The first field deployment of the CARG's Convair-580 aircraft
is in FIRE-ACE/SHEBA, where the aircraft is based in Barrow, Alaska.
Summaries of each flight will be given below within a day or
so of their occurrence.
1) Traverses above, within, and below a stratus deck over the Barrow-ARM site for comparisons with ground-based remote sensing measurements and for cloud absorption and cloud structure measurements. (Due to low cloud base, measurements below cloud were restricted.)
2) Repeated the above series of measurements off-shore over sea-ice in the Chukchi Sea with greater success because of ability to fly with more freedom below cloud base. Also measured BRDF of ice surface with overlying stratus deck.
3) Did some long-track measurements in the stratus cloud over the Chukchi, including possible diffusion-domain measurements with the CAR.
4) Some sampling of aerosol layers above cloud top.
Following the above, the CV-580 made BRDF measurements of sea-ice under clear skies at the most northerly point of the CV-580 track when it was flying beneath the ER-2.
The CV-580 then returned to the ARM site and did a vertical profile from 300-20,000 ft. (Note: PSU Raman lidar is operating at the ARM site.)
Measurements included:
1) Profiles thru three thin stratus layers at different heights with cloud structure measurements, radiative measurements, aerosols etc.
2) BRDF measurements over sea ice and open water under diffuse light with stratus clouds above.
3) Aerosol measurements around clouds.
We obtained radiation measurements in horizontal runs in clear air above the tops of the Ac, between the two Ac layers, and between the base of the lower Ac and the top of the stratus. During the descent, measurements were obtained in horizontal legs of cloud microstructures in all three cloud layers. The upper Ac layer was at -8C, the lower Ac layer at -4C and the stratus layer -6C.
The flight was terminated prematurely due to a (probably erroneous) indication of the right engine overheating.
2) BRDF measurements over highest Ac layer.
3) Radiative measurements above, between, and below Ac cloud layers.
4) Microphysical measurements in the cloud layers.
5) Followed (in upper cloud layer) beneath ER-2 to ARM site plus 50 miles south of ARM.
6) Climbed to 25 k ft through Ac layers and Cs (did not quite clear tops) near ARM.
7) Spiraled down from 25 k ft to close to surface over ARM site.
8) BRDF centered on ARM with no cloud below but broken Ac above.
9) Level runs at 2 k ft through ARM site over snow-covered tundra, open ocean, and ice-covered ocean, with CAR scanning down on two runs and upward on one run.
2) Low-level measurements over leads for emission measurements. (Nothing apparent in real-time displays, but complex situation.)
3) Climbed to 15 k ft in deep cloud layer over the ARM site, cleared cloud top. (PSU Raman lidar, as well as ARM instruments, operating.)
4) Descended to 500 ft over ARM as as cloud was thinning.
5) Straight and horizontal run through ARM site at 500 ft in clear air for radiation measurements.
6) Ditto at 1500 ft for downward-scanning CAR measurements.
1) 1928-2110 UTC: Transit to SHEBA site mainly in clear air. Haze layers apparent.
2) 2127-2231 UTC: Descended from 21.5 k ft (above Ci) to 500 ft through cloud layers in a step-wise fashion, with horizontal legs centered on SHEBA ship. ER-2 flew parallel legs above. Low visibility and drizzle at the surface (at 76 deg 40 min north in May!!), prevented us from flying below 500 ft over SHEBA.
3) 2233-2245 UTC: Spiral up over SHEBA through clouds to above Ac layers.
4) 2245-2254 UTC: BRDF measurements above highest Ac layer with uniform cloud top.
5) Left SHEBA site at 2254 UTC.
6) 0018-0040 UTC: On way back to Barrow did BRDF measurements (10 turns) above highest Ac layer; clear sky above, glory in Ac.
NOTE: ALL TIMES APPROXIMATE. EXACT TIMES AVAILABLE WITH POST ANALYSIS.
1) 1910-2050 UTC: Transit to ship.
