Deep Moist Convection (DMC) AOS 453 – Spring 2014 4/1/2014.

transcript

Deep Moist Convection (DMC)

AOS 453 – Spring 20144/1/2014

Deep Moist Convection (run DMC)

AOS 453 – Spring 20144/1/2014

Structure Of DMC Lectures This Week

• Tuesday– DMC (Convective/Convection) Initiation

• Chapter 7 in MR09

– Isolated DMC Organization (Part 1)• Chapter 8 in MR09

– SPC Mesoanalysis Introduction

• Thursday– Isolated DMC Organization (Part 2)

– Diagnostic Variables for Isolated DMC forecasting– SPC Mesoanalysis Continued

DMC – What do we already know?

Overshooting Top

Convective “Core”

Towering Cumulus

New ConvectionCAN YOU SEE THE PBL???

DMC – What do we already know?

What is DMC?

• Think tropospheric overturning

• Parcels reach LFC and freely convect throughout significant portion of the troposphere

• What does this mean we would find on a thermodynamic diagram?

• The good stuff

DMC Initiation – How can we predict it?

• (aka Convective Initiation) is a complex problem• NOT just a function of where there’s CAPE

– CAPE is a necessary, but insufficient condition– Therefore, we can NOT just look for a loaded gun sounding on a

Skew-T!!!!!!!

• Synoptic scale interactions with the mesoscale are non-negligible– Can prime, or destroy a favorable environment– But DMC starts out as mesoscale thermals which cannot be

diagnosed simply using QG theory!

• Orographic effects can play a role• Pre-existing trapped, interior waves at the right place and

time can help initiate/maintain DMC• The reason for all the hate (I would argue)

• (aka Convective Initiation) is a complex problem• NOT just a function of where there’s CAPE

– CAPE is a necessary, but insufficient condition– Therefore, we can NOT just look for a loaded gun sounding on a

Skew-T!!!!!!!

• Synoptic scale interactions with the mesoscale are non-negligible– Can prime, or destroy a favorable environment– But DMC starts out as mesoscale thermals which cannot be

diagnosed simply using QG theory!

• Orographic effects can play a role• Pre-existing trapped, interior waves at the right place and

time can help initiate/maintain DMC• The reason for all the hate (I would argue)

• Surface-based parcels need to reach their LFCs• So need to overcome pre-existing CIN

DMC Initiaion –No Mechanical Lifting

• Let’s assume we have an LFC and significant CAPE…(envision a loaded gun sounding)

• We are not guaranteed DMC!

• Assume no possible mechanical forcing• What does this mean?• Convective initiation becomes a function of the

following question: How do surface-based air parcels reach the LFC without any mechanical lifting????

Initiation From a Thermodynamic Perspective

• LFC implies pre-existing CIN• If can’t lift through CIN, need to somehow decrease the CIN• Can massage first law of thermodynamics to come up with a

“Lapse Rate Tendency”

I II III IV

Differential Horizontal Advection

Vertical Advection Stretching

Differential Diabatic Heating

I II III IV

Differential Horizontal Advection

Vertical Advection Stretching

Differential Diabatic Heating

HEATING???

Don’t Kid Yourself – We haven’t solved the problem

• Lapse rate tendency terms largely driven by mesoscale processes

• Still difficult to forecast

• A tendency in the lapse rate is not ONLY a function of the terms in the tendency equation

• Large scale processes can modify a sounding via moisture and temperature advection, associated QG vertical motions, etc.

Large Scale UVM Boundary Layer Moistening Surface Heating

QG Forcing

QG Forcing Low-level ( boundary layer below ~800mb cap)

flow from moisture source(in U.S. Plains, typically the Gulf of Mexico)

Ts -> CTLow-level ( boundary layer below ~800mb cap)

flow from moisture source(in U.S. Plains, typically the Gulf of Mexico)

QG Forcing

DMC Initiation • So far have assumed no mechanical lifting

• No Orographic lifting• No Air-mass boundaries (synoptic frontal boundaries, dry lines, outflow boundaries,

density currents, etc.)• Only looked at how we can erode CIN

– Takes a long time!!!

