1. Robert A. Houze and Socorro Medina
Orographic Precipitation Enhancement in Midlatitude Baroclinic Storms: Results from MAP and IMPROVE II
Robert A. Houze and Socorro Medina
As we gather here in Brig this morning at the site of one of the most famous orographic flooding events in the Alps, it is appropriate that we start by reviewing some recent progress on understanding orographic precipitation enhancement in midlatitude baroclinic storms.
One of the major challenges of understanding & forecasting midlatitude precipitation, runoff, and flooding is to understand first how the mountains interact with the airflow and the airflow with the microphysics to enhance the precipitation on the windward slopes.
In this talk, I will review the progress that is coming out of two recent field programs: MAP and IMPROVE II; MAP in the Alps, IMPROVE II in the Cascades. Both of these projects employed aircraft, ground radars, and various other special instruments over mountain ranges.
Some of this work will be contained in the Ph. D. dissertation research of Socorro Medina.
MAP and IMPROVE II both were carried out in locations prone to flooding as a result of baroclinic systems passing repeatedly over a mountain range near an ocean.

2. 20-year Alpine Autumn Precipitation Climatology (rain gauge analysis by Frei and Schaer 1998)
The next slide shows the autumn precipitation climatology for the Alps—a season when rainfall is associated with the passage of baroclinic waves..
Here the Alps are outlined by the 800 m topographic contour.
Note that the amounts are greatest on the lower slopes and minimum along the crest of the range.
This is the primary flooding season on the Mediterranean side of the Alps. Pix from 2000.
Amounts are largest within mesoscale indentations in the larger mountain range.
The lower level flow ahead of the baroclinic waves is from the oceans, and these indentations appear to be downstream of the primary entry points of the moist low level flow into the region.
The maximum on the lower slopes is typical of major mountain ranges…Ron Smith (1979) and back into the 19th century.
Precipitation min on crest of Alps, max on lower slopes

3. Major issue
Understand HOW microphysical processes are invigorated to produce quick and efficient orographic enhancement in windward side flow
…Ron Smith was intrigued by this fact in his review paper in 1979 (see quote ** below). He noted that, according to the literature, % of the water condensed on the windward slope fell out there (as opposed to being carried over the crest). He spoke of this effect as an “efficiency.” And he wondered how this could happen.
Since microphysical processes control the growth rate of precipitation particles, the answer to Ron’s question must be a matter of determining how the microphysical processes are invigorated to accelerate the growth of precipitation particles in the upslope flow and thus produce quick and efficent enhancement of the precipitation in windward side flow.
Current research on orographic enhancement of precipitation must focus on answering this question.
In this talk I will review some recent progress toward this end.
_________
**Smith’s exact wording:
“Even if we accept the idea that large scale orographic lifting of a deep warm moist air current can cause some release, it is still surprising in light of the difficulties in forming precipitation-size particles, to find release efficiencies of 70% to 100%, such as reported by Sawyer (1956) . Myers (1962), and Browning et al. (1975). Is it possible to convert such a high fraction of the condensed water into precipitation.” (quote from Smith 1979)

4. The Cascade Project (Hobbs et al. 1973, Hobbs 1975) Streamlines
Liquid Water Content
Trajectories of ice particles growing by deposition and riming
The problem of fallout of precipitation on the windward slope was addressed 30 years ago in the Cascade Project, led by Peter Hobbs at the UW. This collection of figures from that project shows some key calculations that they did.
They ran a simple steady-state 2D model of the flow over a barrier of the height of the Cascades using an upstream sounding. Ice particle trajectories were computed for particles growing by vapor deposition and riming. These growth processes determined the fall velocity which in turn affects where the precip falls out (Roe & Baker).
They ran these calculations in real time and used them to direct the project aircraft to sample the ice particles at various points along the trajectories. They also sampled ice particles at the ground in the mountains.
High concentration (small particles)
Low concentration (large particles)

5. What microphysical processes can grow precipitation particles quickly?
Coalescence
T > 0 deg C
The previous slide indicates how the microphysical growth mechanisms have a profound effect on the pattern of fallout. The microphysical processes that can produce a particle big enough to fall out rapidly are the accretion processes: Coalescence, aggregation, and riming. Coalescence and riming increase the fallspeed directly. Aggregation may be a more indirect effect by making particles that if they can melt will be big rapidly falling drops. Whatever mechanism enhances the fallout on the windward side must favor one or more of these accretion processes.
Aggregation
Riming
T < 0 deg C
“Accretion”

