Mountain Forced Flows - University of Oklahomatwister.caps.ou.edu/MM2015/docs/chapter2/chapter2_presentation4.… · Mountain Forced Flows Jeremy A. Gibbs University of Oklahoma [email protected]

Mountain Forced Flows

Jeremy A. Gibbs

University of Oklahoma

[email protected]

February 3, 2015

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Overview

Orographic PrecipitationCommon Ingredients of Heavy Orographic PrecipitationFormation and Enhancement Mechanisms

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Common Ingredients of Heavy Orographic Precipitation

The total precipitation (P) is determined by the average rainfallrate (R) and the duration (D),

P = RD . (1)

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The rainfall rate can then be determined by the precipitationefficiency (E ) and the vertical moisture flux (wqv ),

R = E

(ρ

ρw

)(wqv ) , (2)

where ρw is the liquid water density, ρ is the lower moist layeraverage air density, and q is the water vapor mixing ratio.

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Substituting Eq. (2) into Eq. (1) yields

P = E

(ρ

ρw

)(wqv )D . (3)

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The duration of heavy rainfall may be estimated by

D =Lscs

, (4)

where Ls and cs are the horizontal scale of the convective systemand its propagation speed in the direction of the systemmovement, respectively.

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Combining Eq. (3) and Eq. (4) leads to

P = E

(ρ

ρw

)(Lscs

)(wqv ) . (5)

Equation (5) implies that when the value of any combined factorson the right hand side is large, there is a potential for heavyrainfall.

In addition, a set of essential ingredients for flash floods over a flatsurface have been proposed based on Eq. (5)(Doswell et al. 1998).

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For flow over a mountain range, the low-level upward verticalmotion is induced by either orography or the environment (e.g.,upper-tropospheric divergence),

w = woro + wenv . (6)

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The orographically-forced upward vertical motion may be roughlyestimated by the lower boundary condition for flow over mountains,

woro =Dh

Dt≈ VH ·∇h , (7)

where h(x , y) is the mountain height and VH is the low-levelhorizontal wind velocity.

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The environmentally-forced upward vertical motion (wenv ) can bedetermined by the transient synoptic setting, such as thedivergence associated with an approaching trough or conditionalinstability. Combining Eqs. (5)-(7) gives

P = E

(ρ

ρw

)(Lscs

)(VH ·∇h + wenv ) qv . (8)

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Based on the Eq. (8), extremely heavy orographic rainfall requiressignificant contributions from any combination of the followingcommon ingredients:

I high precipitation efficiency of the incoming flow (large E )

I a low-level jet (large |VH | )

I steep orography (large ∇h )

I favorable (e.g., concave) mountain geometry and a confluentflow field (large VH ·∇h )

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continued:

I strong synoptically forced upward vertical motion (large wenv )

I a moist unstable low-level flow (large wenv )

I a high moisture flow upstream (large qv )

I presence of a large, pre-existing convective system (large Ls)

I slow/impeded movement of the convective system (small cs)

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The precipitation efficiency may be controlled by several differentfactors, such as the advection of hydrometeors, evaporationassociated with rainfall below the cloud base, entrainment rate ofenvironmental air into the cloud, and environment wind shear(e.g., Fankhauser 1988).

Large CAPE is not consistently observed for the US and Alpinehistorical events of extremely heavy orographic precipitation.

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Formation and Enhancement Mechanisms

Formation and enhancement mechanisms of orographicprecipitation are complicated and are highly dependent on thedynamical and thermodynamic conditions of the upstream airflow,interactions between different layers of clouds, interactions betweendynamical and cloud microphysical processes, and mountaingeometry.