• Local maxima in 2D convergence can preferentially force vertical motion

• Ever notice how there are convective “hot spots” along a boundary

• Air mass boundaries play a huge role in initiation DMC

• Throwing these factors into the mix can help convective initiation even more and significantly increase its forecastability.

Check and Mate…We solved the problem, right?

• With all these tools, we should be able to find a relationship between them that consistently favors convective initiation, right?!

Um, No

A Dangerous Assumption

• What have we supposed about the convective plumes thus far?

A Dangerous Assumption

• What have we supposed about the convective plumes thus far?

• Entrainment (mixing) of surrounding dry air into the convective plume can crucially diminish its positive buoyancy and inhibit DMC initiation altogether regardless of whether the LFC is reached

Entrainment

Is That It Then?

• Since we can’t really ever neglect entrainment (unless have unique circumstances which we’ll get to in the next lecture), do we just give up on the problem?

• No. Ask yourself, what would decrease the destructive behavior of ever-present entrainment?

Moisture Convergence

• How destructive the entrainment is is a function of the difference in mixing ratio across the plume interface with surrounding environment.

• So if we can moisten the environment throughout a deeper layer, entrainment will have a less destructive impact on buoyancy of the plume!

• Not necessarily an easy diagnostic, can’t just find local max in mixing ratio.

• Typically, look for differential advection of moisture to determine favorable regions for initiation and sustainment

Vertical Wind Shear

• Vertical wind shear can increase entrainment and can inhibit DMC initiation by increasing turbulence along the periphery of convective plumes.

• But what about supercells????? They have rotation and last for a longgggg time! Much longer than ordinary T-storms!

• Saving this for our next talk

DMC Organization (Part 1) –The role of vertical wind shear

• We will ask ourselves the question, once DMC is initiated (we’ve seen how this is a very difficult forecasting problem), will/how-will it organize?

• What possible organizations can you think of?• Are there associated time scales with these organizations that you

can think of off the top of your head?

• Then ask, what is causing the preferential organization of the initiated DMC?

• Studies show the environment in which it was initiated plays a key role, what environmental variables do you (know of) / (can think of) that would play a role?

• What IS vertical wind shear?

• What IS vertical wind shear?• Eulerian (fixed location) vertical derivative (gradient) of

wind field» Typically just want vertical gradient of horizontal wind» So can vary with SPEED and DIRECTION

• Often times simplify the diagnostic to a simple euclidian difference (i.e. v6km-vsfc)

• Lots of associated diagnostic parameters• 0-6km wind shear is a very typical environmental

parameter used to forecast the favorability for certain DMC organizations• 0-1km wind shear, often correlated with supercellular

tornadogenesis

• How do we visualize/diagnose vertical wind shear?

• Hodographs come in all sorts of flavors

• In the literature and observations, there appears to be a relationship between vertical wind shear and the time scale of DMC

• A sensitive relationship, but for the most part, the more shear the longer lived the DMC is.• Why would this be observed if we just discussed that

vertical wind shear INCREASES entrainment?» Increasing the vertical wind shear ALONE is detrimental to

DMC initiation due to increased entrainment» Need an associated increase in updraft strength to

accommodate the increase in wind shear to increase longevity of the DMC

• Bulk Richardson Number

Background /EnvironmentalKinetic Energy

Updraft PotentialEnergy

Storm Motion Storm Relative Inflow Proxy

Storm RelativeOutflow (Low Level)

Storm Motion Storm Relative Inflow Proxy

Storm RelativeOutflow (Low Level)

So, for:

Big BRN (> 50): Outflow overwhelms inflow Short-lived, ordinary

Small BRN (< 50): Outflow comparable to inflow Longer-lived,

organized, severe cells

• Role of wind shear is not really just as a proxy for inflow/outflow strength.

• Wind Shear Does Two Things:– Displaces precipitation from updraft core and slows

propagation of killer low-level outflow (gust fronts) – When “tilted” by an updraft, can ENHANCE DMC

through the development of a dynamic pressure gradient

Deep Moist Convection (DMC) AOS 453 – Spring 2014 4/1/2014.

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