2) 2056-2120 UTC: BRDF measurements above the Ac layer over the ship.
3) 2120-2125 UTC: Level pass above Ac layer through center of the BRDF turns (for SFFR measurements).
4) Descended through Ac layer.
5) ~2135-2140 UTC: Ditto #3 above on same line but below Ac layer.
6) ~2140-2200 UTC: Two straight and level runs above both legs of ground-based, L-shaped, albedo site for radiation measurements.
7) BRDF circles in clear air, with broken Ac above, centered on the SHEBA ship.
8) 2221 UTC: Ascended over ship through Ac layer.
9) Flight over ship in Ac layer.
10) 2230 UTC: Left SHEBA site for return to Barrow.
SUMMARY: Surface albedo and BDRF measurements over ARM site with cloudless sky, followed by spiral to 14 k ft over ARM. BRDF and structural measurements of Ci. Structural measurements in Ci.
TIME SEQUENCE:
1) 2013-2030 UTC: Level flights at 2 k ft over ARM under clear sky.
2) 2037-2100 UTC: BRDF measurements centered on ARM (ten turns)
3) 2112 UTC: Over ARM at 2 k ft.
4) 2112-2035 UTC: Spiral to 14 k ft above ARM in clear air. Weak aerosol layer at 9.5 k ft.
5) 2135-2142 UTC: Level run at 9.5 k ft in aerosol layer.
6) 2218-2240 UTC: Climb to above cirrus tops.
7) 2240-2250 UTC: Level runs above cirrus.
8) 2252-2351 UTC: BRDF of cirrus.
9) 2325-2335 UTC: Level runs beneath cirrus.
10) 2400-2410 UTC: Level runs in cirrus.
11) 2410-2430 UTC: Descent through cloud to altocumulus layer followed by a level run through the cloud.
This was a coordinated flight with the Convair-580 flying beneath the ER-2 over the ARM site in the presence of a broken stratocumulus layer.
1) 2030-2100 UTC: Level runs above Sc layer for radiation measurements.
2) 2100-2110 UTC: Descent in clear at SE point of run below cloud base.
3) 2110-2129 UTC: Return to NW point of run beneath Sc layer for radiation measurements.
(2116 UTC: ER-2 AND CONVAIR-580 OVER ARM SITE SIMULTANEOUSLY. SC PRESENT.)
4) 2129-2131 UTC: Climb thru Sc at NW point of leg.
5) 2131-2151 UTC: Measurements in Sc fromm NW to SE points of leg.
6) 2151-2156 UTC: Climb in clear air at SE point.
7) 2156-2219 UTC: BRDF measurements above Sc.
8) 2219-2245 UTC: Study of unusual precipitation event.
NOTE: CONVAIR-580 WAS ALMOST EXACTLY BELOW THE ER-2 AT SEVERAL TIMES IN THESE LEGS.
SUMMARY: Radiation and cloud measurements over SHEBA ship with ER-2 overhead.
SEQUENCE:
1) ~1912-2050 UTC: Transit from Barrow to SHEBA ship.
2) 2050-2053 UTC: Level run 2,000 ft above cloud top in principal plane centered on ship.
3) 2058-2100 UTC: Level run as in (2) above but perpendicular to principal plane.
4) 2104-2119 UTC: Six CAR turns above Ac cloud top (and over ship) for BRDF.
5) 2119-2131 UTC: Descent to below cloud bases (some stratus below).
6) 2134-2141 UTC: Level runs at 200 ft above surface along same two perpendicular lines as in (2) and (3) above, and centered on ship.
7) 2144-2157 UTC: Six CAR turns (for BRDF) drifting over ship.
8) 2200-2210 UTC: Level runs above the two arms of the L-shaped surface albedo site.
9) 2210-2217 UTC: CLimb to top of upper Ac layer.
10) 2219-2221 UTC: Level run above Ac (A'-B').
11) 2224-2225 UTC: Level run below Ac (B'-A').
