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Structure of mid-latitude cyclones crossing the California Sierra Nevada as seen by vertically pointing radar. Socorro Medina, Robert Houze, Christopher Williams and David Kingsmill. International Conference on Alpine Meteorology (ICAM), Rastatt , Germany, 15 May 2009.

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Structure of mid-latitude cyclones crossing the California Sierra Nevada as seen by vertically pointing radar

Socorro Medina, Robert Houze, Christopher Williams and David Kingsmill

International Conference on Alpine Meteorology (ICAM), Rastatt, Germany, 15 May 2009


Previous studies of extratropical cyclone passage over complex terrain
Previous studies of extratropical cyclone passage over complex terrain

Alps (MAP)

Cascades (IMPROVE-2)

NOAA vertically

pointing (VP) radar

ETH vertically

pointing (VP) radar


REFLECTIVITY (dBZ) complex terrain

REFLECTIVITY (dBZ)

Alps VP radar data

Cascades VP radar data

09:00 08:00

01:00 00:00

 Time (19991021)

 Time (20011214)

RADIAL VELOCITY (m s-1)

RADIAL VELOCITY (m s-1)

OBSERVATIONS IN MIDDLE SECTOR OF

STORM, IN STATICALLY STABLE LAYER

WITH STRONG VERTICAL SHEAR

 TURBULENCE

09:00 08:00

01:00 00:00

 Time (19991021)

 Time (20011214)


Conceptual model of dynamical and microphysical mechanisms active when the middle sector of extratropical cyclone passes over a windward slope

Updrafts ~1-3 m/s

Cell width ~1-5 km



Houze and Medina 2005


Objectives
Objectives active when the middle sector of extratropical cyclone passes over a windward slope

  • Extend the studies of extratropical cyclone passage over complex terrain to California’s Sierra Nevada

  • Inquire about repeatability of overturning, turbulent cells

  • Seek a deeper understanding of this mechanism


Dataset
Dataset active when the middle sector of extratropical cyclone passes over a windward slope

  • From the HydrometeorologicalTestbed that the US National Oceanic and Atmospheric Administration (NOAA) has been conducting in the Sierra Nevada (Ralph et al. 2005)

  • Expands over three winter seasons (2005/06, 2006/07, and 2007/08)

  • Main dataset from the vertically pointing (VP) S-band radar located at ‘Alta’

  • Complemented by GOES, North American Regional Reanalysis (NARR), soundings and weather surveillance radar data


Analysis of the alta vp dataset
Analysis of the Alta VP dataset active when the middle sector of extratropical cyclone passes over a windward slope

We find all the time periods that have stratiform precipitation that is deep (reflectivity > 0 dBZ from surface to 4 km height) and long lasting (> 2 hours)  57 “events” are found

 Time (4 Jan 2008)


Analysis of the alta vp dataset1
Analysis of the Alta VP dataset active when the middle sector of extratropical cyclone passes over a windward slope

We find all the time periods that have stratiform precipitation that is deep (reflectivity > 0 dBZ from surface to 4 km height) and long lasting (> 2 hours)  57 “events” are found

We investigate the upward motions for these events


Vp alta data for 04 jan 2008 estimate of upward motions using perturbation radial velocity
VP Alta data for 04 Jan 2008 active when the middle sector of extratropical cyclone passes over a windward slope Estimate of upward motions using perturbation radial velocity

Reflectivity

(Z = [dBZ])

Radial

Velocity [m/s]

1 hr running mean

radial velocity [m/s]

Perturbation radial

velocity [m/s]


Analysis of the alta vp dataset2
Analysis of the Alta VP dataset active when the middle sector of extratropical cyclone passes over a windward slope

We find all the time periods that have stratiform precipitation that is deep (reflectivity > 0 dBZ from surface to 4 km height) and long lasting (> 2 hours)  57 “events” are found