6. How can the airflow make the accretion processes more active?
“Cellularity”
accretion
How do we get the accretion processes active? One way is by what Smith referred to as cellularity. When overturning occurs in the upslope flow, accretional growth is favored in the updrafts. Precip particles not re-evaporated in the downdrafts—so there is a net enhancement of precip particle growth. When does cellularity occur? Usually we think of a generally saturated upslope flow that is potentially unstable. Overturning occurs as the instability is released. It turns out that cellularity can occur even in stable upslope flow! Today we will see examples of cellularity in potentially unstable flow in Socorro’s talk. In this talk we will focus on the more surprising fact that we get cellularity also in stable flow. But first let’s look at some potentially unstable soundings—all from MAP cases…
(Smith 1979)

7. Potentially unstable upstream flow:
MAP IOPs 2b, 3, and 5
IOP2b was the case illustrated by the synoptic maps in previous slides. IOP3 and 5 had similar characteristics though the trough was not as sharp and the flow not as orthogonal to the mountains.

8. Equivalent Potential Temperature
IOP2b
IOP3
IOP5
12Z 20 Sep 99
00Z 26 Sep 99
12Z 03 Oct 99
Milan sounding

9. Stable cases: IMPROVE II Case 11 MAP IOP8
Now I want to examine what happened in several cases of orographic enhancement of the precipitation on the lower slopes of a mountain range when the upstream flow was absolutely stable. The first case is over the Cascade Mountains in Oregon. Surprisingly—cellularity occurred even though the upstream flow was stable. Let’s see what happened.

10. IMPROVE II Experimental Area 26 November-22 December 2001
WASHINGTON
OREGON
Salem
PACIFIC OCEAN
Newport
NCAR S-Pol
NOAA S-Band UW
MM5
Medford

11. IMPROVE II Case 11: 13-14 December 2001
MM5 12 h forecast 500 mb height, wind, and temperature
IMPROVE II occurred during a time of westerly flow which persisted for ~4 weeks. 16 SW troughs came across with associated frontal cloud systems. This trough was a particularly well defined example. Note the strong SW flow over the Cascades.
Valid 00 UTC 14 Dec 01

12. IMPROVE II Case 11 Upstream soundings
Upstream Soundings of equivalent potential temperature
Upstream soundings were obtained at the four locations shown. All represent upstream conditions in some way. The red, green, and black are probably most representative for the time and location of the data in the next few slides. What’s plotted here again is theta-e. Notice how different these are from the MAP soundings shown earlier. These soundings are all absolutely stable except for some slight potential instability showing up sometimes in the lowest 50 mb. But this low level air is mostly blocked and not going over the range. Thus, in general the air moving over the range at this time was absolutely stable.

13. 3-hour Mean Radial Velocity
IMPROVE II Case 11
3-hour Mean Radial Velocity
Height (km)
The radial velocity cross section shows flow sloping up over the range. Note that the flow is highly sheared in the layer containing the aggregates &/or graupel.
ESE
Horizontal distance (km)
S-Pol radar

14. 3-hour Mean Reflectivity
IMPROVE II Case 11
3-hour Mean Reflectivity
Horizontal distance (km)
Height (km)
The reflectivity structure averaged over 3 hours was stratiform with a well defined bright band.
ESE
S-Pol radar

15. Polarimetric Particle Identification over 3 hours
IMPROVE II Case 11
Polarimetric Particle Identification over 3 hours
weak echo snow
(low dBZ, low ZDR)
large aggregates and/or graupel
(high dBZ, low ZDR)
melting snow
(high dBZ, high ZDR)
Horizontal distance (km)
Height (km)
P3 aircraft data
Polarimeteric data again show a layered structure with weak snow lying above melting aggregates. In between is a layer where the dBZ and ZDR are both high. But this time, given the stability and the fact that this is a broad layer rather than a single spot above a peak, we think this is more likely aggregates. However, the NOAA P3 aircraft sampled the particles in the layer shown, and there was evidence of riming that suggests that there could be some graupel and certainly some heavily rimed aggregates. We’ll look at these a/c data later.
ESE
S-Pol radar

16. Reflectivity IMPROVE II NOAA/ETL S-band Radar 13-14 December 2001
Also at McKenzie bridge was a vertically pointing S-band profiler with a time resolution of about 5 sec. Note the fine scale cellularity extending both above and below the bright band.

