Planetary Boundary Layer (Convective) Feedback in Climate Change - - PowerPoint PPT Presentation

Planetary Boundary Layer (Convective) Feedback in Climate Change

Igor N. Esau

Outline

• Impact of vertical turbulent mixing in

formation of Earth near surface climate

• Control parameters and integral

measures of the vertical turbulent mixing

• Sensitivity of earth’s climate to variability
• f the vertical turbulent mixing
• Some instructive predictions following

from the PBL-feedback

Mechanisms of Equilibrium and Stationary PBL-feedback

Manabe et al. 1960s discovered cooling effect of turbulent convection (by – 44oK on earth’s surface), which transports heat (moisture, aerosols) above the optically thick atmosphere

Mechanisms of Equilibrium but Non-stationary PBL-feedback

∫

=

π

ρ

2

) ( ) ( 1 dt t H t R c dT

p

sunset sunset sunrise Mean SAT Deep mixing Shallow mixing

dT H

Difficult to see in the atmosphere but easy in greenhouse experiment

Demonstration: PBL-feedback in Greenhouse

• Greenhouse limits mixing (R. Wood, 1909), i.e. H,

amplifying DTR and particularly maximum SAT

• On average greenhouse is warmer than outside air
• Strong irradiation during clear but windy nights can

cause excessive cooling in the greenhouse

Integral Measure of Turbulence

• PBL-feedback could be integrally expressed

through PBL thickness, H

• Difficulty is that H depends on external,

variable parameters

Zilitinkevich, Esau, Baklanov, QJRMS 2007 Zilitinkevich 1991

Verification of Theoretical Results (black line) versus LES (red) and SHEBA (blue)

Statement

• Given distribution of the radiative heat flux

divergence in the absence of advection and changes in the atmospheric optical thickness produces higher (lower) surface temperatures in shallow (deep) PBL

• The effect should be the most pronounced

in stably stratified PBL as the relative variability dH/ H and cp ρ dH/ R are much larger in those PBL

PBL-feedback in Stably Stratified Layers

• Convection and its

feedbacks were studied in details

• ver last 40 years
• Nocturnal and polar

shear-driven PBL feedbacks were studied fragmentary

• Impacts of several

important governing parameters were missing

Surface sensible heat flux for December 1959-1997 from NCEP/NCAR Reanalysis project.

Correlation between H and T in wintertime Arctic

Shallow PBL is colder

PBL Depth: CHAMP versus ERA-40

• Convective layer thickness (PBL depth) as the altitude of

minimum relative humidity gradient: left – by the CHAMP (GPS) satellite for all (87598) occultations during 2002-03, data is averaged over a 5 by 5 grid; right – by ERA40 ECMWF data (same time). Courtesy Engeln and Teixeira (2004; 2005)

Less sensitive – More sensitive

Physical Inconsistency of Models

• H is difficult to routinely determine (remote sensing)

in the real atmosphere (ocean?)

• In models, H is strongly inconsistent with theory,

LES and field campaigns

Too deep PBL – Reduced sensitivity

Insufficient Sensitivity of IPCC models

• Rahmstorf et al. (Science, 2007) revealed

insufficient sensitivity of the IPCC models relative to observed climate sensitivity

Climate Sensitivity to PBL-feedback

• PBL-feedback gives inverse climate

sensitivity to mixing layer depth

( )

      − ≡ Ψ Ψ = =

∫ ∫

dR dH H R H dR d where dt dR d c dt dR t H t R d c dR dT

p p

1 1 , 1 ) ( / ) ( 1

2 2 π π

ρ ρ

H and Temperature Trend

Temperature trends are larger in shallow PBL

Global Climate Sensitivity to PBL-feedback in ERA40

Predictive Signature of PBL-feedback

• Where the PBL-

feedback could be find?

• Is there any distinct

signatures of the PBL- feedback?

• PBL-feedback relatively

increases in shallow layers

• Climate change in

shallow PBL should be amplified

Diurnal Temperature Range

• Observed: Min T is

increasing nearly 3 times as fast as the mean T

• Result in decrease
• f DTR
• Observed:

Wintertime mean T increase nearly 2 times as fast as the annual averaged T

Sensitivity to Lapse Rate from LES

• Sensitivity to Lapse Rate depends on heat flux
• Changes in surface T is not necessarily reflect

direction of the global climate change

Conclusions

• Earth’s climate needs cooling
• Cooling is regulated by PBL-feedback
• PBL-feedback depends on limitations
• f the convective layer thickness
• Limitations are strong
• Stronger limitations makes climate

more sensitive to shifts in radiation balance

Auxiliary Materials

To answer questions