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

1
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
2
Sponsored in part by NSF Award ATM-0505739 NSF
Award ATM-0820586 NASA Award NNX07AD59G
3
Previous studies of extratropical cyclone passage
over complex terrain
Alps (MAP)
Cascades (IMPROVE-2)
NOAA vertically pointing (VP) radar
ETH vertically pointing (VP) radar
4

• Alps VP radar data

• Cascades VP radar data

OBSERVATIONS IN MIDDLE SECTOR OF STORM, IN
STATICALLY STABLE LAYER WITH STRONG VERTICAL
SHEAR ? TURBULENCE
5
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
6
Objectives

• Extend the studies of extratropical cyclone
  passage over complex terrain to Californias
  Sierra Nevada
• Inquire about repeatability of overturning,
  turbulent cells
• Seek a deeper understanding of this mechanism

7
Dataset

• From the Hydrometeorological Testbed 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

8
Analysis of the Alta VP dataset

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

? Time (4 Jan 2008)
9
Analysis of the Alta VP dataset

1. We find all the time periods that have stratiform
   precipitation that is deep (reflectivity gt 0 dBZ
   from surface to 4 km height) and long lasting (gt
   2 hours) ? 57 events are found
2. We investigate the upward motions for these events

10
VP Alta data for 04 Jan 2008 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
11
Analysis of the Alta VP dataset

1. We find all the time periods that have stratiform
   precipitation that is deep (reflectivity gt 0 dBZ
   from surface to 4 km height) and long lasting (gt
   2 hours) ? 57 events are found
2. We investigate the upward motions for these
   events
3. 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

12
Analysis of the Alta VP dataset

1. We find all the time periods that have stratiform
   precipitation that is deep (reflectivity gt 0 dBZ
   from surface to 4 km height) and long lasting (gt
   2 hours) ? 57 events are found
2. We investigate the upward motions for these
   events
3. 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
4. Example of 4 Jan 2008 (case shown before)

13
Synoptic setting of 4 Jan 2008 caseInfrared
satellite temperature (15 UTC)
14
Synoptic setting of 4 Jan 2008 case 850 mb
temperature (15 UTC)
NARR
15
Oakland sounding - 15 UTC 04 Jan 2008
16
Conclusions

• 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