12) 2227-2239 UTC: Measurements in upper Ac. Left SHEBA site at 2240 UTC.
13) Some measurements in Ac on return to Barrow.
This flight was devoted to a study of "Aerosol-Cloud Shading" (U. of Wash. Flight Scenario #4) over the Chukchi Sea northwest of Barrow.
The aerosol layer extended from near the top of a shallow stratus layer (near 1100-1600 ft) to about 3600 ft. Although the aerosol layer was not particularly dramatic (light-scattering coefficients about three times above background), absorption coefficients reached 50% of the scattering. Measurements were made in the aerosol layer of ionic and carbonaceous species and humidification factors, as well as the usual size distributions etc.
The stratus layer extended to the the surface; measurements were made at 500ft above the surface. BRDF measurements were made above cloud top. Cloud structure measurements were obtained. There were no clouds above the stratus in the vicinity of the measurements.
Radiation measurements were obtained above the aerosol layer, between the aerosol layer and cloud top, and near cloud base.
The above set of measurements was repeated twice.
This flight invoved two missions: flying beneath the ER-2 in the vicinity of the ARM site, and an "Aerosol-Cloud Shading" study.
1) Co-ordinated flight with the ER-2
The cloud situation was stratocumulus from ~1500 to 2500 ft with broken stratus below. We obtained BRDF measurements of the Sc just prior to the arrival of the ER-2. We then flew a "cross-pattern", centered on the ARM site, beneath the path of the ER-2, with legs above, in, and below the Sc. The ER-2 was sighted.
2) Aerosol-Cloud Shading
West of Barrow we found a good situation for this study: an aerosol layer overlying fairly uniform Sc but with a "hole" in the cloud to the west. We flew above the main aerosol layer, between the aerosol layer and cloud top, beneath cloud base, and in the cloud. These various level runs included the cloud on either side of the "hole" and the "hole" itself. Underlying surfaces varied from sea-ice, open ocean, tundra, and snow covered tundra.
This flight was concerned primarily with measurements over the SHEBA ship, now located at 77 deg 00 min N/166 deg 40.6 min W. A front passed over the ship during the period of our measurements. After returning to the Barrow area, aerosol measurements were made in a vertical profile under cloudless conditions in the vicinity of the ARM site.
Measurements Over the SHEBA Ship:
1) 2048-2109 UTC: Spiral descent over SHEBA ship from 19,000 ft to below cloud base (~1500 ft) through several cloud layers.
2) 2108-2157 UTC: Ten CAR circles, centered on ship, for BRDF measurements under diffuse lighting (i.e. overcast sky). Also, several level runs over ground L-shaped surface albedo array for albedo measurements.
3) 2157-2238 UTC: Spiral ascent over ship. Level run, centered on ship and extending 6 nm on either side of ship, at 1400 ft in stratus layer. Level run at 19,300 ft in cirrus, also centered on ship and 6 nm on either side of ship.
2238-2412 UTC: Return to Barrow
Measurements in Barrow Area:
Vertical profile from 22,000 ft to surface in cloudless sky. Intensive aerosol measurements in haze layers at 9,300 and 8,400 ft, including filter samples.
This flight was concerned with a cloud-aerosol-radiation study over the Chukchi Sea.
1) ABSORPTION OF SOLAR RADIATION BY A STRATUS LAYER.
With clear sky above, and sea ice below, radiation measurements were made above and beneath a fairly uniform stratus layer consisting mainly of water drops. Measurements were then made within the cloud.
1) "AEROSOL-CLOUD SHADING" EFFECT
Since the cloud referred to above had a significant aerosol layer above it, and the cloud also had a hole in it (to the west), it corresponded closely to Fig. 7 in "Hobbs Flight Scenarios for FIRE-ACE". Therefore, immediately following 1) above, we moved into a study of the "Aerosol-Cloud Shading" Effect. Detailed radiation and aerosol measurements were obtained above and in the aerosol layer, between the aerosol layer and cloud top, below cloud base , and within the cloud. These various runs extended above and below both the cloud and the "hole" in the cloud.
