Tropical Convection: A Product of ConvergenceTropical Convection: A

Product of Convergence

But What Drives Convergence?

But What Drives Convergence?

ONE THEORY: CISK Conditional Instability of the Second

Kind A Positive Feedback Mechanism . . .

ONE THEORY: CISK Conditional Instability of the Second

Kind A Positive Feedback Mechanism . . .

CISK

Increased Evaporation

Increased Winds

Surface Low:Low Level CONV

Cum. Towers:Upper Level DIV

LHR

Evaporation

CISK

Increased Evaporation

Increased Winds

Surface Low:Low Level CONV

Cum. Towers:Upper Level DIV

LHR

Evaporation

CISK: Convergence Driven by LH Release Aloft

CISK: Convergence Driven by LH Release Aloft

Is this the Whole Story?

Other Process….Other Process….

Barotropic Instability Sea Surface Temperature

Gradients (Lindzen and Nigam)

*All processes play a role to some extent*

Barotropic Instability Sea Surface Temperature

Gradients (Lindzen and Nigam)

*All processes play a role to some extent*

But how do they compare? But how do they compare?

General Circulation:Conv driven by upper-level

Div

Local Circulation:Conv driven by SST

gradient

General Circulation:Conv driven by upper-level

Div

Local Circulation:Conv driven by SST

gradient

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Basic Hypothesis:Basic Hypothesis:

-Momentum Balance of Hadley circulation aloft does not account for total low-level moisture Convergence

-SST directly influence Convection apart from thermodynamic properties

-Variation or Gradient in SST pattern important for Convection

In TropicsSmall Changes Large Influence

-Momentum Balance of Hadley circulation aloft does not account for total low-level moisture Convergence

-SST directly influence Convection apart from thermodynamic properties

-Variation or Gradient in SST pattern important for Convection

In TropicsSmall Changes Large Influence

Environment of Tropical OceanEnvironment of Tropical Ocean

Basic Approach/Methodology

Basic Approach/Methodology

ATS capped at 700mb (height of inversion) Inversion decouples upper ATS from below No influence from LHR in cumulus towers (CISK) Convergence in lower layer driven by SST Gradient Pressure Gradient

Well Mixed BL SST and gradients correlated in vertical

Model Eddy (anomalous) surface flow Zonally averaged flow well represented by Hadley Circulation

Compare model with observational data (FGGE) in order to determine relative importance of low-level forcing in eddy convergence

ATS capped at 700mb (height of inversion) Inversion decouples upper ATS from below No influence from LHR in cumulus towers (CISK) Convergence in lower layer driven by SST Gradient Pressure Gradient

Well Mixed BL SST and gradients correlated in vertical

Model Eddy (anomalous) surface flow Zonally averaged flow well represented by Hadley Circulation

Compare model with observational data (FGGE) in order to determine relative importance of low-level forcing in eddy convergence

Model DevelopmentModel DevelopmentVertical Temp structure of BL linear function of SST:

Flow in Boundary Layer Incompressible:

Given Temp & Density Pressure via Hydrostatic Eq

Vertical Temp structure of BL linear function of SST:

Flow in Boundary Layer Incompressible:

Given Temp & Density Pressure via Hydrostatic Eq

Momentum Equations: Balance of PGF, Coriolis,

Friction

Momentum Equations: Balance of PGF, Coriolis,

FrictionZonal Component:

Coriolis PGF Turbulent

Stress (friction)

Meridional Component

Zonal Component:

Coriolis PGF Turbulent

Stress (friction)

Meridional Component

Compute Eddy SLP from Observed temperature using:

Compute Eddy SLP from Observed temperature using:

Initial Results :Initial Results :

Major Approximation/Error:

Major Approximation/Error:

-Lindzen & Nigam assume top of Boundary Layer (taken to be 700mb or 3km) is flat and does not

varyin time

-Convection occurs instantaneously

-These simplifications are later revised in order to Get realistic flow pattern in the model (back-pressure effect)

