Radar signatures in complex terrain during the passage of mid-latitude cyclones
Socorro MedinaDepartment of Atmospheric Sciences
University of Washington
MSC/COMET Mountain Weather Course, Boulder CO , 7 December 2007
Observational Perspective Field Experiments
• “MAP” – Mesoscale Alpine Programe
• “IMPROVE-2” – Second phase of the Improvement of Microphysical PaRameterization through Observation Verification Experiment
MAPEuropean Alps
September-November 1999
Orography500-mb geopotential height (black lines) and temperature (shaded)
IMPROVE-2Oregon Cascade MountainsNovember-December 2001
Orography500-mb geopotential height (black lines) and temperature (shaded)
Synoptic conditions of MAP and IMPROVE-2 storms:
◘ Baroclinic system approaching orographic barrier
◘ Flow far upstream nearly perpendicular to terrain
MAP and IMPROVE-2 radar observations
NOAA WP-3D
S-Pol = NCAR S-band polarimetric radar
Mean crest = 3 km MSL Mean crest = 2 km MSL
• Range-Height Indicator (RHI)– Fix the azimuth and scan in elevation
Radar scanning modes
Horizontal distanceRADAR
Ho
rizo
nta
l d
ista
nce
Azimuth (fixed)
range
Horizontal distance
Ver
tica
l d
ista
nce
RADAR
Elevation (scan)range
• Range-Height Indicator (RHI)– Fix the azimuth and scan in elevation
Radar scanning modes
Horizontal distanceRADAR
Ho
rizo
nta
l d
ista
nce
Azimuth (fixed)
range
Horizontal distance
Ver
tica
l d
ista
nce
RADAR
• Plan Position Indicator (PPI):– Fix the elevation angle and scan in azimuth
Radar scanning modes
Horizontal distanceRADAR
Ho
rizo
nta
l d
ista
nce
Azimuth (scan)
range
Horizontal distance
Ver
tica
l d
ista
nce
RADAR
(Elevation fixed)range
• Plan Position Indicator (PPI):– Fix the elevation angle and scan in azimuth
Radar scanning modes
Horizontal distanceRADAR
Ho
rizo
nta
l d
ista
nce
Horizontal distance
Ver
tica
l d
ista
nce
RADAR
(Elevation fixed)range
Radar measurements
• Reflectivity factor (often called reflectivity): Quantity proportional to the sixth-power of the diameters of all the raindrops in a unit volume
• Radial velocity: The flow component in the direction of the radar beam
Methodology: Time-averaged vertical cross-sections (from RHI data)
MAP IMPROVE-2
NNW
Type A low-level flow rises over terrain
S-POL
Mean Radial velocity (m s-1; from RHIs)
MAP Case (IOP2b, 3-hour mean) IMPROVE-2 Case (IOP6, 2-hour mean)
E S-POL
NNW S-POL
IMPROVE-2 (IOP1, 3-hour mean)
E S-POL
MAP (IOP8, 3-hour S-Pol mean)
Type B low-level flow doesn’t rise over terrain ; shear layer
Mean Radial velocity (m s-1; from RHIs)
IOP8 (Type B) Airborne radar-derived low-level winds
Bousquet and Smull (2006)
IOP8 (Type B) Airborne radar-derived down valley flow
Bousquet and Smull (2003)
Summary of terrain-modified airflow in MAP and IMPROVE-2
storms:
◘ Type A: Low-level jet rises over the first peaks of the terrain
◘ Type B: Shear layer rises over terrain
Measure of stability in moist flow
moist Brunt-Vaisala frequency to include latent
heating effects (Durran and Klemp, 1982)
Type A cases stability profiles
STABLEUNSTABLE
Type B cases stability profiles
STABLEUNSTABLE
Summary of static stability in MAP and IMPROVE-2 storms:
◘ Type A: Potential instability
◘ Type B: Statically stable
NNW
Type A Maximum over first major peak
S-POL
Mean Reflectivity (dBZ)
MAP Case (IOP2b, 3-hour mean) IMPROVE-2 Case (IOP6, 2-hour mean)
E S-POL
NW
Type B Bright band
S-POL
Mean Reflectivity (dBZ)
MAP Case (IOP8, 3-hour mean) IMPROVE-2 Case (IOP1, 3-hour mean)
E S-POL
Summary of reflectivity patterns in MAP and IMPROVE-2 storms:
◘ Type A: Localized maximum on terrain peak
◘ Type B: Bright band
TYPE A conceptual model of precipitation enhancement for flow rising over terrain
TERRAIN
snow
rain
0ºC
cloud dropletsgraupel growingby riming
rain growingby coalescence
Low static
stability
Medina and Houze (2003)
Slightlyunstable
air
Medina and Houze (2003)
Small-scale cells in Type B (Case 01)Vertically pointing radar
Kevin-Helmholtz billows in Type B (Case 1)
RAIN OVERTURNING CELLS
0°C
Shear layer and overturning cells
SNOW
Region of enhanced growth by riming and aggregation
Region of enhanced growth by coalescence
TYPE B conceptual Model of precipitation enhancement for cases with statically stable and retarded low-level flow
Houze and Medina (2005)
Results shown so far from RHI scans, but RHIs are not available in
operational scanning
- What do Type A and B flow structures look like in PPIs?