3) BRDF MEASUREMENTS were then made of the stratus tops.
4) FURTHER IN-CLOUD measurements were obtained on the return to Barrow.
1) CLOUD MICROPHYSICS MEASUREMENTS (~1830-1925 UTC)
Measurements of structures in Sc, Ac, As, Ci over Chukchi Sea. The complex multi-layered clouds confounded any simple radiative investigations.
2) MEASUREMENTS OVER ARM SITE (1946-2034 UTC)
Spiral descent from 21,000 ft to 700 ft over ARM site through five cloud layers. Intensive measurements in an aerosol layer at ~11,000 ft.
SUMMARY: Cloud and radiation measurements around SHEBA ship.
DETAILS:
1) 1933-1956 UTC: BRDF measurements over single stratus layer just to NW of ship. (Immediately above the ship there was an encroaching Ac layer above the stratus. Therefore, decided to do cloud absorption measurements on the stratus layer in a region unaffected by the Ac).
2) 1958 - 2005 UTC: 20 nm level run above top of the stratus (very uniform cloud layer), a/c drifted with wind (A -> B).
3) 2008 - 2016 UTC: 20 nm level run below cloud base (B-> A).
4) 2020 - 2031 UTC: level run within St cloud.
5) 2037 - 2042 UTC: Level runs in clear air at 300 ft AGL over both arms of surface L-shaped albedo site.
6) 2045 - 2053 UTC: BRDF measurements of surface, centered on the ship, in diffuse light, centered on ship.
7) 2054 - 2058 UTC: Spiral over ship through St plus two Ac layers (another broken Ac layer above which we did not climb to).
8) 2100 - 2103 UTC: Measurements in main Ac layer (one from top).
SUMMARY:
This flight took place over the Chukchi Sea for stratus cloud microstructure measurements. Long in-cloud legs were flown near cloud top, in mid-cloud, and just above cloud base. The cloud was observed to vary in thickness over 60 nm legs from about 400 feet to about 1400 feet. The cloud had a stable lapse rate (isothermal near cloud top). The cloud exhibited modest inhomogeneities in structure along these legs.
DETAILS:
1) 1950-2010 UTC: First leg near cloud top (-2 C). Liquid water contents reached 0.3 g m-3.
2) 2010-2015 UTC: Vertical profile from 1000 feet above cloud top to 100 feet above surface.
3) 2019-2040 UTC: Mid-cloud leg.
4) 2041-2050 UTC: Vertical profile as in 2).
5) 2053-2109 UTC: Leg just above cloud base. Cloud bases varied from around 800 feet to 200 feet AGL.
6) 2109-2121 UTC: Vertical profile.
7) 2122-2130 UTC: Search for new substantial stratus region for sampling.
8) 2121-2134 UTC: "Porpoise" maneuver in stratus layer.
9) Return to Barrow at 500 feet above stratus layer for aerosol and CAR measurements.
PERIOD OF FLIGHT (ENGINES ON TO ENGINES OFF): 1856-2414 UTC
SUMMARY:
This was our 6th flight over the SHEBA ship (now located 405 nm from Barrow). The flight took place as a trough with multi-layered clouds overspread the SHEBA site, a preferred scenario for the purpose of comparing aircraft data with that obtained from the vertically-pointed radar/lidar on the SHEBA ship. Two vertical profiles were made over the ship. BRDF measurements were made above the highest cloud layer, and single passes for surface albedo measurements were made over the L-shaped surface array.
DETAILS:
1) 1908-1911 UTC: The climb at takeoff was through unusually transparent stratus clouds that were producing drizzle episodes at Barrow. The lowest droplet concentrations yet encountered in Arctic stratus during this campaign (10-30 cm-3) were measured.
2) 1911-2055 UTC : The transit leg of the flight was largely cloud free, with only a few ice crystal cloud encountered in the otherwise cloudy, multi-layered scene. (The multi-layered clouds were either too high or too far below the aircraft for efficient sampling while enroute).
