Lessons on Orographic Precipitation from MAP - MMM
Lessons on Orographic Precipitation from MAP - MMM
Lessons on Orographic Precipitation from MAP - MMM
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<str<strong>on</strong>g>Less<strong>on</strong>s</str<strong>on</strong>g> <strong>on</strong> <strong>Orographic</strong> Precipitati<strong>on</strong> <strong>from</strong> <strong>MAP</strong><br />
R. Rotunno<br />
NCAR<br />
<strong>MAP</strong> Objective<br />
R. A. Houze<br />
U. of Washingt<strong>on</strong><br />
“To improve the understanding of orographically<br />
influenced precipitati<strong>on</strong> events and related flooding<br />
episodes…” (<strong>MAP</strong> Design Proposal)
<strong>Orographic</strong> Precipitati<strong>on</strong><br />
• Large – Scale (Wind, Humidity, Stability)<br />
• Mesoscale dynamics of orographic air flow<br />
• Fine-scale air moti<strong>on</strong>s and microphysics<br />
Sawyer (1956, Weather) )
Dynamics:Stable Flow<br />
Weak Stability, No Blocking<br />
Str<strong>on</strong>g Stability, Blocking
Coalescence<br />
T > 0 deg C<br />
What microphysical processes can grow<br />
precipitati<strong>on</strong> particles quickly?<br />
Aggregati<strong>on</strong> Riming<br />
“Accreti<strong>on</strong>”<br />
T < 0 deg C
How can the airflow make the accreti<strong>on</strong><br />
processes more active?<br />
“Cellularity”<br />
Smith (1979)<br />
Cells due to c<strong>on</strong>vecti<strong>on</strong> or<br />
turbulence embedded in<br />
upslope cloud can<br />
accelerate particle growth<br />
by coalescence, riming, &<br />
aggregati<strong>on</strong><br />
Smith (1979))
Liquid water c<strong>on</strong>tent over the Cascade Mountains (Hobbs 1975)<br />
Trajectories of ice particles growing by depositi<strong>on</strong> and riming (Hobbs et al. 1973)<br />
Large particles<br />
Small<br />
particles<br />
Similar distributi<strong>on</strong><br />
found over the<br />
Sierra Nevada<br />
(Marwitz, 1987)
Dynamics: Unstable Flow<br />
•Str<strong>on</strong>g Updrafts<br />
•Cold Outflows<br />
•Moving Cells
Echo maximum<br />
locked to terrain with<br />
maximum intensity at<br />
low levels as a result<br />
of warm growth<br />
processes<br />
(coalescence)<br />
Colorado Rockies Big Thomps<strong>on</strong> Storm 1976<br />
Caracena et al. (1979)
<strong>MAP</strong><br />
Lago Maggiore Target Area<br />
Warm, Moist
Pre-SOP Results <strong>on</strong> Alpine Heavy Rain Events<br />
• Slow Moving, N-S El<strong>on</strong>gated Troughs<br />
• Ample Moisture<br />
• C<strong>on</strong>v. Inst. (Brig, Vais<strong>on</strong>-La-Romaine)<br />
• Stable Precip (Piedm<strong>on</strong>t)<br />
• Models <strong>Orographic</strong> Enhancement
Pre-SOP<br />
Piedm<strong>on</strong>t Flood<br />
1994<br />
00 Z 6 Nov 1994 Milan<br />
Nearly<br />
Moist<br />
Neutral<br />
Doswell et al. (1998)<br />
Buzzi et al. (1998)<br />
also Ferretti et al. (2000)<br />
C<strong>on</strong>trol: “Flow Over”<br />
No C<strong>on</strong>densati<strong>on</strong>: “Flow Around”
Pre-SOP<br />
Flow Over or Flow Around?<br />
Alps<br />
Latent Heating Weak Stability Flow Over<br />
Alps<br />
