Transcript 090515Rastatt_ICAM_Medina.ppt

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
Sponsored in part by:
NSF Award# ATM-0505739
NSF Award# ATM-0820586
NASA Award# NNX07AD59G
Previous studies of extratropical cyclone
passage over complex terrain
Alps (MAP)
ETH vertically
pointing (VP) radar
Cascades (IMPROVE-2)
NOAA vertically
pointing (VP) radar
Alps VP radar data
REFLECTIVITY (dBZ)
09:00
08:00
 Time (19991021)
RADIAL VELOCITY (m s-1)
Cascades VP radar data
REFLECTIVITY (dBZ)
01:00
00:00
 Time (20011214)
RADIAL VELOCITY (m s-1)
OBSERVATIONS IN MIDDLE SECTOR OF
STORM, IN STATICALLY STABLE LAYER
WITH STRONG VERTICAL SHEAR
 TURBULENCE
09:00
08:00
 Time (19991021)
01:00
00:00
 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
• 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
•
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
Analysis of the Alta VP dataset
1. 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 dataset
1. 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
2. We investigate the upward motions for these events
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]
Analysis of the Alta VP dataset
1. 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
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
Analysis of the Alta VP dataset
1. 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
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)
Synoptic setting of 4 Jan 2008 case
Infrared satellite temperature (~15 UTC)
Synoptic setting of 4 Jan 2008 case
850 mb temperature – (15 UTC)
NARR
Oakland sounding - 15 UTC 04 Jan 2008
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