3) ~2055 UTC: Arrived at SHEBA ship. Altocumulus cloud layers were present over SHEBA, with the highest tops intercepted at about 14,000 ft with isolated mounding cloud tops of altocumulus castellanus clouds 100 feet or so above this level. Cirrus clouds and cirrocumulus clouds were scattered above the altocumulus clouds.
4) 2059-2116 UTC: A BRDF maneuver consisting of seven circles was carried out over the altocumulus layer a few miles northeast of the SHEBA ship (to avoid higher clouds). However, a patch of nearly transparent cirrocumulus cloud extruded itself into the clear region around the second or third orbit. A nearly invisible cirriform wisp was also present overhead.
5) 2120-2143 UTC : A level pass was made over the ship in the higher altocumuli. A spiral descent was then made through the various cloud layers to about 300 feet above the surface. The clouds over the ship consisted of four layers: three patchy layers of altocumulus clouds with bases at 14,000, 12000, and 10,000 ft. (the lowest were the bases of the Ac cas), and a stratus layer (with breaks) with tops at 1100 feet and bases between about 200 and 500 feet. An unusual aspect of the stratus clouds was the extremely low droplet concentrations (~10 cm-3).
6) 2143-2148 UTC: Level passes were made over the L-shaped surface albedo array.
7) 2148-2220 UTC: A spiral ascent was then made over the ship to within a 100 feet of the highest cloud top (a very thin, ice shedding patch of cirrocumulus droplet clouds at about 20,000 ft) before departure from the site was required.
8) 2220-2330 UTC: Extensive ice clouds (various forms of cirrus (uncinus, fibratus), and ice shedding altocumulus clouds) were sampled on the return trip to Barrow.
9) 2358-2410 UTC: The landing at Barrow found much different clouds than at takeoff: elevated stratocumulus/stratus with tops at 2500 feet and bases at 1500 feet. Droplet concentrations were ~100 cm-3.
PERIOD OF FLIGHT: 2051-2450 UTC
SUMMARY:
The goal of this flight was to perform BRDF orbits over cirrus clouds with no cloud below, a rare situation which presented itself at Barrow on 19 June during the mid-day hours. A downward spiral over the ARM site was also made.
DETAILS:
1) 2104-2155 UTC: Takeoff and climbout to 31,000 ft! From below the cirrus appeared amorphous, producing a strong halo with no visible structure (cirrostratus nebulosous). However, as the spiral ascent continued, more structure in the forms of striations and chaotic patchiness began to appear. By the time the flight level reached 31,000 ft, three layers had been detected, with the highest, very thinnest layer (cirrus fibratus/cirrostratus fibratus) 1000-2000 ft higher than the aircraft could reasonably reach. Cloud top heights of the lower, mainly amorphous, cirrus mounded to 31,000 ft, or slightly higher, with saddle regions where tops dipped to below 29,000 ft. Cloud opacity ranged from "ground barely visible" through the mounding cirrus portions to "cloud barely detectable" in the saddle regions.
2) 2200-22300 UTC: After a brief search for the best region to perform the BRDF orbits, seven BRDF orbits were carried out over a region of barely visible cirrostratus nebulosus. Heavy CV-580 contrails were intercepted on numerous occasions. Also, a region that was large enough for the orbits, but was also free of both isolated altocumulus clouds and the more widespread higher layer of Cs fibratus above the aircraft, could not be found even though they covered less than 10% of the region where the orbits were performed.
3) 2300-2310 UTC: Transit to ARM site in cirroform clouds.
4) 2310-2440 UTC: Straight 10 min legs at the top (29,500 ft), middle (variable height), and bottom (variable height) through cirriform clouds over the ARM site followed by spiral descent to the surface over the ARM site. A haze layer extended from near the lowest cirrus cloud base (18,000 ft nearly over ARM) to below 4,000 feet. Extremely clean conditions were noted at the lowest pass (200 ft) over the ARM site; a scruff of stratus clouds, just upwind of ARM, had droplet concentrations of about 10 cm-3!
