WMOD00502

• Title: Orographic Production of Supercooled Liquid Water and Precipitation over the Mogollon Rim of Arizona
• Date: 7/1/1990
• Weather Modification - Doc Type: Report

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rem~nder of the cloud was generally ice free with
significant quantities of SLW suggesting the potential
for cloud seeding. These regions match the model fields
of SLW and IPC rather well as discussed in the next
section.

The SLW was rather low and variable during the
cloud base pass at 3.6 km; however, above this level
well defined zones of SLW, with continuous zones with
values from 0.2 to 0.4 g m-3, were observed upwind of
the crest at flight levels of 4.2 and 5.2 km, mid-cloud
and cloud-top levels respectively. A higher cloud deck
was observed well above the 5.2 km level. The SLW
production zones appear to match the regions of cloud
water production described by the model upwind of the
crest as shown in Figure 7. The average maximum value
of 0.3 g m -3 with peak values from 0.4 to 0.5 g m -1 SLW
were observed at a flight level of 4 km (-700) on the
upwind portion of the cloud about 18 km northwest
of HJK. The cloud generally appeared to have an
orographic wave structure with significant turbulence
making it difficult to fly at a constant altitude. Large
quantities of liquid water found on the upwind side of
the crest indicated rapid -droplet growth in this SLW
production zone in contrast to the lower values of 0.05
to 0.1 g m-l found in the lee evaporation zone. The
structure indicated by the model in Figure 7 matched
this pattern.

The ice particle distributions observed by the 2D-
o and FSSP instruments suggest the possibility of
Hallet-Mossop ice multiplication processes in the lower
portion of the cloud. The Threshold Diameter of the
droplet spectrum as introduced by Hobbs and Rangno
(1985) was measured at 20fLm near cloud base, where
a low concentration of cloud droplets greater than
24fLm were observed. This case was similar to that
on 31 January 1987 where the cloud was almost ice
free except for the warm temperature needle zone with
relatively high IPO. The model analysis appeared to
show a similar maximum of ice particle production
in the lower to middle part of the cloud on the lee
side. Further analysis is required to fully describe the
dynamic and microphysical mechanisms responsible for
this structure.

Airflow in this case was generally strong from the
2400 with speeds from 20-30 m s-l. Vertical velocities
were estimated at an average of 1 m S-1 upwind of the
crest, but sinking motion of -.5 m s-1 was observed
in a wavelike pattern to the lee of the crest in the

evaporation zone as shown in Figure 4. The vertical
velocity computed from the aircraft navigation system
shows positive upward motion plotted above the flight
tracks and downward negative motion plotted below
the tracks. The vertical scale on the left shows the
relative magnitude of the speed from :f:5m s-l. Note
the strong evidence of lifting upwind of the crest and
sinking on the downwind side (arrows). The simple
dynamic wave pattern also characterized the model
simulation. Observed average lifting ahead of the crest
reached maximum peak values of 4 m S-1 with sinking
of -2 m S-1 just to the lee of the crest.

4. SIMULATED RESULTS:

The present results are for a two domain
simulation. The horizontal and vertical resolution for
each domain are 5.0 and 0.5 km for domain 1 and
2.5 and 0.5 km for domain 2. The simulation was
initialized using a potential flow deviation from the

initial horizontal wind field measured by local mesoscale
rawinsondes and aircraft soundings such that mass
continuity was satisfied. No surface heat, moisture,
or momentum fluxes were applied, and all thc~ present
simulation results will be for 1 hour of time integration.
The model results produced vertical motions upwind
crest of the Rim. This lifting produced stable orographic
cloud.

Figure 5 shows the horizontal field of cloud liquid
water content at the 4 km level. This figure illustrates
the horizontal extent of the orographic cloud and
corresponds closely to the aircraft observations. The
diagonal line AB that intersect the X, at HJK" and lies
along the mean horizontal flow direction of the cloud
and approximately marks the path of the Wyoming
King Air when the observations were taken. Note that
the leading edge is approximately 40 km from HJK
along the flight path as reported.

The next three figures are vertical cross sections
of this plane. Figures 6-8 show the supercooled liquid
water concentration, vertical velocity and ice crystal
mass concentration. In both the observations and the
model the liquid water ranged from .3 to .S g m-3
near the leading edge of the cloud and with decreasing
values eastward as ice phase developed as seen in
figures 7 and 8. The supercooled liquid water and ice
crystal horizontal variation with a light snow shower
reaching the summit of the rim matched remarkably
well the observations from cloud physics aircraft and

radar. The precipitation size liquid water observed in
the upper portions of the cloud was not found in the
model results. There are large discrepancies between
the cloud base and cloud top heights reported by the
aircraft and that reported by the radar just bef.Dre 0830
MST when the aircraft observations were made. It
should be noted that the model predicted cloud base
and top matched the earlier radar echo bases and tops
within 0.5 km. The differences in the vertical structure
observed by the aircraft and model may be due to the
cloud decaying after the split in the echo at 08:10 MST.
The precipitation particles observed at the cloud base
were possibly formed in the original deeper cloud as
simulated by the model.

The vertical velocity field in figure 6 shows a
distinct orographic wave with strong lifting at the base
of the rim and sinking over the crest on the rim. Model
results produced vertical motions of 1.5-4. 111 s-1 the
upwind crest of the Rim and compare very well with
the observations sh~wn in figure 4.

5. SUMMARY AND CONCLUSION:

The predominant mechanisms responsible for the
evolution of liquid water and precipitation over the
barrier are the production of supercooled liquid water
by forced orographic uplifting followed by nucleation,
diffusional and rime growth of ice hydro meteors. These
present preliminary model findings compare very well
with many of the details of the observed cloud physical
and dynamical structures for the case study of 28
January 1987 over the Mogollon Rim of Arizona.

Future work will include a more complete
comparison with the observations and additional
calculations with improved vertical and horizontal
resolution, and surface boundary layer. Further
examination of numerous microphysical assumptions
used by the model such as auto conversion thresholds
and ice nucleation rates is in progress. Other case