Radiative Impacts of Cirrus on the Properties of Marine Stratocumulus

M. Christensen1,2, G. Carrió1, G. Stephens2, W. Cotton1

Department of Atmospheric Science, Colorado State University1

Jet Propulsion Laboratory, California Institute of Technology2

A53G-0204

Determine the extent to which cirrus affect the radiation budget and dynamics of stratocumulus clouds using data from the Afternoon-Train (A-Train) of satellite observations and large eddy simulations (LES).

• Cirrus clouds decrease net radiative fluxes above stratocumulus clouds by an amount which depends on the cirrus base altitude and optical thickness.

Interregional Annual Average: - Optically thin cirrus (τ < 0.03): -8 ± 4 W m-2

- Optically thick cirrus (τ > 0.03): -27 ± 12 W m-2

• Diurnal radiative response is twice as large as the effect caused by cirrus.

• LES simulations demonstrate that the decreased net radiation causes weaker radiative cooling rates and turbulence, resulting in shallower stratocumulus clouds with less liquid water and precipitation.

• LES results are consistent with A-Train observations.

• These results illustrate that cirrus can affect the radiation budget of stratocumulus, thus global circulation models must have comparable skill (to LES) in simulating both low-level stratocumulus and high-level cirrus clouds to provide reliable assessments of low-cloud feedbacks in the climate system.

CIRRUS RADIATIVE FORCING: The radiative effect depends on the optical depth and cloud base altitude of cirrus. Large values are observed in storm track regions where cirrus are typically thick and have low cloud base heights. Radiative effect is assessed at 3.5 km and defined as the difference in net radiation between profiles containing only stratocumulus and those containing both stratocumulus and cirrus cloud layers. Boxes denote regions that were used in the analysis.

INTERREGIONAL MONTHLY AVERAGE DIFFERENCES: Cloud top heights, radiative cooling rates, precipitation rates, and free-troposphere subsidence rates significantly decrease in response to overlying optically thick cirrus layers. Differences in stratocumulus properties between day (1:30 pm) and night (1:30 am) are nearly twice as large as the effects caused by cirrus. Histograms are constructed from the monthly means derived from subtropical and midlatitude regions.

Atmospheric Modeling System (RAMS)• Grid: 80 X 5 X 300 levels at Δx = Δy = 50 m and Δy = 10 m resolution (24 hour simulation)

• Microphysics: Double moment bulk prognostic scheme (Saleeby and Cotton 2004, J. Appl. Meteorol., 43, 182-195)

• Initial Conditions: CCN = 100 cm-3 and GCCN = 1e-4 cm-3

• Radiation scheme: Nighttime simulation (Harrington et al. 1999, Atmos. Res., 51, 45-75)

• Vertical sounding: European Centre for Medium-Range Weather Forecasts (ECMWF) coincident profile with cirrus and stratocumulus observed by CloudSat on 12/5/2007 at 45° N and 128° W (see above image).

• 9 Experiments: Nudge pristine ice into the radiation scheme to simulate the effects of cirrus.

• Multiple levels: 4, 6.5, and 9 km

• Cirrus optical depths: 0.5, 2, 20 ( τ = 2 for the simulations results shown below)

• Period: June 2006 – June 2011 (60 months of data)• CloudSat: radiative fluxes and precipitation rates• CALIPSO: cloud layer altitude and cirrus cloud optical depth• ECMWF re-analysis: lower troposphere stability and free-troposphere subsidence rates• Vertical profiles: stratocumulus only, stratocumulus & thin cirrus, stratocumulus & thick cirrus• Stratocumulus: cloud layer top less than 3 km• Thin cirrus: cloud layer base greater than 4 km and optical depth less than 0.03• Thick cirrus: cloud layer base greater than 4 km and optical depth greater than 0.03

CLOUD AMOUNT: Stratocumulus amount is strongly tied to the lower troposphere stability, free-troposphere subsidence rate, and the cloud top radiative cooling rate. Cirrus clouds and solar radiation can affect these processes. Based on the linear fit from these controlling variables we infer that optically thick cirrus clouds reduce stratocumulus cloud amount by 2.7 ± 0.6% on average.

LES RESULTS: Cirrus significantly decrease turbulence and cloud top radiative cooling in stratocumulus thereby causing shallower clouds with less liquid water and precipitation. The effect is more pronounced as the cirrus cloud base altitude decreases. Vertical plots were constructed for the final time step of the simulation (24 hours).

Cirrus base = 4 km

Control: No Cirrus

Cirrus base = 6.5 km

Control: No Cirrus

Cirrus base = 4 km

Annual Average Interregional Monthly Average

Stratocumulus Height Rain Rate Cloud Top Radiative Heating 500 hpa Subsidence RateSUMMARY

A-TRAIN OBSERVATIONSOBJECTIVE

LARGE EDDY SIMULATIONS

Conceptual Diagram (interregional average)

ΔQR

ΔLTS

Δω

ΔαRAD

ΔαLTS

ΔαSUB