17. Radial Velocity IMPROVE II NOAA/ETL S-band Radar 13-14 December 2001
Ri»0.25
The radial velocity uncorrected for fall speed shows intermittent upward motions in small cells shown by the blue marks. Upward radial velocities occurred repeatedly with durations typically of sec, or about 1-3 km in width. These occurred in the layer of the shear shown by the radial velocity of the S-Pol and by the profiler. The updrafts may have extended below the melting level but would have been masked by raindrop fall velocity. The Ri in this layer was ~.25, so this could be shear induced turbulence. Or it could be turbulence owing to the stable air flowing over rough terrain.
__________________________-
Ri=N^2/(du/dz)^2
du/dz (sfc-3km) = 1E-3
N^2 (dry, sfc-3km) = 1.75E-4
N^2 (sat, sfc-3km) = 0.25E-4
Ri (dry) = 4.46E-3 = 1.75
Ri (sat) = 6.37E-4 = .25

18. IMPROVE II Case 11 Time series at McKenzie Bridge during Shear at
km (profiler)
Radial velocity
(VP S-band)
Min radial velocity at 2-3 km (VP S-band)
This figure combines the wind profiler, VP Sband radar, and the S-Pol data in a time series. The upper panel shows the shear in the km layer at McKenzie Bridge from the profiler. The period from roughly was when the main frontal rainband was passing over the region. During this period of several hours the VP S-band showed intermittent UPWARD radial velocities, as we saw in the previous slide. The third panel indicates the updraft magnitudes. It shows the negative (upward) radial velocity extremes as a function of time. The values regularly reached several m/s, with 2 m/s being typical. Such updraft velocities are sufficient to promote rapid condensation & riming. Rimed particles, even graupel, would be expected. The turbulent air motions also would favor aggregation, and this is a temperature layer where aggregation would be expected. Thus conditions favored both aggregation and riming in this layer during this several hour period. The bottom panel shows the frequency of occurrence of aggregates or graupel according to the S-Pol polarimetric particle ID algorithm. These signals registered exactly where & when the shear and turbulent updrafts were maximum. 2%
Occurrence of graupel &/or aggregates (S-Pol)

19. Track of P3 aircraft & S-Pol reflectivity at 1.5 deg elevation
IMPROVE II Case 11
Track of P3 aircraft & S-Pol reflectivity at 1.5 deg elevation
160 km
The NOAA P3 aircraft flew a short time in the layer indicated by the ground based radars to be turbulent and conducive to aggregation and riming. The color coded flight track indicates the altitude of the aircraft. The purple segment was at the 2 km level. Ice particles sampled by the aircraft along the purple segment are shown in the next slide.

20. Ice particle imagery from P3 aircraft
IMPROVE II Case 11
Ice particle imagery from P3 aircraft
1.6 mm
9.6 mm
These images from the high and low resolution probes both indicate aggregates with probable riming. Large irregularly shaped particles with gaps indicating aggregates and filled in blobby spots indicating riming.

21. Stable cases: MAP IOP 8
Now we can return to MAP and check whether we saw similar echo structure and inferred precipitation mechanisms when the conditions were stable, as in the IMPROVE Case 11. The cases from MAP which fit that description are IOP8 and IOP14.

22. Equivalent Potential Temperature
IOP8
18Z 20 Oct 99
These soundings of theta-e show the potentially unstable upstream air in all three cases
Milan sounding

23. 34-hour Mean radial velocity
MAP IOP8
34-hour Mean radial velocity
The radial velocity in IOP8 shows the sloped flow with strong shear, as in IMPROVE II Case 11. There was blocking at low levels, and the greens show flow away from the mountains. NW
S-Pol radar

24. 34-hour Mean Reflectivity
MAP IOP8
34-hour Mean Reflectivity
This is a _____ hour average reflectivity cross section taken more or less parallel to the low level flow in IOP8, which is a stable case studied by several investigators. It has a well defined bright band, similar to IMPROVE II Case 11. NW
S-Pol radar

25. Polarimetric Particle Identification over 34 Hours
MAP IOP8
Polarimetric Particle Identification over 34 Hours
weak echo snow (low dBZ, low ZDR)
melting aggregates (high dBZ, high ZDR)
The polarimetric data in IOP 8 showed a simple two-layer structure with weak echo snow above the melting level and melting aggregates below. There was apparently not enough turbulence to generate enough large aggregates or graupel to register NW
S-Pol radar

26. Reflectivity from vertically pointing S-band radar at Locarno Monti
MAP IOP8
Reflectivity from vertically pointing S-band radar at Locarno Monti
Height (km)
Although the polarimetric algorithm did not pick up the aggregation/graupel signature as in IMPROVE Case 11, the vertically pointing S-band radar showed cellularity in the reflectivity. Note all the fine scale cells and fall streaks in and below the bright band. It is often hypothesized that this structure is the result of melting induced cooling and overturning. This is possible. However, It is also possible that turbulence induced growth is producing these intermittent cells since they occur in the shear layer. The fact that the cells do extend up into the snow layer suggests that these are not melting induced, but we cannot be certain of this.
Time UTC
OPRA radar
Yuter & Houze 2003

