Boundary layer parameterization and climate

Chris BrethertonUniversity of Washington

Some PBL-related climate modeling issues• PBL cloud feedbacks on tropical circulations, climate

sensitivity and aerosol indirect effect in all latitudes.• Wintertime land surface temperature biases

– stable boundary layer – cloud biases– role of land surface vs. PBL

• PBL interactions with deep convection – gustiness, enhanced surface flux over warm oceans – cumulus cloud base properties– diurnal cycle over land

• Surface wind stress– ocean (warm pool, coasts, cold side of ocean fronts)– complex terrain

• Optimal resolution tradeoff – z and t vs. x-y.

PBL params improving (with large-scale feedbacks); biases remain.

EPIC2001ECMWFNCEPAM2.10CAM2.0

Bretherton et al. (2004)

Yu and Mechoso (2001)

PBL cloud feedbacks on climate sensitivity

Is every geographical region different?

Lack of published physical mechanisms:

(-) In warmer climate, adiabatic dLWC/dz larger, so PBL clouds of given thickness are more reflective (Somerville and Remer 1984).

… but not much evidence for such trends with T.

Tselioudis and Rossow (1994)

• (-) In warmer climate, steeper moist adiabat raises lower tropospheric stability, increasing low cloud cover (Miller 1997 based on Klein and Hartmann 1993).

…but how to apply ‘Klein line’ to changed climate (or even its robustness in current climate) is questionable.

• Observed TOA net CRF moderately correlated to LTS.• In CAM3, LTS increases ~2 K in 2xCO2 climate with little

NCRF change.

Alternative LTS measures more climate-stable?

• Estimated Inversion Strength (Rob Wood)

EIS = LTS - (z700-LCL)*(d/dz)moistad(700 mb)

LTS = 700 – 1000 EIS = +LCL,ma –

700

1000

LCL

• Collapses midlatitude vs. low-lat. Sc regimes better. • EIS less sensitive than LTS to steeper moist adiabat

predicts less low cloud feedback on climate sensitivity.

(Klein and Hartmann 1993)

Anthropogenic aerosol-CTBL feedbacks

• Major IPCC uncertainty (current FTOA= 0-2 W m-2).

• CTBLs likely a major contributor.

• Manifest in POCs (pockets of open cells; Stevens et al. 2005)?

• Limited understanding of aerosol feedbacks on PBL cloud thickness, lifetime thru drizzle, radiative feedbacks.

POCs

Bretherton et al (2004)

POCS, cloud droplet size, and drizzle

Rob WoodSandy Yuter

DYCOMS RF02 nocturnal drizzling stratocumulus

Nd ~ 45-70 cm-3

(Stevens et al. 2003; vanZanten et al. 2005)

LES

Extremely diverse dependence of Sc drizzle on LWP

GCSS RF02 SCM/LES intercomparison

High-latitude wintertime PBL

• AGCMs often have large high-latitude surface temperature biases. These can feed back on snow-albedo feedback and biogeochemistry.

CAM3DJFT2m bias

Low cloud bias

• Inadequate attention has been paid to understanding the role of stable BL turbulence vs. clouds vs. land-surface in these biases.

• Often addressed by large-Ri tuning of stability functions, which may compensate other errors.

• NWP-mode testing of climate models would help!

• Sensitive to underresolution of PBL depth & terrain.

M-Y stability fns

Enhanced hi-Ri mixing

PBL interactions with deep convection• The PBL under precipitating deep convection is highly

inhomogeneous, enhancing surface fluxes and complicating Cu parameterization.

Kuang and Bretherton (2005)

Cu base properties

• Narrow range (±0.1K, 0.3 g kg-1) of cloudy updraft properties• Moist (but not most buoyant) ‘tail’ of PBL air forms Cu bases.• Cold pools affect near-surface (but not upper PBL) air properties.• Suggests prediction of PBL T/q PDF would help Cu parameterization.

Cold pool tail

(Kuang and Bretherton 2005)

Diurnal cycle biases (Yang and Slingo 2001)

UKMO Unified Model

Satellite

Local time of peak precipitation• Satellite shows early evening peak over land, early morning peak over

ocean ITCZ.• Models show late morning peak over land, midnight peak over ocean.• Mainly Cu param, but maybe also PBL parameterization issue?

Wind stress in EPIC2001 95W cross-equatorial flow

aircraft obs. NCEP GDAS

Shearin SBL

Surface jetin CBL

Observed dy/dy absent

GCM resolution badly smears wind stress gradients

Raymond et al. (2004)

PBL wind stress errors II

Momentum transport problems in stable warm-advection PBLs (Brown et al. 2005).

ug

EC ctrlM-O stabsonde

Resolution

• Vertical resolution is a concern for both stable and cloud-topped boundary layers.

• In GCMs, we typically screw up our climate when changing vertical resolution. The role of PBL processes in this is hard to disentangle, but likely significant.

• More GABLS/GCSS-style benchmark LES/SCM/regional model simulations (including katabatic flows over undulating terrain) are needed for us to better appreciate our discretization errors.

Some perspectives on these problems

• In the U.S. we have great field expts and data, but almost no full-time PBL parameterization developers and closely connected to major GCMs. This is a social crisis.

• Broad PBL issues (e. g. high latitude low cloud/land temperature biases) are going almost unstudied in the U.S. climate community. Cross-talk with NWP models and NWP-mode simulations are valuable strategies.

• These biases assume increasing importance as we couple more systems (e.g. plants, ice) into our models.

• We often explore horizontal resolution, but we must also routinely push GCMs to much higher vertical and time resolution to understand how well converged their PBL climatology is, even if we are afraid of the answers.