Chapter 11: Thermally forced orographic circulations

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

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

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.

Med Bow range

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

CuPIDO composite: July-August 2006

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

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

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

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

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

2-month composite: horizontal pressure difference

impact on/of thunderstorms (27) (31)

… 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.

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

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

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

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

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

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

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

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

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

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

diurnal cycle of valley & slope flow diurnal wind rose Fig. 11.11 Fig. 11.10