Convection over a heated mountain
B. Mapes, Feb 2008, from
http://catalog.eol.ucar.edu/cgi-bin/cupido/research/date_browse?dateUTC=20060810
and papers (BAMS and MWR) & a .ppt by
Bart Geerts and J. Cory Demko
University of Wyoming
Joseph A. Zehnder
Creighton University
2786 m
20 km
600-800 m
facilities
University of Wyoming King Air
digital cameras (2 stereopairs)
NCAR/EOL
University of Arizona:
GPS water vapor
U of A and NCAR & NOAA FSL:
WRF modelling
2 M-GAUS mobile radiosondes
facilities
Mt Bigelow
flux tower on
the mountain
10 ISFF/PAM stations around the mountain
monsoon onset
.
* 10 aug
monsoon onset
Part 1: August 10 convection development
• Paper by Zehnder et al.Zehnder, J.A., J. Hu and A. Razdan,
2009: Evolution of the vertical thermodynamic profile during
the transition from shallow to deep convection during CuPIDO
2006. Mon. Wea. Rev., 137, 937-953.
10am
12:30 (1930 TC)
•
http://catalog.eol.ucar.edu/cgi-bin/cupido/research/date_browse?dateUTC=20060810
•
Movie on laptop (huge)
12Z (5am) Tucson
3:45pm (2245 TC)
Soundings evolution
1530UTC = 8:30, 9:30,
10,1030,11,1130,12,1230 = 1930
1930 at right
Soundings evolution - poormans buoyancy
Aside - buoyancy reversal as
cloudy + dry air mix
Grabowski 1993
thermo/mixing/cu powerpoints
http://www.atmos.umd.edu/~russ/syllabus620.html
10:30 - 12:30 (1730-1930) change
adiabatic vertical displacements?
would conserve theta-e as well as theta
theta-e
saturation
theta-e
What I think happened
• Adjustment of the sounding toward a moist adiabat by nearby but
separate convection (Bretherton and Smolarkiewicz JAS 1989).
Bretherton and Smolarkiewicz JAS 1989
c = N/m =
L/t_BV
Plenty of convection to adjust the sounding
Evolution and neighbors
1800
1830
25 m/s ring expansion
1845
1900
1915
1930
Cooling by evaporation would require liquid injection
by updrafts in this situation - which would bring
higher theta-e. Light rain could be another story...
still reqs
fairly
undilute
ascent
looking for 346 -> 344 (theta-e-s) cooling
stereo analysis
ascent
4km/9’
~7 m/s
Mixing diagnosis: “Paluch diagrams”
scatter of 2 conserved, linearly mixing variables
e.g. http://ucsu.colorado.edu/~kuesterm/paluch/paluch.html
Can we outright resolve entrainment
events? 1646 UTC Doppler radar
1751, just under sounding lid
Another day, another situation
moistening of midlevel dry layer,
but with little T change. Detrainment of
moist but neutrally buoyanct mixtures?
saturation
theta-e
theta-e
1738, 1800, 1822 UTC
moistened air blown
downwind (theta-e is color)
17:55 UTC
Paluch diagram for this case
Again, mixing seems to be at cloud top, not from lateral
entrainment on sides of cloud!
Collapsing cloud top
Yet another day - aircraft data
(from BAMS article, to appear soon)
not all updrafts
buoyant...
Dry convection
small-scale updrafts locally buoyant
Hot air rises, cold air sinks?
“hot”, “cold” in a very local sense
Part 2: role of the mountain
• Does a toroidal BL circulation develop over a heated mountain?
• What drives the anabatic flow & mass convergence?
• Is orographic convection initiated & maintained by heat &
moisture convergence in the BL?
Orographic forcing of boundary layer flow: mass and moisture convergence
25 July 2006
1600 – 1650 UTC
Orographic forcing of boundary layer flow: mass and moisture convergence
25 July 2006, 16:00-16:47 UTC (~4 hours after sunrise)
Equivalent Potential Temperature (°K)
N↑
5
Mixing Ratio (g/kg)
15
Mountain scale convergence
25 July 2006
1810Z
1910Z
2020Z
2130Z
25 July 2006
25 July 2006
CAPE
6 August 2006
18:10
19:20
20:30
21:50
6 August 2006
6 August 2006
average surface values
for 4 hours centered on solar noon
19 July
anabatic flow
(m
airmass convergence
s-1)
(10-4
0.13
s-1)
0.21
CBL depth zi (km)
2.6
sfc sensible flux SH
(Wm-2)
200
sensible heat convergence SHC *zi/2
sfc latent flux LH
(Wm-2)
(Wm-2)
latent heat convergence LHC *zi/2
7000
60
(Wm-2)
430
25 July
6 Aug
1.1
0.4
1.8
0.7
1.4
1.5
225
140
38200
15360
30
190
3900
1780
pressure
perturbations
conclusions
• A toroidal circulation generally is present around the mountain at
the time of first Cu growth. This circulation is associated with a
heat convergence towards the mountain that far exceeds the
surface sensible flux. Thus, once such circulation develops, it
contributes far more to the development of orographic cumuli than
the surface heat flux over the mountain.
• Anabatic flow and SHC peak approximately 2-3 hours prior to local
solar noon and the peak in local SH. This appears driven by a
pressure gradient towards the mountain. This force is present even
before sunrise, but drainage flow prevails until 1.5 hours after
sunrise. Presumably the anabatic flow develops at an earlier time,
but above the very thin drainage flow layer measured by the surface
stations.