ElevationDependent Trends in Precipitation Observed during NAME

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Elevation-Dependent Trends in Precipitation
Observed during NAME

• Angela Rowe, Steven Rutledge, and Timothy Lang
• Colorado State University
• Department of Atmospheric Science
• 26 February 2008

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The North American Monsoon
Shift in warm-season climate in June in semiarid
regions of SW U.S. and W Mexico
Figure from Douglas et al. (1993)
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North American Monsoon Experiment (NAME)
Goal To characterize and understand
precipitation processes occurring in the complex
terrain of the core region of the NAM for
improved prediction of warm season
rainfall. IOPs from 1 July 15 August 2004
conducted to focus on studying variability of
convective systems over the diurnal cycle in the
Tier-1 domain
Figure from Higgins et al. (2006)
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NAME Event Rain gauge Network (NERN)
Gochis et al. (2003, 2004)

• Core region of frequent, but moderate intensity
  precipitation over SMO
• Elevation-dependent diurnal cycle where rainfall
  occurs earliest and most frequently over higher
  elevations and later in the day across lower
  elevations with less frequency and higher
  intensity

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NAME radar network

• 2-D composites
• 0.02 lat/lon resolution
• Available every 15 min from 8 July 0000 UTC
  through 21 August 2345 UTC
• Created for Tb, Zh, and rain rate
• S-Pol rain rates computed from CSU blended
  rainfall algorithm (Cifelli et al. 2002) SMN
  rain rates uses Z133 R1.5

• 3-D composites
• S-Pol only
• Includes all major polarimetric variables
• Available on same temporal and horizontal
  resolution as 2-D composites

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Partitioning
Partitioning scheme following Yuter and Houze
(1997, 1998)
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CDFs

• Transition to heavier precipitating events with
  decreasing elevation
• Greatest instantaneous and daily rain totals
  confined to lowest elevations compared to over
  peaks of SMO
• Possibility for intense precipitation to occur
  over the SMO, but duration not as long as over
  lower terrain

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Diurnal trends

• Frequent, low-intensity precipitation initiating
  over the SMO around 1600 LDT
• Less frequent, but higher intensity
  precipitation over the lower terrain during the
  morning (0800 LDT)
• Consistent with NERN results

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Diurnal trends
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Reflectivity profiles
Tendency for convection to be more intense over
the lower terrain
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Hourly profiles
The most intense convection occurs over the lower
terrain during the evening and early morning.
Convection over the SMO is less intense and
concentrated in the afternoon.
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Echo-top height distributions
Convection occurs less frequently over water than
over land. Preferred peaks at 5 km, 9 km, and 12
km revealing trimodal-like structure (Johnson et
al. 1999).
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Echo-top heights
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CSU NAME Gridded Analyses

• Obtained from radiosondes, wind profilers,
  QuikSCAT, METAR surface reports, and others
• Surface and upper-air fields available at 1
  horizontal and 25-hPa vertical resolutions at
  0000, 0600, 1200, and 1800 UTC for 7 July through
  15 August
• Upper-air analyses over SMO interpolated from
  sounding sites on either side of SMO crest

Johnson et al. (2007)
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Lifting Condensation Levels
Photo courtesy of Dave Gochis
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Warm-cloud depths
There is a decreasing trend in warm-cloud depth
with increasing elevation, reflecting the
shallower convection over the SMO compared to the
lower terrain.
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Simulated Results

• Used a simplified model of stochastic droplet
  growth from the CSU RAMS microphysics algorithm
  (Saleeby and Cotton 2006)
• Initial Marshall-Palmer distribution of
  hydrometeors allowed to grow through collection
  of cloud droplets in the warm-cloud layer
• Typical values of mixing ratio, drop
  concentrations, and drop size diameters specified
  at the melting level ( 5 km)

Simulated trend of decreasing convective
precipitation intensity with increasing elevation
(decreasing depth)
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S-Pol Near-Sfc D0 via D0 1.529(ZDR)0.467
Trends in Convection Water Smaller D0s, less
ice mass, large liquid water - Importance of warm
rain processes Land D0, ice water mass
increase with decreasing elevation - Upscale
growth of convection toward lower elevations
(Lang et al. 2007).
S-Pol Liquid Ice Water Mass via Cifelli et al.
(2002)
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Summary

• Statistics from the two-dimensional composites
  of rain rates from the NAME radar network reveal
  a transition to heavier precipitating events with
  decreasing elevation and a pronounced
  elevation-dependent diurnal trend in
  precipitation consistent with NERN results.