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In the past, several major formation and enhancement mechanismshave been proposed:

I stable ascent

I release of moist instabilities

I effects of mountain geometry

I combined thermal and orographic forcing

I seeder-feeder mechanism

I interaction of dynamical and microphysical processes

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Figure: Conceptual models of the formation and enhancement mechanisms oforographic precipitation: (a) stable ascent, (b) release of moist instabilities, (c)effects of mountain geometry, (d) combined thermal and orographic forcing, (e)seeder-feeder mechanism, and (f) interaction of dynamical and thermodynamicprocesses

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Stable Ascent Mechanism

I The earliest and simplest mechanism proposed to explain theformation of orographic rain is the stable ascent mechanism,which assumes the precipitation is produced by the forcedascent of a stable, moist airstream along the upslope of amountain.

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Release of Moist Instabilities

I Most of the heavy orographic precipitation that often leads toflash flooding is produced by convective cloud systemstriggered by orographic lifting on an incoming flow throughthe release of moist instabilities.

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I The possibility that conditional instability may occur in anorographic precipitation system is often detected by inspectingwhether there is positive CAPE, in a sounding observedupstream of the threat area.

I Occasionally extremely heavy orographic precipitation isproduced by slantwise convection associated withsymmetrically unstable airstream in an area of strong verticalshear.

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I In addition to the above-mentioned instabilities, thewave-CISK mechanism might operate in the combinedorographic forcing and latent heating in the orographic cloudand precipitation system, especially when both forcing are inphase along the upslope. The elevated thermal forcing andorographic forcing might interact with each other in acoherent manner to enhance the upslope response, andproduce strong lee slope winds.

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Effects of Mountain Geometry

Orographic forcing on incoming airstream is highly dependent onthe detailed geometry of a mountain, which is oftenthree-dimensional and complex.

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I The geometry of the mountain, in turn, affects the amountand distribution of orographic precipitation if the incomingairstream is moist, especially in a moist unstable condition.

I The terrain geometry influences orographic precipitation invarious ways, such as small-scale orographic features,large-scale terrain shapes, and three-dimensional flow blockingand splitting.

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I When a low-Froude number airflow passes over atwo-dimensional mountain ridge, it may produce wavebreaking and severe downslope winds over the lee slope. Inthis situation, strong convergence is generated at the leadingedge of the severe downslope winds and forms a hydraulicjump. When the air is moist, the hydraulic jump becomesvisible as clouds form above their lifting condensation levels.Occasionally, these waves are trapped on the lee side androtor clouds may form

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I If the mountain is isolated, steep and tall, orographic blockingis strong on the impinging moist airstream. In this situation,the flow tends to split and go around, instead go over, themountain and may converge on the lee side and triggerconvection, as sketched schematically in the conceptual modelshown earlier.

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I Moist flow tends to converge in concave regions of amountain range and initiate convection which may lead toheavy orographic rain or flooding.

I Complex terrain is often composed by numerous irregularpeaks and valleys, resulting in fine-scale, complicateddistribution of orographic precipitation.

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Figure: Triggering of convection in the lee of the Olympic Mountains underconditions of the Puget Sound Convergence Zone of Washington State.Streamlines represent surface winds. (Adapted from Mass, 1981) 26 / 45

Combined Thermal and Orographic Forcing

I During the day, a mountain serves as an elevated heat sourcedue to the sensible heat released by the mountain surface.

I In a quiescent atmosphere, this may induce upslope winds,which in turn may initiate cumuli or thunderstorms over themountain peak and produce orographic precipitation.

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I The cumulus could be advected to lee side of the mountainand produce precipitation over the lee slope if the ambientwind is strong enough.

I This type of orographic cumuli, as well as that induced byorographic lifting alone, has been observed byphotogrammetry, instrumented airplanes in the 1950s, and byradars since 1960s.

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I The understanding of the dynamics of these thermally andorographically forced cumuli and their associated precipitationhas been advanced significantly by numerical modelingsimulations since 1970s.

I The next figure shows an example of an orographic rainstormsimulated by an early cloud model.

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I Besides sensible heating, cooling over the mountain slope atnight may influence the airflow and the formation of cloudsand precipitation.

