The regional cloud and meteorological conditions are described for this case using satellite imagery (GOES), dual polarization lidar data, NWS radar, NMC analyses, rawinsonde data including special soundings, and analyzed vertical motions. These observations are interpreted with respect to relationships between the observed cloud characteristics and corresponding atmospheric structure. Similarities with the 27 to 28 Oct. FIRE Cirrus Case Study are noted

Sassen, Kenneth

Starr, David

English

NASA Technical Reports Server

N90-28287 '
Regional Conditions During The 25 October 1986 FIRE
Cirrus/Altocumulus Case Study
David O'C. Starr
NASA Goddard Space Flight Center
Greenbelt, MD 20771
Kenneth Sassen
Department of Meteorology
University of Utah
Salt Lake City, UT 84112
The regional cloud and meteorological conditions are described for this case using satellite imagery
(GOES), dual polarization lidar data (from Wausau), NWS radar, NMC analyses, rawinsonde data
including special soundings, and analyzed vertical motions. These observations are interpreted
with respect to relationships between the observed cloud characteristics and corresponding
atmospheric structure. Similarities with the 27-28 October FIRE Cirrus Case Study are noted.
Visible satellite imagery reveals overcast conditions over the upper Midwest at 2000 UTC
on 25 October (Fig. 1a). Radar observations showed a band of light precipitation across south-
eastern Wisconsin at that time (echo tops to 7 km). Drizzle and light rain were also found to the
south where the clouds were brightest. The decreased cloud albedo over southern Wisconsin
indicates the presence of overlying cirrus clouds. The cirrus axe more evident in the infrared
imagery where a band of cold clouds stretches from western Iowa to Michigan (Fig. lb). To the
north and south of this feature, the high clouds appear more variable and were responsible for the
patches of lower albedo seen over northern Illinois (Fig. la).
These clouds were associated with a nearly stationary occluded cyclone at the surface and
closed low aloft (Fig. 2). The cyclone was centered in southern Illinois with the occluded front
lying through southern Indiana and then southward to Georgia. The high clouds were located in
the exit region of the upper level vortex and were bounded on the west by the interface between
northerly flow (entrance) and southerly flow (exit). The northern boundary of the cirrus (Fig. lb)
lay along the transition between southerly flow out of the closed low and westerly flow in Canada.
The cloud features moved steadily to the NNW during the afternoon.
Surface-based, dual-polarization lidar observations at Wausau prior to 1730 UTC detected
fairly dense midlevel cirrostratus between 5.5 and 8 km (Fig. 3a). Observed polarization ratios (s)
of 0.55 to 0.65, corresponding to rimed (or very complex) ice crystals, were widespread in the
lower portion of the cloud (Fig. 3b). At about 1730 UTC, cloud base height began to increase
rapidly and evidence of supercooled liquid water was found between 7.8 and 8 km (s < 0.15). An
overlying layer of cirrus between 9.5 and 11.1 km was observed at this time. Concurrent satellite
observations indicate that this higher cirrus layer was present earlier but obscured by the midlevel
cloud. Lidar observations after 1900 UTC may have also been significantly attenuated by the
midlevel cloud layer. Subtropopause cirrus were sporadically detected above 10.5 km and another
cirrus layer was observed between 8.5 and 9.8 km around 2000 UTC (Fig. 3a). Airborne
observers noted the distinct clear region between this cirrostratus layer and the underlying midlevel
cloud (Shanot, personal communication).
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From 1900 to 2030 UTC during the King Air flight operations near Wausau, altocumulus
clouds were observed between 7.8 and 8.1 km (-31 to -33 C) capping a 2.5-km deep ice cloud
with a variable base around 5.5 km. Values of s in the ice cloud were generally near 0.3 which is
characteristic of some forms of cirrus. Low depolarization ratios (_ < 0.15) in the ice cloud were
likely associated with oriented plate crystals in sheared fallstreaks (in contrast to the liquid phase
altocumulus layer, significantly increased s and decreased return energy were found when the lidar
was tipped off vertical by a few degrees - points labelled "x" along the bottom of Fig. 3). The
strongest orientation effects were apparent below 7 km and well below the base of the altocumulus
layer. This indicates that growth of the crystals to sizes greater than 200 mm, as required to
maintain a stable orientation, or a change to a less complex crystal habit occurred during
sedimentation. The flight crew of the King Air reported a continuous sloping cloud base during
transit from Wausau, where cloud base was near 5.5 km, to Madison (2030 UTC), where the base
height was about 3 km and drizzle was observed from a lower cloud layer.
Vertical velocities were estimated using the adiabatic method (three-station technique with
an estimated uncertainty of+_2 cm s-1) applied to rawinsonde data from eight stations as in Starr
and Wylie (1989). The location of the analyzed band of weak ascent (Fig. 4) on the 321 K
isentropic surface (7.7 km at Green Bay and 8.2 km at Platteville in southwestern Wisconsin)