-Lindzen & Nigam assume top of Boundary Layer (taken to be 700mb or 3km) is flat and does not

varyin time

-Convection occurs instantaneously

-These simplifications are later revised in order to Get realistic flow pattern in the model (back-pressure effect)

Back-Pressure adjustmentBack-Pressure adjustment

-In original model, BL (700mb sfc) is a rigid sfc that can’t be modified

-In reality, vertical motion above SFC LOW raises the top of the BL (700mb sfc) and this adiabatic expansion acts to cool the lower tropopause raises pressure Negative feedback

-This cooling is eventually dampened by ample LHR ;But it takes time for convective clouds to develop

(~30mins)

-In original model, BL (700mb sfc) is a rigid sfc that can’t be modified

-In reality, vertical motion above SFC LOW raises the top of the BL (700mb sfc) and this adiabatic expansion acts to cool the lower tropopause raises pressure Negative feedback

-This cooling is eventually dampened by ample LHR ;But it takes time for convective clouds to develop

(~30mins)

2 Major New Variables Introduced:2 Major New Variables Introduced:

= Deviation of 700mb layer from flat 3km sfc Proportional to uptake of mass via convergence

Proportional to cooling of tropopause* If large cooling offsets warm SST Convergence suppressed

= Time Scale ~ Cloud development time

Represents adjustment time of ATS to reach steady state*If small, LHR quickly compensates cooling from h’ Convergence excessive

= Deviation of 700mb layer from flat 3km sfc Proportional to uptake of mass via convergence

Proportional to cooling of tropopause* If large cooling offsets warm SST Convergence suppressed

= Time Scale ~ Cloud development time

Represents adjustment time of ATS to reach steady state*If small, LHR quickly compensates cooling from h’ Convergence excessive

Revised Equations in ModelRevised Equations in Model

- Allows for modulation of 700mb sfc with upward vertical motion

variation in top of BL

- Allows for modulation of 700mb sfc with upward vertical motion

variation in top of BL

Note new variables directly proportional to each other:

Note new variables directly proportional to each other:

time scale conv/div time scale conv/div

New SolutionsNew Solutions

If tau=30s looks like old model (excessive convergence)

If tau=3hrs Weak to no convergence (Big back-pressure)

If tau=30mins resembles flow from real data

If tau=30s looks like old model (excessive convergence)

If tau=3hrs Weak to no convergence (Big back-pressure)

If tau=30mins resembles flow from real data

Solution with tau=30mins :Solution with tau=30mins :

Both Gradients ImportantBoth Gradients Important

Forcing from Meridional -Represents ITCZ better

Forcing from Zonal -Represents SPCZ better

Forcing from Meridional -Represents ITCZ better

Forcing from Zonal -Represents SPCZ better

Criticisms/NotesCriticisms/Notes

Questionable parameterizations-3km can be considered too high for mean Boundary Layer-Time Adjustment of 30 mins chosen b/c it looks the ‘nicest’

(No theoretical Justification)

Poor Results for NH Winter-Boundary Layer is shallower-Greater influence from motions aloft

Are Results repeatable-How does model compare against other reanalysis

and data sets (future work)

*Conceptual Problem*

Questionable parameterizations-3km can be considered too high for mean Boundary Layer-Time Adjustment of 30 mins chosen b/c it looks the ‘nicest’

(No theoretical Justification)

Poor Results for NH Winter-Boundary Layer is shallower-Greater influence from motions aloft

Are Results repeatable-How does model compare against other reanalysis

and data sets (future work)

*Conceptual Problem*

Inherent Ambiguity: What drives what?

Low level vs. Upper Level

Inherent Ambiguity: What drives what?

Low level vs. Upper Level

SST gradient Pressure gradient Low-level flow(Lindzen Nigam)

Deep Convection/LHR Pressure gradient Low-level flow

(Gill & others)

*Different Forcing can yield similar results *Each Mechanism only valid given assumptions

made

SST gradient Pressure gradient Low-level flow(Lindzen Nigam)

Deep Convection/LHR Pressure gradient Low-level flow

(Gill & others)

*Different Forcing can yield similar results *Each Mechanism only valid given assumptions

made