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Figure: A rainstorm simulated by an early cloud model. The rainstorm isinitiated by a combined orographic and thermal forcing. The cloud boundary isdepicted by solid curves and the rainwater content exceeding 1g/kg is shaded.The vertical domain of the model extends to 10 km (Adapted after Orville andSloan 1970)

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Seeder-Feeder Mechanism

I Over small hills, there is a remarkably strong dependence ofrainfall with height for stratus rain, although topography doesnot seem to have much effect on convective showers.

I The stable ascent mechanism could not explain the heavyorographic rainfall observed over such small hills, and thephenomenon was then explained by the seeder-feedermechanism (Bergeron 1949).

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I Based on this mechanism, cloud droplets in the low-level(feeder) clouds formed by orographic lifting over small hillscould be washed out by raindrops or snow from the midlevel(seeder) clouds through coalescence or aggregation process,respectively. In some cases, the feeder clouds may contain iceparticles.

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I The seeder-feeder mechanism can significantly enhance therainfall over the small hills, which could not be produced bythe feeder clouds alone, which has been applied to explain therainfall over the Welsh Hills (Browning et al. 1974), where theseeder clouds were associated with potentially unstable airduring the passage of a wintertime warm sector.

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I Note that the classic seeder-feeder mechanism of Bergeron(1949) yields rain over cloud-covered hills that would nototherwise exist without the presence of the seeder particles.However, many cases of orographic precipitation representenhancements of pre-existing disturbances that producerainfall prior to their interaction with topography. x

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I The seeder-feeder process may also combine with otherprocesses that independently yield raindrops at low levels.

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Dynamical-Microphysical Interaction Mechanism

Microphysical processes may interact with dynamical processesinduced by orography to enhance precipitation in various ways

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such as:

I increase of vertical transport of water vapor, hydrometeors

I enhancing the accretion processes, such as coalescencebetween water (rain or cloud water) drops, aggregationbetween ice particles and riming between ice particles andwater droplets, through the reduction of the liftingcondensation level over the mountain surface

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continued:

I changes in microphysical pathway in the production ofprecipitation

I interception of hydrometeors by a mountain surface at ahigher level than that without a mountain

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continued:

I changes in the horizontal advection scale of air motioncompared to the time scale for microphysical process maychange the precipitation distribution over the mountainsurface

I changes in precipitation efficiency

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continued:

I enhanced evaporation over the downslope with the addition ofadiabatic warming

I unsteadiness of small-scale orographic precipitation viaturbulent motion. In fact, the seeder-feeder mechanism mayalso be viewed as a special case of thisdynamical-microphysical interaction mechanism

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Figure: A conceptual model of the microphysical processes occurring in theorographic cloud and precipitation system. Cloud droplet (C), needle (N), anddendritic particle (D) trajectories in the plane of the cross section are indicatedby arrows. Radar-echo contours (solid) and isotherms (dashed) are also shown.

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In this example, two intersection zones of hydrometeor species overthe upslope during this event can be seen.

One region is located over the middle upslope (about x = 40 to60 km) at about z = 3 km, where large cloud droplets rise fromthe cloud base and interact with the snow dendrites descendingfrom the cloud top. The cloud droplet and snow dendriteinteraction leads to secondary ice production from rime splinteringand the formation of graupel.

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The other region is located near the mountain top (about x = 70to 90 km) in the layer of z = 2 to 3 km. In this region, there isenhanced dendritic aggregate production as the ice crystals createdby secondary ice production interact with the falling snow crystals.

This conceptual model demonstrates how the cloud microphysicalprocesses are influenced by the orography.

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That’s it for mountain forced flows

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Documents

Mountain Forced Flows - University of Oklahomatwister.caps.ou.edu/MM2015/docs/chapter2/chapter2_presentation4.… · Mountain Forced Flows Jeremy A. Gibbs University of Oklahoma [email protected]