No Latent Heating Str<strong>on</strong>g Stability Flow Around
Pre-SOP<br />
Special Effects of Stability Variati<strong>on</strong><br />
Flow Over<br />
Alps<br />
Flow Around<br />
Weak Stability Str<strong>on</strong>g Stability<br />
Schneidereit and Schär (2000); Rotunno and Ferretti (2001)
Pre-SOP<br />
Special Effects of 3D Topography<br />
“C<strong>on</strong>cavity”<br />
Alps<br />
Schneidereit and Schär (2000); Rotunno and Ferretti (2001)
SOP Results<br />
• IOP2a Str<strong>on</strong>g C<strong>on</strong>vecti<strong>on</strong>/Ouflows<br />
• IOP2b Flow-Over Regime / Weak C<strong>on</strong>vecti<strong>on</strong><br />
Enhancement Mechanism Coalescence and<br />
Riming at Low Levels in Weak C<strong>on</strong>vective Cells<br />
• IOP8 Blocking / Stratiform Precip<br />
Enhancement Mechanism Turbulence in Shear<br />
Layer Accentuates Accreti<strong>on</strong>
IOP 2a – 17 September 1999<br />
A short, intense, isolated, c<strong>on</strong>vective event<br />
70 mm within 12 hours<br />
<strong>MAP</strong> target area<br />
Richard et al. (2003)
IOP 2a – 17 September 1999<br />
SIMULATION<br />
Richard et al. (2003, QJRMS)
RADAR<br />
IOP 2a – 17 September 1999<br />
SIMULATION<br />
ECMWF: OP. ANA 1999<br />
12 hour accumulated precipitati<strong>on</strong><br />
Richard et al. (2003)
Radar Retrieval<br />
(S-Pol)<br />
(x) hail + graupel<br />
(o) hail<br />
rain<br />
12 km<br />
IOP 2a – 17 September 1999<br />
100 km<br />
18:00 UT<br />
19:00 UT<br />
20:00 UT<br />
Simulati<strong>on</strong><br />
(Meso-NH)<br />
graupel<br />
hail<br />
rain<br />
Richard et al
Comparis<strong>on</strong> of IOP2b and IOP8<br />
Rotunno and Ferretti (2003, QJRMS)
“<strong>Orographic</strong> Effects <strong>on</strong> Rainfall in <strong>MAP</strong> Cases IOP2b and IOP8”<br />
Rotunno and Ferretti (2003, QJRMS)<br />
Trajectories passing through T at levels 1.5 (red), 2.25(blue) and 3.0 (purple) km<br />
Observati<strong>on</strong>s and MM5 Simulati<strong>on</strong>s <br />
IOP 2b<br />
Heavy Rain in Target Area<br />
Nearly Neutral Flow Over Alps<br />
Later Period of Deep C<strong>on</strong>vecti<strong>on</strong><br />
IOP 8<br />
Light Rain in Target Area<br />
Low-Level Flow Blocked in Po Valley<br />
No Period of Deep C<strong>on</strong>vecti<strong>on</strong>
Height (km)<br />
1 2 3 4 5 6<br />
1 2 3 4 5 6<br />
Precipitati<strong>on</strong> Enhancement Mechanism in IOP2b<br />
REFLECTIVITY<br />
1 2 3 4 5 63h MEAN S-Pol RADAR DATA<br />
FREQUENCY OCCURRENCE<br />
Dry snow (50 %)<br />
Wet snow (30 %)<br />
Graupel - Shaded<br />
dBZ<br />
54<br />
44<br />
34<br />
24<br />
14<br />
4<br />
-6<br />
-16<br />
-26<br />
m/s<br />
RADIAL VELOCITY 36<br />
30<br />
24<br />
18<br />
12<br />
6<br />
0<br />
-6<br />
-12<br />
%<br />
16<br />
14<br />
12<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
120 90 60 30 0<br />
Distance (km) <strong>from</strong> S-Pol radar<br />
snow<br />
0ºC<br />
rain<br />
Slightly<br />
unstable<br />
air<br />
cloud droplets<br />
rain growing<br />
by coalescence<br />
TERRAIN<br />
graupel growing<br />
by riming<br />
Medina and Houze (2003) &<br />
Houze and Medina (2005)
1 2 3 4 5 6<br />
