Stratus. Outline Formation –Moisture trapped under inversion –Contact layer heating of fog –Fog induced stratus –Lake effect stratus/strato cu Dissipation.

StratusStratus

OutlineOutline

FormationFormation– Moisture trapped under inversionMoisture trapped under inversion– Contact layer heating of fogContact layer heating of fog– Fog induced stratusFog induced stratus– Lake effect stratus/strato cuLake effect stratus/strato cu

DissipationDissipation Model ParametersModel Parameters Model TendenciesModel Tendencies Forecasting ToolsForecasting Tools Conditional Climatology Conditional Climatology

Formation Formation

Low level moisture becomes trapped Low level moisture becomes trapped underneath an inversion underneath an inversion – Cyclonic flow below 800mbCyclonic flow below 800mb– Low level liftLow level lift– Weak instability below the base of the inversionWeak instability below the base of the inversion– Warm/moist air advection above the inversion Warm/moist air advection above the inversion

and cold air advection below the inversionand cold air advection below the inversion– Subsidence (not too strong) above the inversionSubsidence (not too strong) above the inversion

FormationFormation Large-Scale conditions favorable for stratusLarge-Scale conditions favorable for stratus

– Overriding of warm, moist air along and north of a Overriding of warm, moist air along and north of a warm/stationary front, often in advance of an warm/stationary front, often in advance of an approaching surface lowapproaching surface low

– In the wake of a cold front where shallow cold air In the wake of a cold front where shallow cold air undercuts warm and moist airundercuts warm and moist air

FormationFormation

Contact layer heating Contact layer heating of fogof fog– Conduction from the Conduction from the

groundground– Latent heat of Latent heat of

condensationcondensation– Incoming radiation from Incoming radiation from

the fog layerthe fog layer– This all leads to This all leads to

decreasing stability for decreasing stability for weak mechanical/ weak mechanical/ turbulent mixing to turbulent mixing to developdevelop

FormationFormation Fog induced stratusFog induced stratus

– Absorption of outgoing Absorption of outgoing fog layer radiation can fog layer radiation can induce another induce another inversion under which a inversion under which a stratus deck may stratus deck may developdevelop

– Radiation from stratus Radiation from stratus and fog absorbed by and fog absorbed by water vapor warms the water vapor warms the layer between, lowering layer between, lowering the surface-based the surface-based inversion through inversion through hydrostatic expansion.hydrostatic expansion.

FormationFormation

Lake effect stratus/strato cuLake effect stratus/strato cu– Nocturnal lake-effect stratocumulus will Nocturnal lake-effect stratocumulus will

propagate eastward from LM once surface-propagate eastward from LM once surface-based mixing ceases (decoupling)based mixing ceases (decoupling)

– Elevated moist mixed layer is the duct for cloud Elevated moist mixed layer is the duct for cloud advancementadvancement

– Layer is capped above and below by highly Layer is capped above and below by highly stable layers (usually surface and subsidence stable layers (usually surface and subsidence temperature inversions)temperature inversions)

FormationFormation

Lake effect stratus/strato cuLake effect stratus/strato cu– Sufficient low level instability over the lake Sufficient low level instability over the lake

capped by a stable layer (cool season)capped by a stable layer (cool season)– Advection of moist air over cold water (Td > Tw)Advection of moist air over cold water (Td > Tw)

FormationFormation

- Frictional Convergence- Frictional ConvergenceAir moving from relatively smooth water to rough land leads to more cross Air moving from relatively smooth water to rough land leads to more cross

isobaric flow, lighter wind speeds, speed/directional convergence, and isobaric flow, lighter wind speeds, speed/directional convergence, and enhanced lift.enhanced lift.

