Convective Precipitation in the Rocky Mountains:

1
Convective Precipitation in the Rocky Mountains
Variability of Postfrontal Rain During the
Summer Months Created by Luke Peffers for
Mesofest 2007

• 3) The Preferred Environment
• The annual variability of precipitation in the
  Four Corners region was found to be highly
  correlated to the Southwestern U.S. Monsoon (Fig.
  3), which is responsible for moisture transport
  from the Gulf of Mexico and the Gulf of
  California (Higgins, Mo, and Yao 1998).
• In addition, it was found that variations in
  the precipitation rate are in response to the
  magnitude of the wind speeds and available
  moisture below 600 mb. The most accumulated
  precipitation was recorded when the mean sfc-600
  mb winds were weak (i.e. less than 15 knots) and
  the sfc-500 mb relative humidity was high (i.e.
  above 50). However, morning and midday CAPE
  values rarely exceeded 100 J/(kg).

1) Introduction Convective
precipitation is common in the Four Corners
region during the summer months (Fig.1). A large
portion of the accumulated rainfall is due to
postfrontal storms that occur in the absence of a
synoptic-scale front (i.e. postfrontal). The goal
of this study is to analyze the atmospheric
conditions associated with such precipitation in
order to determine the mechanisms responsible for
the daily and spatial variability.
Fig. 1. Accumulated rainfall (mm) derived from
daily TRMM satellite data averaged over the Four
Corners area (Lat 32 N-40 N, Lon 114 W-104 W).
Fig.2. Preferred regions for postfrontal
convection include the Sangre de Cristo and San
Juan mountain ranges on the Colorado and New
Mexico border.

• 2) Data and Methodology
• Images from the NCAR image archive were used to
  visually identify convective cells and track
  their trajectories in order to identify
  convection genesis regions.
• Archived RUC model data from the NOAA National
  Operational Model Archive Distribution System
  was used to evaluate multiple variables within
  the interior Rocky Mountains.
• TRMM satellite data from NASAs data archive
  was used to plot precipitation rate/accumulation
  data.
• Climatological patterns using NCEP/NCAR
  Reanalysis data from the NOAA Earth System
  Research Laboratory (ESRL) were analyzed to
  determine seasonal variability of precipitation
  in the Southwest.
• NSSL

Fig. 3. Precipitation rate seasonal correlation
with the Southwestern U.S. Monsoon. NCEP/NCAR
Reanalysis from 2000 through 2006.
Fig. 4. Time series of sfc-500 mb mean relative
humidity (10, blue), sfc-600 mb mean wind speed
(10 knots, Yellow), and daily accumulated
precipitation (mm/day, green) for grand Junction
CO. from Jun-Sep 2006.

• 4) A Closer Look at a typical case
• Figure 5 is a time/height RUC model output
  diagram for July 1st, 2006 centered over the San
  Juan Mountains northwest of the Wolf Creek Pass
  ASOS station.
• Ascending motion began at approximately 1700
  UTC at upper levels and was most intense at 600
  mb by 1900 UTC. Relative humidity reached a
  maximum of 95 at 560 mb by 2230 UTC.
  Thunderstorms were observed in the area from 1715
  UTC throughout the evening.
• At approximately 2130 UTC descending motion
  began at the surface followed by a descending
  motion up through 500 mb by 2300 UTC.

• 5) Conclusion
• Postfrontal convective precipitation in the
  interior Rocky Mountains appears to be in
  response to daytime diabatic heating of elevated
  terrain, which causes convergence over peaks and
  broad ranges.
• The daily variability of accumulated rainfall
  is highly dependant on two variables 1) The
  available moisture below 500 mb, which is
  supplied by monsoonal-wind-driven moisture flux
  from the Gulf of Mexico and/or the Gulf of
  California and 2) the magnitude of the winds
  below 700 mb. Weak winds along the heated
  terrain provides parcels with a longer residence
  time, which causes more heating induced
  convection (Tucker, and Crook 2005).
• Preferred regions for postfrontal convective
  precipitation include broad mountain ranges that
  have peaks that extend well into the
  moisture-rich lower troposphere.
• The elevation of the moist layer also plays a
  role in the spatial variability when the
  moisture is confined to levels below peaks, it is
  less likely that the vertical motion above the
  moisture will cause condensation.

Fig. 5. (left) RUC model output, solid lines are
vertical velocity (Pa/s), dashed lines are
relative humidity (). (below) Corresponding
time series of surface obs. at Wolf Creek Pass.
X
O
X
O
X
O
References Banta, R. M., and Schaaf, C. B.,
1987 Thunderstorm Genesis Zones in the Colorado
Rocky Mountains as Determined by Traceback of
Geosynchronous Satellite Images. Mon. Wea. Rev.,
115, 463476. Hales, J. E., Jr., 1974 Southwest
United States summer monsoon sourceGulf of
Mexico or Pacific Ocean? J. Appl. Meteor., 13,
331-342. Higgins R.W., Mo K. C., and Yao, Y.,
1998 Interannual Variability of the U.S. Summer
Precipitation Regime with Emphasis on the
Southwestern Monsoon. J. Climate, 11,
25822606. Kalnay, E., and Coauthors, 1996 The
NCEP/NCAR Reanalysis 40-year Project. Bull. Amer.
Meteor. Soc., 77, 437-471. Tucker, D. F.,
and Crook, N. A., 2005 Flow over Heated Terrain.
Part II Generation of Convective Precipitation.
Mon. Wea. Rev., 133, 25652582.
Fig. 6. Surface observations on July 1st at 1800
UTC (left) and 2300 UTC (right) for the Four
Corners region. Areas marked with an X
indicate convergence associated with strong
convection. Areas marked with a O indicate
divergence associated with drainage winds.