Tropical Convection: A Product of 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: Convergence Driven by LH Release Aloft Is this the Whole Story?

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? 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

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 Tropics Small 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 Tropics Small Changes Large Influence

Environment of Tropical Ocean

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 Development Vertical 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 Zonal Component: Coriolis PGF Turbulent Stress (friction) Meridional Component Zonal Component: Coriolis PGF Turbulent Stress (friction) Meridional Component

Compute Eddy SLP from Observed temperature using:

Initial Results :

Major Approximation/Error: -Lindzen & Nigam assume top of Boundary Layer (taken to be 700mb or 3km) is flat and does not vary in 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 vary in time -Convection occurs instantaneously -These simplifications are later revised in order to Get realistic flow pattern in the model (back-pressure effect)

Back-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: = 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 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: time scale conv/div

New 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 :

Both 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/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 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