Height (km)<br />
1 2 3 4 5 6<br />
Precipitati<strong>on</strong> Enhancement Mechanism in IOP 8<br />
REFLECTIVITY<br />
1 2 3 4 5 63h MEAN S-Pol RADAR DATA<br />
SPOL P3 RADIAL VELOCITY<br />
dBZ<br />
54<br />
44<br />
34<br />
24<br />
14<br />
4<br />
-6<br />
-16<br />
-26<br />
m/s<br />
36<br />
30<br />
24<br />
18<br />
12<br />
6<br />
0<br />
-6<br />
-12<br />
%<br />
FREQUENCY OCCURRENCE 16<br />
Dry snow (50 %)<br />
Wet snow (30 %)<br />
Graupel and/or<br />
dry aggregates<br />
- Shaded<br />
14<br />
12<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
120 90 60 30 0<br />
Distance (km) <strong>from</strong> S-Pol radar<br />
Height (km)<br />
0 2 4 6 8 10<br />
VERTICAL POINTING RADAR<br />
RADIAL VELOCITY<br />
Updraft cells occur in shear layer<br />
0600 0800 1000 1200<br />
Time (UTC) 21 Oct<br />
This structure leads to a sec<strong>on</strong>d<br />
paradigm for fine-scale moti<strong>on</strong>s<br />
& microphysics in stable flow<br />
over the barrier
More <strong>on</strong> IOP2b…
Cellularity in <strong>Orographic</strong> Flow<br />
Smith et al. (2003)<br />
Also <br />
Georgis et al. (2003);<br />
Asencio et al. (2003)
Three Views of Mesoscale Airflow Patterns in IOP2b<br />
Air-Mass Scrambling<br />
C<strong>on</strong>tradicts Classic<br />
2D C<strong>on</strong>ceptual Model<br />
(Smith et al. 2003)<br />
Precipitati<strong>on</strong> Intensity<br />
~ Strength of Low-<br />
Level Easterlies<br />
(Asencio et al. 2003)<br />
C<strong>on</strong>vergence of Easterlies<br />
with Southerlies Important<br />
(Rotunno and Ferretti<br />
2003; also Georgis et al.<br />
2003)
More <strong>on</strong> IOP8…
Bousquet and Smull (2003, QJRMS)
Down-valley flow under persistent upslope precipitati<strong>on</strong><br />
Steiner et al. (QJ 2003)<br />
Bousquet & Smull<br />
(QJ 2003; JAM 2003)<br />
IOP8
<str<strong>on</strong>g>Less<strong>on</strong>s</str<strong>on</strong>g>
Miglietta and<br />
Buzzi (2004)<br />
Rakovec, Gaberšek<br />
and Vrhovec (2004)<br />
3D<br />
Mesoscale<br />
Topographic<br />
Effects<br />
Matter<br />
Gheusi and Davies (2004)
Moist Stability Matters<br />
Fundamental N<strong>on</strong>linearity<br />
g<br />
δ<br />
Upward Displacement:<br />
Parcel Remains<br />
Saturated<br />
Downward Displacement:<br />
Parcel May Desaturate
46.0<br />
Latitude ( °N )<br />
45.0<br />
Small-Scale Topo Effects Matter<br />
Observati<strong>on</strong>s, Oreg<strong>on</strong> Idealized Flow / Secti<strong>on</strong> of Real Topo<br />
-124.25 L<strong>on</strong>gitude ( °E ) -122.75<br />
U = 10m<br />
/<br />
s<br />
Weak Instability<br />
Courtesy D. Kirshbaum<br />
also Cosma et al. (2002)
Fine-scale air moti<strong>on</strong>s & microphysics matter<br />
snow<br />
0ºC<br />
rain<br />
Slightly<br />
unstable<br />
air<br />
0°C<br />
cloud droplets<br />
SNOW<br />
rain growing<br />
by coalescence<br />
TERRAIN<br />
graupel growing<br />
by riming<br />
RAIN Shear layer and turbulent cells<br />
2 <strong>MAP</strong> Paradigms<br />
need to be tested<br />
Unstable case<br />
Stable case<br />
2008 SHARE<br />
is an opportunity
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