PERIOD OF FLIGHT (ENGINES ON TO ENGINES OFF): 1800-2327 UTC
SUMMARY:
This was a flight to the SHEBA ship, now located 420 nm northwest of Barrow. The flight took place in a band of scattered to broken layers of cirriform and mid-level altocumulus clouds, which stretched from Barrow to the SHEBA ship. The main objectives were to obtain over BDRF over the ship, measure surface albedos over the L-shaped surface albedo array, and to sample clouds over the ship.
DETAILS:
1) 1807-1955 UTC: Takeoff and transit to the ship. Altocumulus layer sampled over Barrow during climb out. For the remainder of the flight the aircraft was in and out of the bases and mid-points of cirrus clouds at 19,000 ft. No stratus clouds were seen below the aircraft during the entire transit leg.
2) 1955-2009 UTC: Descent from 20,000 ft to 2,000 ft over ship. Portions of tenuous cirrus clouds were sampled at the top of the descent spiral. Measurements in the tops of the cirrus (SHEBA-indicated 23,000 ft AGL) were not obtained due to time constraints.
3) 2010-2031 UTC: Ten BDRF orbits were carried out upwind, over, and downwind of the ship. Scattered thin cirriform and altocumulus clouds were present, which occasionally crossed the sun's disk during the orbits.
4) 2032-2046 UTC: Two 10 nm legs (one in each direction) over the L-shaped surface albedo array.
5) 2047-2240 UTC: Climbout and transit to Barrow at 20,000 ft. Considerable cirriform clouds sampled at mid-levels.
6) 2240-2310 UTC: Due to better than expected weather at Barrow, a climb to near cirriform tops (26,000 ft) over the ARM site was made, followed by a slow descent (500 foot per minute) through the patchy cirriform cloud overhead of ARM. When exiting cloud "base," the descent was accelerated to 1500 feet per minute until the lowest pass at 400 feet was made. A DMPS sample was taken at 7,200 ft in a deep haze layer that extended from the 15,000 ft to just above stratus cloud tops (2,100 ft).
7) 2311-2323 UTC: Stratus clouds were sampled during the set up for the approach and landing. Droplet concentrations were about 130 cm-3. Tops were 2,100 ft and bases 1,500 ft. The cloud was orographic in nature, forming at the coastline in 20-30 kts easterly winds. A clearing to the east of the airport persisted for several hours. Hence, the cloud sampled near the airport was "young".
PERIOD OF FLIGHT (ENGINES ON TO ENGINES OFF): 2120-2609 UTC
SUMMARY:
In this, our final flight over the SHEBA ship, we were successful in obtaining our first BRDF and surface albedo measurements under a clear-sky.
DETAILS:
1) 2140-2142 UTC: Takeoff through very clean broken stratus clouds with bases near and on the surface at the east end of the runway and about 300 feet at the west end of the runway. Stratus tops were at 1000 feet. Droplet concentrations were about 10-20 cm-3, and some drizzle drops were present. (Takeoff was considerably delayed due to incoming air traffic.)
2) 2142-2332 UTC: Transit to SHEBA in clear air. Haze was unusually thick, with tops in excess of the cruising altitude of 18,000 ft. Occasionally, undulations (rolls) in the heights of the haze layers were observed when the haze tops were near the flight level. The main body of the haze was confined above 10,000 ft. (Back trajectories suggest a European origin for the haze). The stratus cloud at Barrow extended as an overcast to 75 deg N 163 deg W, where an abrupt clearing occurred and no further stratus was seen. During this entire time, the stratus was unusually transparent, being barely visible when looking straight down at the surface. Leads and other surface features were easily seen through the stratus out to about 10 nm from the aircraft. Also drawing attention on the transit flight was a large region (a few to 10s of square km) of discolored ice and water having a brownish or muddy hue located about 10 nm north of Barrow (at 71.28 deg N and 156.79 deg W). Many photos were taken of this feature.
