Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Problems related to absorption dependent retrievals and their validation

Marc Schröder1, Rene Preusker1, Ralf Bennartz2, Rasmus Lindstrot, and Jürgen Fischer1

1 Institut für Weltraumwissenschaften, Freie Universität Berlin, Berlin, Germany2 Atmospheric and Oceanic Sciences Department, University of Wisconsin, Madison, USA

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Overview

• Introduction

• Some items affecting absorption dependent retrievals

• Various LWC profiles• Photon path length distributions and their parameterisation

• Validation and evaluation: Horizontal averaging effects on nadir reflectance and reflectance ratios

• Results based on casi measurements• Results retrieved from radiative transfer simulations

• Conclusions and outlook

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Introduction

Photon path length distributions and cloud top height retrievals

cloud

aircraft / satellitephoton path

L(761 nm) ~ T L: radianceT: transmission

T = exp(- l) l : photon path: extinction

difference between L for an adiabatic and a cloud top entrained profile (optical thickness = const.)

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

IntroductionValidation and evaluation efforts under different spatial resolutions

0

10

10

20

MODIS: 1 km, casi: 10 m, Lidar (Polis, MIM, B. Heese): 10 m

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Effect of various LWC profiles on absorption intensities: Methods

• LWC profiles from LES model (F. Chosson, Meteo France, CNRM)

• conversion of LWC to microphysical parameters: Chosson et al., 2004 Brenguier, 1991 Schueller et al., 2004

• Radiative transfer with a Monte Carlo model: Petty, 1994 Schroeder, 2004

constant liquid water path in all cases

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Effect on absorption

• Difference mainly depends on absorption intensity and extinction at cloud top.

• Layer thickness and mixing scheme are of minor importance.

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Parameterisation of mean photon path length

In addition to optical thickness, < l > depends significantly oneffective radius.

N

< l >

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Horizontal averaging effects: Results based on observations

cloud land surface

• use casi images

• Calculate reflectance: R = *L / (F cos())

• Use reflectance ratio of R(761 nm) / R(755 nm) as a measure for cloud top height

• Artificially degrade resolution

land

cloud

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Smile Effect

temperatures as observed by radiosondes:FL100: -4 to –5°CFL60: +2°C

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Overview of cases

Stratus / Stratocumulus

1+2 layer clouds

broken clouds

increasing cloud thickness

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Results

Stratus/Stratocumulus

other (e.g. broken clouds)

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

1D versus 2D averaging

• take time series at left and right side

• average over each 1 km series

• average over each 1 km x 1 km area

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

1D versus 2D averaging: Results

time series at left and right side of image

black, red: 1 km averagesgreen: 1 km x 1 km averages

• Very high spatial variability of clouds possible.• Reduction of resolution of time series might not be sufficient.• Exact allocation of pixels required.

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

2D averaging based on RT simulations

cloud droplet number spectra, n(r), from M. Leporini, LaMP, CNRS

directly from n(r) homogeneous

< > 2.57 3.20 < R > 0.16 0.12

dire

ctly

fro

m n

(r)

hom

ogen

eous

mix

ing

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Conclusions

• Extinction at cloud top strongly affects the signal at absorbing channels.

• Significant to strong dependence of the mean photon path on cloud optical thickness AND effective radius.

Potential improvements for retrieval schemes, either through direct simulation or subsequent adjustments.

It may increase the accuracy of gas absorption estimation in GCMs.

• Stratus and Stratocumulus: possible impact of averaging at scales below 1 km, hardly reaching the significance level

Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04

Conclusions and outlook

• Complicated cases: significant effects on scales up to 25 km, effect up to 15%

• A simple reduction of the resolution must be accompanied by an exact allocation of considered observations.

• The microphysical properties, in particular the phase function, can have strong effects on the overall reflectance and absorption intensity.

• Validation of cloud top height (continued)

• Evaluation of forecast models with satellite data

• Intensify analysis of different microphysical schemes