Over Water

PGFF

WindC

Over LandF

PGF

WindC

DissipationDissipation

Advection of cold air aloft or warm air at the Advection of cold air aloft or warm air at the surface (weaken the inversion)surface (weaken the inversion)

Dry air advecting into the lower levelsDry air advecting into the lower levels Solar heating to mix out the inversion (warm Solar heating to mix out the inversion (warm

season)season) Strong subsidence forcing the inversion to Strong subsidence forcing the inversion to

groundground

DissipationDissipation

Mechanical mixingMechanical mixing– Shear induced mixing Shear induced mixing

promotes momentum promotes momentum transporttransport

– This increases the This increases the mixingmixing

– The inversion layer The inversion layer lowers and weakenslowers and weakens

DissipationDissipation

Model ParametersModel Parameters

Low level cyclonic vorticityLow level cyclonic vorticity– Ekman layer develops which leads to cyclonic Ekman layer develops which leads to cyclonic

flow with low-level liftflow with low-level lift

Boundary layer convergenceBoundary layer convergence

Model ParametersModel Parameters

Presence of moisture below 800mbPresence of moisture below 800mb Moist isentropic ascent in the lower levels of Moist isentropic ascent in the lower levels of

the atmospherethe atmosphere Lapse rates near the inversion levelLapse rates near the inversion level

– Weak low level dry air advection will likely not Weak low level dry air advection will likely not be enough to diminish the stratus without a be enough to diminish the stratus without a change in the inversionchange in the inversion

Model ParametersModel Parameters Differential temperature advection centered Differential temperature advection centered

on the inversion levelon the inversion level– In winter, temperature advection is the most In winter, temperature advection is the most

efficient way to weakening the inversionefficient way to weakening the inversion

Model ParametersModel Parameters

Time sections of omega/theta e to diagnose Time sections of omega/theta e to diagnose mid-level subsidencemid-level subsidence– Strong subsidence will cause dry air Strong subsidence will cause dry air

entrainment at cloud top and will push the entrainment at cloud top and will push the inversion to the groundinversion to the ground

– Weak subsidence however may not be enough Weak subsidence however may not be enough to break up the clouds, but can lower inversion to break up the clouds, but can lower inversion causing lowering of the cloud bases.causing lowering of the cloud bases.

Model ParametersModel Parameters

Model soundings to diagnose the height and Model soundings to diagnose the height and strength of the inversionstrength of the inversion

Model TendenciesModel Tendencies

Tendency to dry out the low level RH too Tendency to dry out the low level RH too fastfast– Compare 24-hour forecast differences in RH Compare 24-hour forecast differences in RH – Look for gradients in RH rather than forecasting Look for gradients in RH rather than forecasting

a specific valuea specific value

Model TendenciesModel Tendencies

Tendency to lower or weaken the inversion Tendency to lower or weaken the inversion too fasttoo fast– Compare temperature advectionsCompare temperature advections– Compare observed soundings to model Compare observed soundings to model

soundingssoundings

Forecasting ToolsForecasting Tools

Observed soundings, including Observed soundings, including ACARS/TAMDARACARS/TAMDAR– Compare postfrontal sounding to “near front” Compare postfrontal sounding to “near front”

soundingsounding– Examine the changes in inversion depth Examine the changes in inversion depth – Compare observed sounding to model soundingCompare observed sounding to model sounding

Is the inversion as strong in the model as the Is the inversion as strong in the model as the observedobserved

Does the model have the sub-inversion layer as Does the model have the sub-inversion layer as saturated as the observed soundings?saturated as the observed soundings?

Forecasting ToolsForecasting Tools

Satellite imagerySatellite imagery– Compare location of cloud edge to low level RH Compare location of cloud edge to low level RH

in modelsin models– Compare evolution of cloud edge movement to Compare evolution of cloud edge movement to

changes in RH field in the modelschanges in RH field in the models

Conditional ClimatologyConditional Climatology

AVNFPS 3.0 now has the capability of AVNFPS 3.0 now has the capability of displaying climatological data gathered displaying climatological data gathered since 1972since 1972

This should be used as a guide to determine This should be used as a guide to determine the “family of possibilities”the “family of possibilities”

The higher the percentage frequency of a The higher the percentage frequency of a given ceiling or visibility, the more given ceiling or visibility, the more confidence.confidence.

The larger the sample size the more reliable The larger the sample size the more reliable the forecast.the forecast.

Conditional Climatology - DrawbacksConditional Climatology - Drawbacks

It does not tell the cause, especially the It does not tell the cause, especially the meso scale influences. meso scale influences.

It does not tell about the coverageIt does not tell about the coverage– The data is gathered at a specific pointThe data is gathered at a specific point

Does not account for changes in surface Does not account for changes in surface cover (urban heat island)cover (urban heat island)

Not good for forecasting a rare event (snow Not good for forecasting a rare event (snow in late April)in late April)