We investigate the upward motions for these events

These calculations indicate that ~10 (out of a total of 57 events) have clearly defined, intermittent up and downdrafts, similar to what was seen in the Oregon Cascades and the Alps


Analysis of the alta vp dataset3
Analysis of the Alta VP dataset active when the middle sector of extratropical cyclone passes over a windward slope

We find all the time periods that have stratiform precipitation that is deep (reflectivity > 0 dBZ from surface to 4 km height) and long lasting (> 2 hours)  57 “events” are found

We investigate the upward motions for these events

These calculations indicate that ~10 (out of a total of 57 events) have clearly defined, intermittent up and downdrafts, similar to what was seen in the Oregon Cascades and the Alps

Example of 4 Jan 2008 (case shown before)


Synoptic setting of 4 Jan 2008 case active when the middle sector of extratropical cyclone passes over a windward slopeInfrared satellite temperature (~15 UTC)


Synoptic setting of 4 jan 2008 case 850 mb temperature 15 utc
Synoptic setting of 4 Jan 2008 case active when the middle sector of extratropical cyclone passes over a windward slope850 mb temperature – (15 UTC)

NARR


Oakland sounding 15 utc 04 jan 2008
Oakland sounding - 15 UTC 04 Jan 2008 active when the middle sector of extratropical cyclone passes over a windward slope


Conclusions
Conclusions active when the middle sector of extratropical cyclone passes over a windward slope

  • Intermittent cells were clearly defined in at least ~17% of the deep, long lasting stratiform events in the Sierra Nevada

  • Case study analysis suggest that the updrafts occur as the middle sector of the storm passes over the windward slopes, in a statically stable layer and in a region of strong vertically shear  i.e., similarly to what was observed in other mountain ranges

FUTURE STEP: Analyze the distribution of updraft magnitude

in relationship to shear, static stability, precipitation, etc


Vp alta data for 04 jan 2008 estimate of upward motions using perturbation radial velocity1
VP Alta data for 04 Jan 2008 active when the middle sector of extratropical cyclone passes over a windward slope Estimate of upward motions using perturbation radial velocity

Perturbation radial

velocity [m/s]


Aircraft observations across the snowy range wyoming
Aircraft observations across the Snowy Range, Wyoming active when the middle sector of extratropical cyclone passes over a windward slope

Geerts et al. 2008


Aircraft observations across the snowy range wyoming1
Aircraft observations across the Snowy Range, Wyoming active when the middle sector of extratropical cyclone passes over a windward slope

Geerts et al. 2008


Precipitation pattern typical of maritime cyclones moving toward us west coast
Precipitation pattern typical of maritime cyclones moving toward US west coast

Nagle and Serebreny 1962


Farber (1972) found a pronounced orographic effect in the middle sector:

“pockets of moisture are observed in liquid water content meter”

“There are pockets of excessively high crystal concentration which

appear to be directly proportional to the amount of turbulence in the region”

“turbulence near the surface, originating from local topographical

features as well as strong vertical wind shears”

Houze and Medina (2005)


Results from Farber (1972) and Hobbs (1975): Aircraft study of mid-latitude cyclones over the WA Cascades

LATE

(Post-frontal)

EARLY

(Pre-frontal)

MIDDLE

(Transitional)

Instability increases

Cloud tops decrease

Ice crystals decrease

LWC:

low

high

moderate

# Aggregates:

moderate &

constant

variability,

locally high

low

Turbulence:

low

high

high


Upstream stability profiles for of mid-latitude cyclones over the WA Cascades

storms with shear layer

STABLE

UNSTABLE


Richardson number (Ri) of mid-latitude cyclones over the WA Cascades

for storms with shear layer

Ri = Nm2 / [dU/dz]2


Shear layer of mid-latitude cyclones over the WA Cascades

m s-1

Case 11

m s-1

E

RADAR

E

W


04 jan 2008 alta reflectivity and residual radial velocity
04 Jan 2008 of mid-latitude cyclones over the WA CascadesALTA reflectivity and residual radial velocity