corresponds very well to the position of the band of cold clouds (cirrus over midlevel cloud, Fig.
lb) at 0000 UTC on the 26th. Ascent rates of more than 3 cm s -1 were found at this level.
The corresponding vertical velocity profile computed from soundings at Green Bay (GRB),
St. Cloud (STC) and Platteville (PLA) is shown in Fig. 5. The 1800 UTC profile (supplemental
soundings at those stations) is consistent with upper level cloud generation at two primary levels -
near the tropopause (11 km) where cirrus were detected and near the level where the altocumulus
cloud layer was observed. Later, more uniform uplift is indicated throughout the upper
troposphere when cirrus cloudiness was detected in the intervening layer (Fig. 3a). The fact that
the maximum ascent rate at 1800 UTC was diagnosed above the altocumulus layer is not
necessarily inconsistent with the observed cloud location since, as noted by Gedzelman (1988), the
pre-existing vertical structure of water vapor plays a major role in determining the level of cloud
formation. Rawinsonde-resolved vertical motions were quite weak below the level of the
altocumulus layer where the adjoining 2.5-km deep ice cloud was observed.
Rawinsonde data from three soundings that ascended through the cloud band are shown in
Fig. 6 - see Starr and Wylie (1989) for discussion of data quality. At Platteville (PLA), ice-
supersaturation was observed at all levels colder than 0 C (no data at temperatures colder than -50
C), except for a layer from 8.9 to 9.7 km where relative humidity with respect to ice (RHI)
decreased to 95% (Fig. 6a). Relative maxima of 102% were measured from 7.8 to 8.4 km and at
the 9.9 km level (-50 C) corresponding to relative humidity with respect to pure liquid water (RH)
of 73% and 64%, respectively. Greater humidities were found at lower levels (RHI = 108% at 5.5
km). The Wausau (WAU) sounding (released at 1855 UTC) exhibited more structure with RHI
equal to 112% (RH = 85%) from 7.5 to 7.6 km (RHI > 100% from 6.8 to 7.8 km) and decreasing
to 60% at -50 C and to 67% at -14 C (5.5 km). The lower vertical-resolution (standard) sounding
at Green Bay (GRB) showed ice-supersaturation at 5.6 km (105%) and 7.7 km (101%) with drier
but still humid conditions between these levels (RHI = 91% and 75% at 6.6 and 7.3 km,
respectively) and above (91% and 86% at 8 and 8.9 km, respectively). Thus, each sounding
indicates a relative humidity maximum near 7.8 km corresponding to ice-supersaturation but
subsaturated with respect to the liquid phase. Both the GRB and PLA soundings show moist
conditions at most levels while, except for the layer from 6.8 to 7.8 km, the WAU sounding is
significantly drier. On this basis, it appears that the GRB and PLA soundings were representative
of the more disturbed conditions to the south of Wausau where the base of the altocumulus-topped,
midlevel, ice cloud extended to below 5.5 km.
382
The temperature profiles each show an inversion-capped boundary layer with a second
inversion near the 3 km level at WAU and GRB (Fig. 6a). At WAU, a third inversion was found
at 5 km. Tropopause heights ranged from 10.6 km at PLA to 11.2 km at WAU (11 km at GRB).
The Platteville sounding is generally colder than the other two at a given height but this is mostly
the result of the lower surface pressure (0.4 kPa) at PLA. Static stability structure is more evident
in the potential temperature profiles (Fig. 6b). The high vertical-resolution sounding from Wausau
exhibits a lot of fine structure in the upper troposphere that is not found in the PLA sounding of
comparable resolution.
A striking feature of the GRB sounding is the nearly dry adiabatic stratification found from
6.6 to 7.3 km (9.4 C km-1). This layer was capped by a relatively stable layer (5.8 C km -1)
extending to the 8 km level. There is also evidence for a statically stable layer at both Wausau (6.1
C km -1 from 7.7 to 8 km) and Platteville (7 C km -1 from 8 to 8.2 km) although the feature was
weaker at those locations. The height of the stable layer corresponds closely to that of the thin,
supercooled liquid phase, altocumulus layer observed over Wausau.
If the anomalously cold data point at the 7.3 km level in the GRB sounding is removed
(regarding it as erroneous - possibly due to evaporative cooling of a wetted thermistor), the stable
layer then extends from 7.7 to 8 km in accord with the other two soundings. In this case, the
stratification below 7.7 km is also comparable in each of the soundings - lapse rates within 0.5 C
km -1 of the ice-pseudoadiabatic lapse rate down to the 6.6 km level. The Green Bay sounding is
still conditionally unstable in this layer but only slightly. Below 6.6 km, the GRB and PLA
soundings become increasingly stable while the lapse rate at Wausau remains close to ice-
pseudoadiabatic down to the inversion at the 5 km level. It is also of note that lapse rates were near
ice-pseudoadiabatic in the layer where the overlying cirrostratus layer was observed - within 0.3 C
km -l from 8.5 to 9.8 km at WAU.
Upper level winds at Platteville were generally stronger than at Green Bay (Fig. 6c),
especially from 8 km to the tropopause (unfortunately, the wind observations from Wausau were
unusable). Of particular note is the more southerly wind directions found at GRB in the layer
