0900, 1030, 1200, 1330, and 1500.

The NOAA Twin Otter and the Wyoming King Air will have three missions:

1. Commencing about 1500 CDT. Will fly 'Mixed Cat's Ears and Stack' patterns with a Triangle at the beginning and end of the mission. In the Mixed Cat's Ears and Stack pattern, one aircraft will fly two legs just below the top of the boundary layer, punctuated by pop-up soundings to locate the top of the boundary layer, and one leg just above the surface, while the other aircraft will fly multiple levels between the surface and boundary layer top. The triangle pattern will be flown by both aircraft (one aircraft will fly it before the major pattern and one will fly it after) between the three profilers at Beaumont, Oxford, and Whitewater.
2. Commencing about 0800 CDT and ending between 1200 and 1230, Tuesday, April 29. Aircraft will fly Mixed Cat's Ears and Stack.
3. Commencing between 1300 and 1330 and ending around 1700 CDT, the two aircraft will fly Stacks and Triangles. In this pattern, one aircraft will fly the triangles, while the other flies the stack pattern. Both will be punctuated by pop-up soundings to verify boundary-layer depth.

In all cases, the pop-up soundings will not only help dictate the heights at which the aircraft will fly, but will be used for later comparison with radiosondes and profilers.

We have requested support from the S-POL radar for at least the daytime. The radar would do PPI scans to enable determination of the wind field from tracking of eddies (TREC) and RHI scans for still another look at the vertical structure of the boundary layer.

The first IOP studied a situation with increasing southerly winds ahead of a developing low to the west and a slowly advancing cold front to the north. The IOP was successful and should yield a good data set. The JETEX group at Iowa State, under the leadership of Ray Arritt and Moti Segal, joined us in the IOP.

The IOP commenced at 1500 LDT April 28 with the coordinated takeoffs of the NOAA Twin Otter and the University of Wyoming King air, and ended at 1900 LDT when the aircraft landed.

1. Three coordinated flight patterns were flown:
2. ~1500-1930, April 28. One aircraft flew straight-and-level crosswind legs at several levels in the boundary layer, while the second alternated between low-level legs and legs just below the top of the boundary layer. Triangle patterns between the three ABLE profilers were flown at the beginning and end of the pattern. The first will provide vertical profiles of vertical fluxes; the latter pattern will yield horizontal gradients over the experimental domain.
3. 0900 - 1430. The NOAA Twin Otter (NOAA) and the Wyoming King Air (WY) flew coordinated stacks to obtain flux profiles. WY completed their pattern with a triangle. NOAA landed at Augusta Airport between missions, while WY returned to Ponca City to refuel.
4. 1500 - 1700. NOAA continued flux stacks, while WY flew triangles at the lower, middle, and upper parts of the boundary layer. The triangles are particularly important since the radiosonde launches are over. At the end of the mission, the two aircraft flew triangles together.

   During the patterns, the aircraft checked in to CASES Ops for comparisons of boundary-layer depths to what we were seeing from profiler and radar. The comparisons were fair, but it is clear we need to learn to interpret both radar and profiler data better. The flying scientists noted at the end of the mission that the humidities were increasing rapidly through the day, and that a low-level jet was observed upon landing at Ponca City. A great set of missions.

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1. NCAR: All eight NCAR stations were operating, all systems operative except CO2 at the ASTER supersite (Site 7). There were short outages that caused loss of 13 minutes of data at ASTER Site 8, and 1.5 h of data were lost at ASTER Site 7. Soil moisture profiles were taken at both the sites the morning of April 28.
2. ABLE: The Smileyburg site weather data were successfully collected. Flux data were more intermittent, but problems should be resolved (this station was just installed).
3. NOAA: Station should be installed in the next few days on pastureland.

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