The 500 mb pressure level was dominated by a large low-pressure trough over the western US with a closed low centered over the Idaho-Utah-Wyoming triple point. Ample moisture and vorticity rotating around the closed low were the source of widespread cloudiness over the western US. The 1200 UTC upstream (Denver) sounding had ample mid-level moisture between 600 mb and 400 mb. Strong southwesterly flow (with 75 knots at 250 mb and 50 knots at 400 mb) dominated the wind profile from 200 mb down to 700 mb over the TERRA overpass region.
A large area of early morning mid-level cloudiness over the Nebraska Panhandle region was associated with a vorticity maximum rapidly rotating around the eastern side of the upper level low. Prior to the TERRA overpass at 1800 UTC, this cloud feature began to break up and rapidly rotate northward (cyclonically) into the Dakotas (1815 UTC VIS satellite image). Much of the remaining area of mid-level clouds was now topped by cold cirrus and was raining lightly. Most of the PVA associated with this cloud was already well into North Dakota by 1800 UTC. Despite the lack of obvious dynamical forcing, a relatively warm (-2° to –3° C IR tops) mid-level (13kft) AC field developed east of Rapid City, SD after 1715 UTC. The 1815 UTC IR satellite image can be seen here.This AC field was cloud target #1 for the Learjet during the TERRA overpass, which was nearly directly overhead. After the overpass, the Learjet sampled colder (-18° C) topped mid-level clouds to the northeast that were associated with the tail end of the PVA area rotating around the 500 mb closed low. WSR-88D radar summaries indicated that both of the sampled clouds were precipitation free.
Observational Platform: SPEC (Stratton Park Engineering Company Inc.) Learjet 25 (N999MF, Call Sign: Ice 9)
Base of operation: Jefferson County Airport, Broomfield, Colorado
Flight Scientists: Adam Kankiewicz (CIRA) and Cleon Biter (SPEC)
CIRA Forecast Team: Larry Carey (lead), John Forsythe and John Davis
Instruments:
SPEC Learjet was equipped with complete suite of cloud microphysical instruments (e.g., 2D-C, King
LWC, FSSP, Nevzorov LWC/TWC, and Cloud Particle Imager [CPI]) and state (e.g., T, P) and
aircraft parameter (e.g., altitude, position, heading, airspeed) measurements.
Instrument Status:
All microphysical equipment appeared to be operational and functioning normally. On-board GPS units failed around 1725 UTC during the flight.
Mission Strategy:
Scientific priority for this mission was to sample mixed-phase cloud tops during an 1800 UTC TERRA overpass near the Nebraska panhandle region. Racetrack patterns at cloud top would be run just before, during and just after the TERRA overpass. Cloud sampling would continue if suitable clouds (and fuel) were available.
Mission #1:
1750 - 1900 (all times UTC):
The SPEC Learjet took off from JeffCo Airport at 1710 with an initial target region located NE of Chadron, NE. By 1750 we were NE of Chadron and looking for a suitable cloud deck to sample for the 1800 TERRA overpass. With only a band of cirrus about eighty miles to our northeast the decision was made to sample a cloud deck that was below us. A good digital camera snapshot of the clouds we sampled was taken as we descended. We started our cloud top racetracks at 1754. During the first leg we sampled cloud tops at -3°C (13,800ft). Cloud top was a bit ragged and we descended to 13,200 ft (-2°C for the rest of the leg). Cloud tops were mostly liquid water (0.5 - 1.0 g m-3) with patches of ice (graupel) detected on the CPI periodically. At 1805 we turned around and made another pass through cloud top. We sampled cloud tops during two legs that were ten miles long in a northwest-southeast orientation.
and 100 mm long). We landed at LBF to refuel.
At 1817 we finished our second leg and headed northeast in search of another mid-level cloud deck. A suitable cloud deck was encountered at 1821. The mid-level cloud deck was oriented northwest to southeast and was thicker on the southern end. Cloud tops were near 20 kft (-18°C) with ice falling out of the bases. The closest clouds to us looked AS in nature while the more distant clouds (northeast) looked like AC with some vertical growth observed. We called air traffic control to request permission to sample these clouds and were informed that we were over some hot military air traffic. We were told to stay at our current altitude and heading. After a few minutes we were cleared to sample the northeast end of the mid-level cloud field. This was very disappointing as the healthiest part of the mid-level cloud was to our southeast.
We began a twenty-mile leg through cloud top at 1827. Cloud tops were somewhat ragged. The observed clouds were mostly ice with some liquid water detected (~0.03 g m-3). At 1835 we climbed to 23 kft and began a 500 ft/min descending Lagrangian spiral. The cloud was broken in places and was mixed-phase in nature with mostly ice and some liquid water present. Cloud base was near 18 kft (~ -14°C) and we continued sampling down to about 13 kft (-1°C) where the observed virga ended. Some liquid drops were observed below cloud base. Only thin areas of cloud were left above us through the column that we had just sampled. We then noticed an extensive stratus deck below us. We were getting low on fuel at this point and decided to look for a place to land. We headed towards Huron, SD to refuel. With no mid-level clouds near us, we returned back home to JeffCo after refueling.