where the altocumulus layer and underlying ice cloud were observed (5.5 to 8 km). This may be
interpreted as indicating deformation along the cloud band (see Figs. lb and 2). The computed
vertical velocity profile at 1800 UTC (Fig. 5) implies that horizontal mass convergence existed
from roughly the 5 km level to near 9 km and was strongest between 7.5 and 8.5 km with
divergence from 9 to 10.5 km and convergence from there to the tropopause. [Computed
divergence profiles exhibited large variations depending on the stations used - probably attributable
to the mesoscale character of the situation as discussed by Starr and Wylie (1989). Corresponding
vertical velocity profiles computed by the kinematic method were judged to be relatively useless.]
Similarly, convergence is indicated from 6.5 to 8.5 km at 0000 UTC on the 26th with divergence
below.
Relatively strong wind shear was observed near the tropopause at both locations as wind
direction veered from SE to SSW in the lower stratosphere. Weak veering was found near the 7
km level at PLA in association with a relative minima in wind speed. A similar feature occurred at
GRB but at a lower height (near 6 km). Weak backing was found from there to 8.5 km.
In summary, regional analysis of the available rawinsonde data show that the supercooled
altocumulus cloud occurred in a statically stable layer (7.8 to 8.1 km). Saturation was likely
induced by adiabatic cooling due to weak ascent (-2 cm s -1) and enhanced by infrared cooling as
described by Gedzelman (1988) and Starr and Cox (1985b) - solar warming was suppressed by
the overlying cirrostratus cloud. Cellularity in the cloud was likely induced by infrared cooling at
cloud top (see Starr, 1987b). Dynamic interaction with the adjoining 2.5-km deep ice cloud below
383
may also have been significant given the near ice-pseudoadiabatic stratification observed from 6.6
to 7.7 km which is consistent with weak convective overturning.
Uplift was associated with horizontal mass convergence that was strongest above 7.5 km.
Given that the midlevel moisture supply associated with the convergent flow was fed by the more
disturbed conditions to the south where precipitation was observed (echo tops to 7 km), the stable
layer in which the altocumulus formed would be consistent with maximum detrainment of cloud
water at that level in the disturbed region and maintenance of the midlevel cloud layer downstream.
Furthermore, downward growth of the underlying ice cloud via ice particle sedimentation would be
consistent with horizontal moisture supply near cloud top and maintenance of convective
overturning (Starr and Cox, 1985a; Starr and Wylie, 1989) in the absence of significant mean
ascent below the 7 km level. Maintenance of convective currents in the ice cloud would provide
further moisture to the cloud by recycling vapor from evaporated particles below (Starr and Cox,
1985b).
Some notable similarities to the 27-28 October FIRE Cirrus Case Study (Starr and Wylie,
1989 - conclusions summarized in paper C02.01) are: 1) synoptic control dominated with a
persistent cirrus shield maintained over a ridge axis (exit knee in the case here), 2) significant
mesoscale structure was evident over a variety of horizontal scales (the long-lived 100 x 500 km
altocumulus cloud band and smaller scale [10 km in width] transverse banding in cirrus here, Fig.
la), 3) cloud generation occurred at multiple levels (optically thin subtropopause cirrus layer, an
underlying cirrostratus layer and a lower altocumulus/altostratus/cirrostratus layer), and 4)
relatively shallow (< 0.5 km) cloud generation layers were found.
REFERENCES
Gedzelman, S.D., 1988: In praise of altocumulus. Weatherwise, 41,143-149.
Starr, D.O'C., and S.K. Cox, 1985a: Cirrus clouds, Part I: A cirrus cloud model. J. Atmos.
Sci., 42, 2663-2681.
,1985b: Cirrus clouds, Part II: Numerical experiments on the
formation and maintenance of cirrus. J. Atmos. Sci., 42, 2682-2694.
,1987b: Effects of radiative processes in thin cirrus. J. Geophys. Res., 92, 3973-
3978.
Starr, D.O'C., and D.P. Wylie, 1989: The 27-28 October 1986 FIRE Cirrus Case Study:
Meteorology and Clouds. (accepted by Mon. Wea. Rev.).
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Figure 3: Time-height display of (a) range-corrected rctunw_ power (E)
and Co) polafiz.adon ratio (5) observed at Wausau by the University of
Utah cloud lidar.
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Figure 6: Rawinsondc-obsea'vcd vertical profiles of a) tcrapcratur¢ and mlaLivc humidity b) potential te,mpctatur¢, and
c) horizontal wind vector observed at Plaucvill¢ (PLA) at 1800 UTC, Wausau (WAU) at 2000 UTC on 10-25-86 and
Green.Bay (GRB) at 000(3 UTC on 10-26-86. Tcrapcratum profiles am plotted on a scale reflecting _ 10 C offset
with r_t to the profile to the left and similarly for potential tcrapcratum. Width of the shaded area indicates
relative humidity values ranging from 50% to 100% (5°C width) or greater in the case of relative humidity with
respect to ice (RHI) which is plotted at temperatures colder than -20 C.
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385



Regional conditions during the 25 October 1986 FIRE cirrus/altocumulus case study

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