PPI range as a proxy of height
Z1 < Z2
Range
(b)
Orography (km)
Radial velocity (m s-1); PPI = 3.8°Type A case
MAP IOP2b 10 UTC 20 Sep
32
24
16
8
0
-8
-16
-24
-32
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(b)
Orography (km)
Radial velocity (m s-1); PPI = 3.8°Type A case
MAP IOP2b 10 UTC 20 Sep
32
24
16
8
0
-8
-16
-24
-32
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Radial velocity (m s-1); PPI = 3.8°Type A case
MAP IOP2b 10 UTC 20 Sep
32
24
16
8
0
-8
-16
-24
-32
MAP Case (IOP2b, 3-hour mean)
NNW S-POL
Range < 20 km Height < 1.5 km MSL
(b)
Orography (km)
Radial velocity (m s-1); PPI = 3.8°Type A case
MAP IOP2b 10 UTC 20 Sep
32
24
16
8
0
-8
-16
-24
-32
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(b)
500-mb geopotential height (black lines) and temperature
12 UTC 20 Sep
Radial velocity (m s-1); PPI = 3.8°Type A case
MAP IOP2b 10 UTC 20 Sep
32
24
16
8
0
-8
-16
-24
-32
30 km < Range < 70 km 2 km < Height < 5 km MSL
Orography (km)
32
24
16
8
0
-8
-16
-24
-32
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Radial velocity (m s-1); PPI = 3.8°Type B case
MAP IOP8 06 UTC 21 Oct
(b)4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Orography (km)
32
24
16
8
0
-8
-16
-24
-32
Radial velocity (m s-1); PPI = 3.8°Type B case
MAP IOP8 06 UTC 21 Oct
(b) 32
24
16
8
0
-8
-16
-24
-32
Radial velocity (m s-1); PPI = 3.8°Type B case
MAP IOP8 06 UTC 21 Oct
Range < 10 km Height < 1 km MSL
NNW S-POL
MAP (IOP8, 3-hour S-Pol mean)
32
24
16
8
0
-8
-16
-24
-32
Mid-level flow
Radial velocity (m s-1); PPI = 3.8°Type B case
MAP IOP8 06 UTC 21 Oct
NNW S-POL
MAP (IOP8, 3-hour S-Pol mean)
20 km < Range < 30 km 1.5 km < Height < 2 km MSL
(b)
Orography (km)
32
24
16
8
0
-8
-16
-24
-32
Mid-level flow
Radial velocity (m s-1); PPI = 3.8°Type B case
MAP IOP8 06 UTC 21 Oct20 km < Range < 30 km 1.5 km < Height < 2 km MSL
32
24
16
8
0
-8
-16
-24
-32
Radial velocity (m s-1); PPI = 3.8°Type B case
MAP IOP8 06 UTC 21 Oct
500-mb geopotential height (black lines) and temperature
06 UTC 21 Oct30 km < Range < 70 km 2 km < Height < 5 km MSL
Orography (km)
32
24
16
8
0
-8
-16
-24
-32
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Radial velocity (m s-1); PPI = 3.8°Type B case
MAP IOP8 06 UTC 21 Oct
Low-Level Flow : 1.5 - 2 km height over the closest plane
Cross-Barrier Flow: 2.5 - 3.5 km height just South of the Alpine crest
Upper-Level Flow: 4 - 5 km height in a circle
u p p e r – l e v e l f l o w
cross-barrier flow
low-level flow
Using flows below 5 km (from PPI scans) for nowcasting of precipitation
Work by Panziera and Germann 2007 (MeteoSwiss)
A decrease of the three flows intensities seems to anticipate the end of the heavy rain.
Magnitude (m/s)
Rain (averaged over several basins)
Using flows below 5 km (from PPI scans) for nowcasting of precipitation
Panziera and Germann (2007)
Conclusions
-Two predominant terrain-modified flow patterns during orographic enhancement of precipitation have been identified (Types A and B)
-Both patterns produce strong updrafts (>2m/s)
-During Type A cases static instability is responsible for the updraft generation versus turbulent instability in Type B
-During both Types the enhancement of precipitation is produced by the accretion processes (coalescence, aggregation and riming)
-The flows at low-levels have some potential for nowcasting precipitation
Whistler topography
16 March 2007 Whistler case TYPE B case (MAP IOP8)
16 March 2006 Whistler case
16 March 2006 Whistler case
END