22 May 2001 CLEX-8 Field Operations Summary


Meteorological Conditions:

The synoptic conditions were dominated by a large high-pressure ridge over the western US with a deep low-pressure center located over Wisconsin (500 mb analysis). Moisture riding over the western ridge was the source for the targeted mid-level clouds. The 1200 UTC North Platte sounding had a strong capping inversion at 450 mb (-30°C). By 1800 UTC MM5 model forecast soundings had subsidence pushing the cloud top inversion down to 540 mb (-20°C). A deep mid-level moist layer existed between 700 mb and 500 mb. Below 700 mb, the temperature sounding was nearly dry adiabatic down to a strong capping inversion just above the surface. Strong northwesterly flow (with 100 knots at 250 mb and 50 knots at 600 mb) dominated the entire atmospheric profile.

By 1700 UTC a large mass of mid-level cloud had rapidly moved southeastward into Kansas (associated with a large region of upward vertical motion due to PVA). Patchy areas of mid-level cloud dominated the southwest Nebraska region (1703 UTC IR satellite image). Over this region, the surface temperature had reached 61° F, effectively eliminating the surface inversion and likely connecting cloud base to the surface through a deep mixed layer up to 700 mb. As a result, the mid-level clouds began to take on a more cellular or convective appearance from satellite. No precipitation was reported at the ground or noted on radar over the North Platte region.

Mission Summary:

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: Larry Carey (CIRA) and Paul Lawson (SPEC)

CIRA Forecast Team: Adam Kankiewicz (lead), John Davis, John Forsythe, and Don Reinke

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 with a few minor exceptions. 2D-C operational status was delayed a few minutes during the first cloud penetration. The Nevzorov TWC is too high by some multiplicative constant due to a software problem and was off during the first 10 minutes of the 2nd mission. The Rosemount icing probe occasionally gave unexpected readings.

Mission Strategy:

Given the meteorological and cloud conditions, scientific priority for operations was on the characterization of mid-level cloud microphysics with emphasis on horizontal and vertical structure. Since the clouds over KS were moving out of reach and were producing light precipitation at the surface, the strategy was to fly to vicinity of North Platte, NE to penetrate non-precipitating, mid-level, mixed phase cloud shield as indicated by early morning GOES satellite imagery. Approach mid-level cloud deck from above. Execute a racetrack near cloud top to ascertain general cloud morphology. For vertical structure, execute Lagrangian spiral down through vertical extent of cloud to base or below if virga exists. For horizontal structure, execute racetracks through cloud at various levels (upper, mid and lower) if possible.

Mission #1:

1700 – 1830 (all times UTC)

Learjet took off from JeffCo Airport at 1628. We arrived over the North Platte (LBF) area at 1650 and descended to 24 kft (all heights MSL) to ascertain cloud morphology. General cloud conditions were a broken to overcast AC deck below a thin scattered AS cloud deck. We were given clearance to fly within a 30 nm radius of North Platte Airport between 10,000 and 18,000 ft (occasionally modified to 6,000 to 20,000 ft). We began with a racetrack along cloud tops. First leg sampled AC/AS at about –18° C (17 kft). Peak liquid water contents (LWC) were 0.1 g m-3 in AC. CPI showed some ice in plate form. Noted a very thin AS layer above. Climbed and sampled AS at –20° C (18 kft – 19kft) using a undulating flight leg (or “porposing”). We then began downward Lagrangian spiral through multi-layer mid-level cloud complex. Upper layer contained plate-like crystals and liquid water up to 0.1 g m-3. Entered lower AC layer with ragged cloud top at 15 kft (-14° C). There were peak LWC up to 0.4 g m-3 in AC with plate-like crystals. Some big ice (mm-sized was present as passed through -12°C. There were still lots of cloud liquid water with big ice during descent to -7° C. The clouds seemed convective and well mixed. We exited cloud base during spiral at 9.3 kft (-4°C). Virga was seen visually and photographed. CPI and 2D-C detected large ice. We continued to sample virga below cloud base all the way down to 5 kft (9° C) where concentrations were too low to detect. No surface precipitation was ever reported at North Platte or detected by NEXRAD radar. We made a shallow climb back through cloud. We then executed racetracks through middle portions of AC layer (11 – 13 kft, -9°C to -12°C). Structure seemed cellular with peak LWC up to 0.5 g m-3 in updrafts. Plate like crystals were present as expected. Often in and out of AC cloud elements during final racetrack. Noted strange debris beneath cloud base with filament-like shape (order 10 mm wide and 100 mm long). We landed at LBF to refuel.

Mission #2:

2030 - 2130 (all times UTC)

Before taking off, we noted that cloud bases were darker and the clouds looked like elevated convection with virga but no precipitation reaching the ground. Took off at 2028 UTC. Rain and then small graupel on the windshield in virga streaks below cloud base at 9.5 kft (-2.5° C). Slow climb through line of AC to tops around 15 kft (-15° C). We then proceeded to step down through AC with level flight tracks. The first flight leg was just below cloud top. We dropped about 2 kft for the 2nd flight leg at 13 kft (-11° C). The next flight was about 1.5 kft down just above cloud base at 11.5 kft (-7.5° C). Clouds were very cellular in morphology. We were often in and out of cloud elements. There was light-to-moderate turbulence. LWC was substantial with peaks from 0.5 g m-3 to 1.0 g m-3. Ice was present (mixed-phase) and in the form of rimed plate-like (e.g., stellar) crystals, aggregates and small graupel. Virga was present beneath the cloud still in form of slushy aggregates and small graupel. Finally, we executed a slow climb through cloud and returned to JeffCo Airport.