Radar signatures in complex terrain during the passage of mid-latitude cyclones Socorro Medina Department 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

MAP European Alps September-November 1999 Orography 500-mb geopotential height (black lines) and temperature (shaded)

IMPROVE-2 Oregon Cascade Mountains November-December 2001 Orography 500-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 distance RADAR Horizontal distance Azimuth (fixed) range Horizontal distance Vertical distance RADAR Elevation (scan) range

Range-Height Indicator (RHI) –Fix the azimuth and scan in elevation Radar scanning modes Horizontal distance RADAR Horizontal distance Azimuth (fixed) range Horizontal distance Vertical distance RADAR

Plan Position Indicator (PPI): –Fix the elevation angle and scan in azimuth Radar scanning modes Horizontal distance RADAR Horizontal distance Azimuth (scan) range Horizontal distance Vertical distance RADAR (Elevation fixed) range

Plan Position Indicator (PPI): –Fix the elevation angle and scan in azimuth Radar scanning modes Horizontal distance RADAR Horizontal distance Vertical distance 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) MAPIMPROVE-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 STABLE UNSTABLE

Type B cases stability profiles STABLE UNSTABLE

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 droplets graupel growing by riming rain growing by coalescence Low static stability Medina and Houze (2003) Slightly unstable air Medina and Houze (2003)

Small-scale cells in Type B (Case 01) Vertically pointing radar

Kevin-Helmholtz billows in Type B (Case 1)

RAINOVERTURNING 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

(b) Orography (km) Radial velocity (m s -1 ); PPI = 3.8° Type A case MAP IOP2b 10 UTC 20 Sep

Radial velocity (m s -1 ); PPI = 3.8° Type A case MAP IOP2b 10 UTC 20 Sep 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

(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 km < Range < 70 km  2 km < Height < 5 km MSL

Orography (km) Radial velocity (m s -1 ); PPI = 3.8° Type B case MAP IOP8 06 UTC 21 Oct

(b) Orography (km) Radial velocity (m s -1 ); PPI = 3.8° Type B case MAP IOP8 06 UTC 21 Oct

(b) 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)

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) Mid-level flow Radial velocity (m s -1 ); PPI = 3.8° Type B case MAP IOP8 06 UTC 21 Oct 20 km < Range < 30 km  1.5 km < Height < 2 km MSL

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 Oct 30 km < Range < 70 km  2 km < Height < 5 km MSL

Orography (km) Radial velocity (m s -1 ); PPI = 3.8° Type B case MAP IOP8 06 UTC 21 Oct

Low-Level Flow : km height over the closest plane Cross-Barrier Flow: km height just South of the Alpine crest Upper-Level Flow: 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 caseTYPE B case (MAP IOP8)

16 March 2006 Whistler case

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