Vertical analyses reveal that convection is
more vertically intense over the lower terrain,
occurring during the evening and early morning,
and convection over the SMO tends to be shallower
than over the coast.

• Surface and upper-air data shows a decreasing
  trend in warm-cloud depth with increasing
  elevation.

• Trends similar to observed rainfall were
  simulated by RAMS suggesting that the differences
  in warm-cloud depth between elevation groups
  could explain the differences in rain rates with
  respect to elevation.

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Photos courtesy of Brenda Dolan during NAME
THANK YOU
QUESTIONS?
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References

• Collier, J. C., and G. J. Zhang, 2007 Effects of
  increased horizontal resolution on simulations of
  the North American Monsoon in the NCAR CAM3 An
  evaluation based on surface, satellite, and
  reanalysis data. J. Climate, 20, 1843-1861.
• Douglas, M., R. A. Maddox, K. Howard, and S.
  Reyes, 1993 The Mexican monsoon. J. Climate, 6,
  16651677.
• Gochis, D. J., J. C. Leal, C. J. Watts, W. J.
  Shuttleworth, and J. Garatuza-Payan, 2003
  Preliminary diagnostics from a new event-based
  monitoring system network in support of the North
  American monsoon experiment (NAME). J.
  Hydrometeor, 4, 974981.
• Gochis, D. J., A. Jimenez, C. J. Watts, J.
  Garatuza-Payan, and W. J. Shuttleworth, 2004
  Analysis of 2002 and 2003 warm-season
  precipitation from the North American Monsoon
  Experiment event rain gauge network. Mon. Wea.
  Rev., 132, 29382953.
• Higgins, R. W., Y. Yao, and X. Wang, 1997
  Influence of the North American monsoon system on
  the U.S. summer precipitation regime. J. Climate,
  10, 26002622.
• Higgins, R. W., Coauthors, 2006 The NAME 2004
  field campaign and modeling strategy. Bull. Amer.
  Meteor. Soc., 87, 7994.
• Johnson, R. H., T. M. Rickenbach, S. A. Rutledge,
  P. E. Ciesielski, and W. H. Schubert, 1999
  Trimodal characteristics of tropical convection.
  J. Climate, 12, 23972418.
• Johnson, R. H., P. E. Ciesielski, B. D. McNoldy,
  P. J. Rogers, and R. K. Taft, 2007 Multiscale
  variability of the flow during the North American
  Monsoon Experiment. J. Climate, 20, 1628-1648.
• Nesbitt, S. W., and D. J. Gochis, 2007 The
  diurnal cycle of clouds and precipitation along
  the Sierra Madre Occidental observed during
  NAME-2004 Implications for warm season
  precipitation estimation in complex terrain. J.
  Hydrometeorology, in press.
• Saleeby, S. M., and W. R. Cotton, 2006 A binned
  approach to cloud droplet riming implemented in a
  bulk microphysics model. J. Appl. Meteor., in
  press.

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Elevation groups
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Tendency for convection over the higher terrain
to be shallower than over the coast
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Stability and RH
Contoured frequency by altitude diagram (CFAD) of
relative humidity with respect to ice. (The solid
line is the mean)
Cumulative frequency (percent) of stability
(dT/dz) greater than -3 (solid), -4(dashed),
and -5 C km-1 (dotted)
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Contribution to total convective rainfall
Greater contribution to convective rainfall from
the deeper convection over the lower terrain
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Echo-top heights