Convective Feedback: Its Role in Climate Formation

and Climate Change

Igor N. Esau

Contents:Convective Feedback

• Concept Sketch• Physical Mechanisms• Damping Effect• Atmosphere-Ocean

Control Factors• Climate Sensitivity

Amplification/Damping• Conclusions

Color shading –potential T (density) stratification important for turbulent mixing

Concept: Background

4TR SB

Tropics

atmosphere

atmosphere

ocean ocean

Curves – absolute T stratification important for radiation processes.

pcR

zHS

pT

KHB//1000

H

H

Extra-Tropics

Concept Demonstration:Convective 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

OBS: Erroneously these SAT changes are associated with radiation balance!

Physical Mechanisms: Absorbing Surface

• Solar radiation is mostly absorbed in a thin layer of soil ( ~ 1 mm) or water ( ~ 10 m)

• Local absorption causes strong overheating and convective instability (atmosphere) or stability (ocean)

Physical Mechanisms:Feedback to Local Overheating

• Instability, d/dz < 0, results in fierce convection, transporting heat (moisture, aerosols) above the bulk of the atmosphere (by mass or optical thickness)

Physical Mechanisms:Feedback to Local Overheating

• Stability, d/dz > 0, impedes convection, preventing heat transport and storage in deep ocean

Physical Mechanisms:Summary

• Convective Feedback always cools the system (counter-act heating)

• Efficiency of cooling is controlled by density not temperature gradients

• Side-effects of Convective Feedback are also cooling (PBL clouds, aerosols, evaporation)

• Overall cooling is locally consistent with SAT warming

Earth Climate Needs Cooling

• Earth is located at the inner (hot) edge of the solar habitable zone

• Earth surface as black body ~ -18 C

• Earth surface plus motionless atmosphere ~ +54 C

• S. Manabe and co-authors papers 1960s

What Controls Convective Feedback?

• Damping effect depends reciprocally on H• H is controlled by: (i) mean lapse rate; (ii) surface

temperature difference; (iii) large scale convergence• In their combination and side effects

O(100 m)

O(1000 m)

Control Factors: Physical Inconsistency of Models

• Problem identified in GABLS experiment (Beare, Esau, et al., 2006)

• However, the needed correction is not a constant

Entrainment and Climate Sensitivity

• Entrainment is a rate of involvement of fresh air into convective mixing

• Steinforth et al. (2005, Nature, 433) – 50% reduction in the entrainment rate increase climate sensitivity, i.e probability of higher temperatures, especially in Amazonia

2.5 K

10.5 K

Sensitivity from LES

Without diurnal cycle of the heat flux, the LES sensitivity

is further reduces

Strong nocturnal cooling is compatible with amplified

sensitivity in LES

Convective Layer Thickness: CHAMP Satellite 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 sen

sitive – Mo

re sensitive

Convective Layer Thickness:GLAS Satellite versus ECMWF

• Convective layer thickness (nocturnal PBL depth) as altitude of the first maximum of aerosol concentration from GLAS observations, averaged for 3.10 – 15.11.03 (left); and the average of ECMWF forecasts, 12 GMT 1.10 – 31.10.03. Courtesy Palm and Miller (2004).

• A bit odd intercomparison valid mostly for oceans and Pacific rim.

Conclusions

• Earth’s climate needs cooling• Cooling is regulated by convective

feedback• Convective feedback depends on

limitations of the convective layer thickness

• Limitations are strong• Stronger limitations makes climate more

sensitive to shifts in radiation balance