3) 2332-2412 UTC: BRDF orbits were carried out very near the SHEBA ship. However, the first set of orbits was interrupted for a location adjustment when the surface wind at the ship proved to be quite different from that at the flight level of 2,000 ft, and it became clear that as the aircraft drifted with the wind during the course of its orbits it would miss the ship by perhaps a mile or more. Following a 1 min leg to locate more accurately upwind of the ship, the BRDF orbits were resumed and six were carried from the new location. The ship's lidar confirmed that there was no cloud above the ship (isolated cirrus clouds were present in the northeast quadrant from the ship but were never closer than about 20 nm.)
4) 2414-2420 UTC: Straight passes (at 300 ft) over the L-shaped surface albedo array for airborne albedo measurements.
5) 2420-2609 UTC: Transit leg to Barrow in clear air at 20,000 ft. The top of the polluted layer was near or slightly above this level during the entire transit. Upon descent into Barrow, the main body of the haze was once again found above 10,000 ft. The broken to overcast stratus layer at Barrow had changed little during the time of the flight, with tops again at 1000 feet and droplet concentrations of 20-30 cm-3. A few drizzle drops were also observed. The stratus bases were near the ground just upwind of the airport, but elevated at the downwind end of the field. A large cloud bow was observed upon landing.
PERIOD OF FLIGHT (ENGINES ON TO ENGINES OFF): 2014-2310 UTC
SUMMARY:
BRDF of cirriform clouds in Barrow region, followed by a slow descent from cloud top to cloud "base" to obtain cloud microstructure measurements.
DETAILS:
1) 2014-2044 UTC: Take off and climb to the middle (24,000 ft) of a WNW-ESE oriented line of broken cirriform clouds southwest of Barrow. A broken, and very clean, layer of stratus clouds (bases 300 feet, tops 800 feet) was sampled on climbout. Droplet concentrations in the stratus were about 10 cm-3. Aerosol loading comparable to the previous day was observed at the base of the cirrus clouds (determined from a distance to be 20,000 ft).
2) 2044-2135 UTC: Continued climb to cloud top at 31,000 ft (-52 deg C) and survey of clouds for most suitable region (devoid of upper or lower clouds) for BRDF measurements of the cirrus. The highest tops of the cirrus uncinus and cirrus spissatus reached this level, with saddle top regions several thousand feet lower. A brief apparent nucleation/release of CN was noted at cirrus top.
3) 2135-2145 UTC: Search for best cloud situation for BRDF measurements. A region of subsided cirrus uncinus and cirrus intortus ("tangle of strands") was chosen.
4) 2145-2210 UTC: A modified BRDF maneuvre, for the purpose of avoiding contrails, was carried out over a saddle region of the cirrus tops. While an isolated cirrus uncinus/cirrus intortus top did extend to the flight level, 90 percent of the ice crystals in the region where the BRDF measurements were made were 1000-2000 feet or more below the flight level. Contrail production was heavy in the region where an ice crystal top was intercepted, but virtually non-existent in the regions where the clouds tops were lower and the air drier. To avoid contrails, the roll angle was changed during the BRDF orbits. No visible contrails were intercepted by the aircraft.
5) 2215-2240 UTC: Slow (500 feet per minute) descent through the clusters of cirrus uncinus, cirrus spissatus, and cirrus intortus clouds from cloud tops at 31,000 ft to bases at 21,000 ft. The ice crystals were mainly bullet rosettes.
6) 2240-2307 UTC: Descent and transit to Barrow. Haze layer intercepted. Possible Barrow plume, or cloud top CN nucleation, noted on final approach to Barrow (downwind of Barrow and in evaporating clouds). CNC-3 concentrations were more than 2000 cm-3 at this point near cloud top height.
This was our last research flight for FIRE-ACE/SHEBA.
Peter V. Hobbs
University of Washington
June 25, 1998