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