VP Alta data for 04 Jan 2008 of mid-latitude cyclones over the WA Cascades

Estimate of upward motions by subtractingfallspeed

Reflectivity

(Z = [dBZ])

Radial

Velocity [m/s]

Fallspeed of

snow [m/s] =

0.817 Z 0.063

(Atlas et al. 1973)

Vertical motion

[m/s]


Radial velocity 15 utc 04 jan 2008 kdax
Radial velocity – 15 UTC 04 Jan 2008 (KDAX) of mid-latitude cyclones over the WA Cascades

m s-1

-30.9

-18.0

-7.7

-0.5

0.0

7.7

18.0

30.9

2.4 deg elevation scan, 25 km rings


0700 utc 03 jan 2020 utc 04 jan 2008 accumulated precipitation kdax 50 km rings
0700 UTC 03 Jan – 2020 UTC 04 Jan 2008 of mid-latitude cyclones over the WA CascadesAccumulated precipitation KDAX (50 km rings)


Synoptic setting of 4 jan 2008 case 500 mb geopotential height 15 utc
Synoptic setting of 4 Jan 2008 case of mid-latitude cyclones over the WA Cascades500 mb geopotential height – (15 UTC)

NARR


Synoptic setting of 4 jan 2008 case 900 mb specific humidity narr
Synoptic setting of 4 Jan 2008 case of mid-latitude cyclones over the WA Cascades900 mb specific humidity (NARR)


15 utc 04 jan 2008 800 mb specific humidity narr zoom
15 UTC 04 Jan 2008 of mid-latitude cyclones over the WA Cascades800 mb specific humidity (NARR) - zoom


15 utc 04 jan 2008 700 mb zonal wind narr zoom
15 UTC 04 Jan 2008 of mid-latitude cyclones over the WA Cascades700 mb zonal wind (NARR) - zoom


15 utc 04 jan 2008 700 mb meridional wind narr zoom
15 UTC 04 Jan 2008 of mid-latitude cyclones over the WA Cascades700 mb meridional wind (NARR) - zoom


Synoptic setting of 4 jan 2008 case 850 mb temperature narr
Synoptic setting of 4 Jan 2008 case of mid-latitude cyclones over the WA Cascades850 mb temperature (NARR)


15 utc 04 jan 2008 zonal and meridional winds narr
15 UTC 04 Jan 2008 of mid-latitude cyclones over the WA Cascadeszonal and meridional winds - NARR


4 jan 2008 case shs sounding
4 Jan 2008 case – SHS sounding of mid-latitude cyclones over the WA Cascades


Case study 30 31 may 2005
Case study 30-31 May 2005 of mid-latitude cyclones over the WA Cascades


Case study 30 31 may 20051
Case study 30-31 May 2005 of mid-latitude cyclones over the WA Cascades


Case study 30 31 may 20052
Case study 30-31 May 2005 of mid-latitude cyclones over the WA Cascades


30 31 dec 2005 case shs sounding
30-31 Dec 2005 case – SHS sounding of mid-latitude cyclones over the WA Cascades


Shear and billows in map

Shear and billows in MAP of mid-latitude cyclones over the WA Cascades


Ticino of mid-latitude cyclones over the WA Cascades

DOW

Toce

Po Basin

32

16

S-Pol

0

Height above radar (km)

-16

0 2 4 6

-32

Milan

0 2 4 6 8 10 12 14 16 18 20 22 24

Distance from the radar (km)

DOW radar radial velocity (m s-1) Down-valley direction


Ticino of mid-latitude cyclones over the WA Cascades

DOW

Toce

Po Basin

S-Pol

Milan

DOW Radar observations

Radial velocity (m s-1)

Shear (m s-1 km-1)


Time series of DOW derived observations of mid-latitude cyclones over the WA Cascades


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