27. Microphysical enhancement
Conceptual model for orographic precipitation enhancement in stable, sheared upstream flow
Microphysical enhancement
TURBULENCE
Aggregation
Riming
Coalescence
0°C
Heavy rain

28. Conclusions
Low-level growth by coalescence and/or riming is needed to make precipitation fall out quickly on lower slopes
Cellularity is required to make the coalescence and/or riming occur
Cellularity may occur by EITHER release of potential instability OR by turbulence in stable flow
In stable flow, cellularity is a manifestation of turbulence in sheared flow rising over the terrain.
Cells in stable flow
favor particle growth by accretion
have updrafts >1-3 m/s
contain aggregates and/or graupel
enhance precipitation on lower slopes
TURBULENCE

30. Mixed case: MAP IOP 14
Now we can return to MAP and check whether we saw similar echo structure and inferred precipitation mechanisms when the conditions were stable, as in the IMPROVE Case 11. The cases from MAP which fit that description are IOP8 and IOP14.

31. Equivalent Potential Temperature
IOP14
00Z 4 Nov 99
These soundings of theta-e show the potentially unstable upstream air in all three cases
Milan sounding

32. Mean wind shear from Lonate profiler
MAP IOP14
Mean wind shear from Lonate profiler
…it was also seen in IOP 14.
Mean and SD over 16 hours

33. 34-hour Mean radial velocity
MAP IOP14
34-hour Mean radial velocity
IOP 14 also had sheared and sloping flow.
NNW
S-Pol radar

34. 34-hour Mean Reflectivity
MAP IOP14
34-hour Mean Reflectivity
IOP14 had similar thermodynamics and again a well defined bright band. This is a _____ hour average reflectivity cross section. The enhancement over the first peak is ground clutter.
NNW
S-Pol radar

35. Polarimetric Particle Identification over 34 Hours
MAP IOP14
Polarimetric Particle Identification over 34 Hours
weak echo snow (low dBZ, low ZDR)
melting aggregates (high dBZ, high ZDR)
Ditto in IOP14.
NNW
S-Pol radar

36. Reflectivity from vertically pointing S-band radar at Locarno Monti
MAP IOP14
Reflectivity from vertically pointing S-band radar at Locarno Monti
Height (km)
It is even more evident in IOP 14 that the cellularity extends above the melting layer.
Time UTC
OPRA radar

38. Newport Wind Profiler Data
IMPROVE II Case 11
Newport Wind Profiler Data
This shear is also seen in the profiler data, here at Newport.
Mean and SD over 8 hours

39. McKenzie Bridge Profiler Data
IMPROVE II Case 11
McKenzie Bridge Profiler Data
…and at McKenzie Bridge.
Mean and SD over 8 hours

40. Mean wind shear from Lonate profiler
MAP IOP8
Mean wind shear from Lonate profiler
The shear was also seen in the Lonate profiler data taken in the Po Valley in both IOP8 and…
_____________________________________________________________________________________
Lonate Pozzolo, Italy Station details
Frequency: 1274 MHz, WMO No.: 07613  Latitude: 45.34°N Longitude: 8.43°E Height: 146m Hgt res.: 143m Range: 0.5km to 6.0km Beam Angle: 15° Averaging period: 30 minutes
Lonate Pozzolo, Italy
Mean and SD over 34 hours

41. A Microphysical Question
“Even if we accept the idea that large-scale orographic lifting can cause some release, it is … surprising in light of the difficulties in forming precipitation-size particles, to find release efficiencies of 70% to 100%, … Is it possible to convert such a high fraction of the condensed water into precipitation?”
Ron Smith (1979)
…Ron Smith was intrigued by this fact in his review paper in 1979 (see quote ** below). He noted that, according to the literature, % of the water condensed on the windward slope fell out there (as opposed to being carried over the crest). He spoke of this effect as an “efficiency.” And he wondered how this could happen.
Since microphysical processes control the growth rate of precipitation particles, the answer to Ron’s question must be a matter of determining how the microphysical processes are invigorated to accelerate the growth of precipitation particles in the upslope flow and thus produce quick and efficent enhancement of the precipitation in windward side flow.
Current research on orographic enhancement of precipitation must focus on answering this question.
In this talk I will review some recent progress toward this end.
_________
**Smith’s exact wording:
“Even if we accept the idea that large scale orographic lifting of a deep warm moist air current can cause some release, it is still surprising in light of the difficulties in forming precipitation-size particles, to find release efficiencies of 70% to 100%, such as reported by Sawyer (1956) . Myers (1962), and Browning et al. (1975). Is it possible to convert such a high fraction of the condensed water into precipitation.” (quote from Smith 1979)
Major issue
Understand HOW microphysical processes are invigorated to produce quick and efficient orographic enhancement in windward side flow