1. Chapter 11: Thermally forced orographic circulations

2. Which cloud(s) are driven by a thermally-forced circulation?

3. Banner clouds: driven by a dynamic pressure perturbation (p’D)
interpret this mountain top low using the Bernouilli eqn:
½rv2+p’=constant, following a streamline

4. Rotor clouds: hydraulic jump  dynamically forced
Föhn wall cloud, Front Range
Downslope winds gather dust on
the valley floor and serve as a tracer of the air rising suddenly into the
cloud. Over the mountains themselves, (upper right) a portion of a
Föhn wall cloud can be seen.
B>>0
turbulent rotor cloud in Owens Valley, CA, downwind of the Sierras.

5. Med Bow range

6. Wave clouds: dynamically forced
upper layer: evanescent waves
lower layer: VP waves
source: Durran 1986

7. combined thermal and mechanical forcing
no surface heating
D : with – without surface heating
Kirshbaum and Wang 2014, JAS

8. Thermally forced? we will come back to this question later
animation of Cb development over the Santa Catalina Mountains, AZ, 16 Aug 2002
we will come back to this question later

9. Mahrt (1982) momentum balance of gravity flows
Assume a n s
B sina B a
B cosa

10. Solenoidally-forced circulation
katabatic flow
anabatic flow
Fig. 11.1
p’ is a hydrostatic pressure perturbation:
air near the mountain slope is warmer than air in the free atmosphere to the left
This also explains the positive buoyancy B in the sloping CBL.
convective boundary layer (CBL) top
Fig. 11.3

11. CuPIDO campaign summer 2006: instrument layout
2790 m MSL
20-30 km
airport
Santa Catalina Mountain
Tucson
Rincon
~800 m MSL

12. estimating mass & energy convergence from station data: polygon method
Mt Lemmon vn

13. mountain–wide horizontal flux method
mass convergence (kg m-1 s-1)
divergence theorem (s-1)
mean anabatic wind (m s-1)
A=576 km2 for the station polygon (D~27 km)

14. case study: 6 August 2006
1810Z
1920Z
2030Z
2150Z

15. case study: 6 August 2006 orographic Cu evolution
local
solar
noon

16. case study: 6 August 2006 18:01-18:51 UTC
Equivalent Potential Temperature (°K) (aircraft track)
18:01-18:51 UTC
Mixing Ratio (g/kg) (wind barbs) 15 5

17. case study: 6 August 2006 mass convergence vs SH time series

18. case study: 6 August 2006 mass convergence profile
divergence in the
upper CBL & above
evidence for a mountain-scale solenoidal circulation
low-level
convergence

19. case study: 6 August 2006 solenoidal forcing
CBL top
x= actual distance from Mt Lemmon
z= height above average terrain
qv is expressed as a perturbation from the mean at 700 mb, 780 mb, 300 m AGL, or sfc data

20. CuPIDO composite: July-August 2006

21. 2 month composite: mass convergence and SH flux
up-
slope
down
slope

22. 2-month composite: solenoidal forcing (buoyancy)x
convective boundary layer depth over Tucson
(hourly soundings, 16 days)

23. 2 month composite: solenoidal forcing (pressure)
How do we obtain a horizontal pressure gradient from stations at different elevations?
time (UTC)
time (UTC)
How to remove the hydrostatic pressure?
Remove the 24 hour mean at any station
How to remove the (semi)-diurnal p variation?
Remove the spatial average p’ at any time

24. Can the perturbation pressure field be used to infer a horizontal gradient, and thus a forcing for anabatic flow?
Define
Examine the spatial variation of p’’. Treating the difference as a differential:
Assuming that both p and are in hydrostatic balance:
observed
station
pressure
gradient
horizontal
pressure
gradient
correction term, depends on phase of the diurnal density (mostly temperature) variation, and the terrain slope

25. 2-month composite: temperature
sunrise
LSN
24 hour mean removed at any station

26. 2-month composite: horizontal pressure difference

27. impact on/of thunderstorms
(27)
(31)

28. … back to the question: is orographic Cu convection thermally forced?
Answer: in the absence of much ambient wind, and in the presence of strong surface heating/cooling, a diurnal solenoidal circulation is well-established, with shallow drainage flow at night and deep, ill-detectable anabatic flow during the daytime.
But that may not trigger orographic Cu convection. That convection is triggered by elevated heating. So yes, orographic Cu clouds are thermally forced, but no, they are NOT due to the convergent anabatic flow (Demko and Geerts 2009, 2010a, 2010b). BL convergence becomes important once deep convection moves away from the mountain.

29. Early evening zonal wind in an idealized 2D simulation without background wind (de Wekker et al 1998)
Fig. 11.2

30. Early evening zonal wind in an idealized 2D simulation without background wind (de Wekker et al 1998)
Fig. 11.2

31. Early evening zonal wind in an idealized 2D simulation without background wind (de Wekker et al 1998)
Fig. 11.2

32. diurnal cycle of BL convergence in the presence of mean wind: across-barrier windq0 q0 q0 q0
convergence zone
green zone: CBL
Fig. 11.4, after Banta 1990

33. diurnal cycle of BL convergence in the presence of mean wind: along-barrier wind
Soderholm and Kirshbaum 2014, MWR

34. seasonal temperature profile in a mountain valley vs a plain
Landeck
day
night
Munich
too cold
Fig. 11.6
too warm

35. thermally-forced mountain-valley wind
pMUN-pINN
warm season, synoptically quiescent fair weather
between Munich and Innsbruck
Fig. 11.8
Fig. 11.7
note: daily-mean pressure was removed first to eliminate the effect of station elevation

36. surface heating/cooling and diurnal temperature range
(a) topographic amplification factor
Q: diabatic heating
SH: surface sensible heat flux
The TAF quantifies the amplification of the diurnal T range W
TAF d
W= width of valley in 3rd dimension
Fig. 11.9
 heating rate within the CBL is inversely proportional to the cross-section area Axz

37. surface heating/cooling and diurnal temperature range
The SVF quantifies the reduced LW radiation loss at night, and reduced SW gain at day. A high SVF reduces the diurnal T range
(b) sky view factor

38. drainage flow
Doppler lidar observations of down-valley flow wind out of page, in m/s
Fig. 11.3

39. diurnal cycle of valley & slope flow
diurnal wind rose
Fig
Fig