# Presentation: 033979

## Optical effects of clouds on trace-gas absorption

- Joanna Joiner, Alexander Vasilkov,
- P.K. Bhartia, Robert Spurr, Nick Krotkov,
- Jerry Ziemke, Sushil Chandra,
- Pieternel Levelt, Graeme Stephens

## Parameters that affect atmospheric absorption

- Cloud fraction (geometric)
- Cloud optical depth
- Surface albedo
- Cloud vertical structure

- Radiative cloud fraction

- Radiative cloud pressure

- Joiner, Aura Sci Team Meeting

- 10/04/07

## Radiative Cloud Fraction

- Definition: Fraction of measured radiation that is scattered by clouds.

- Where Ic and Im are the cloudy and measured (total) radiances, respectively (normalized by solar irradiance) and fg is the geometrical cloud fraction
- It can be shown approximately that

- If cloud is brighter than the surrounding atmosphere then fR>fg.
- In a non-absorbing atmosphere, fR increases with Im decreases, IC doesn’t change
- Can be computed with simple cloud models

- Joiner, Aura Sci Team Meeting

- 10/04/07

## Simple cloud models

**Lambertian-equivalent reflectivity (LER):** Surface (cloud/ground) is an opaque and isotropic scatterer with reflectivity R
**Mixed LER:** Pixel is composed of weighted clear and cloudy components; In OMI algorithms, clouds are assumed to have R=0.8; Proper weighting via fr accounts for light scattered from beneath the cloud.
**Plane-Parallel Cloud (PPC):** Use Mie scattering theory with a horizontally infinite and vertically homogeneous cloud with an effective optical depth (related to fr).
**Mixed PPC:** Similar to Mixed LER but uses PPC model for cloud

- Joiner, Aura Sci Team Meeting

- 10/04/07

## How well can we predict Rayleigh scattering with simple cloud models?

- Rayleigh scattering can be described well by simple MLER model with 1 parameter (not significantly affected by cloud vertical structure)
- Implication: May not need subpixel imager on future instrument

**From Ahmad *****et al*****. JGR (2004) **

- Joiner, Aura Sci Team Meeting

- 10/04/07

## How well does Mixed LER (radiative cloud fraction concept) work for trace-gas absorption?

**LambertianCloud**

**No  Cloud**

**Mie  Cloud**

-  **Mixed LER**

**(gives correct column ozone)**

- Joiner, Aura Sci Team Meeting

- 10/04/07

## What do real clouds look like to OMI?Cloudsat (A-train) helps us to answer

- Cloudsat optical depth profiles

**2 cloud decks: thin upper deck-**
**OMI retrieve pressure of lower deck**

**Optical depth peaks in ice portion of cloud**

**MODIS: sensitive to cloud-top**
**(not appropriate for UV-VIS trace-gas retrievals)**

**Optical depth peaks in liquid water portion of cloud**

**Cloudsat radar reflectivity**

**OMI radiative cloud pressure from Raman scattering: UV/visible light penetrates deeper****  **

**OMI simulated from Cloudsat**

-                    **thick upper deck**

- Joiner, Aura Sci Team Meeting

- 10/04/07

## O3, O2-O2 cloudy Jacobians: significant photon penetration inside clouds

- IR cloud-top

- Joiner, Aura Sci Team Meeting

- 10/04/07

## Application: Cloud-slicing

**From Ziemke *****et al*****., in prep. (2007) – see poster **

- Joiner, Aura Sci Team Meeting

- 10/04/07

## OMI Cloud-Slicing: Slope Mixing Ratio; Intercept  Stratospheric Column

**From Ziemke *****et al*****., in prep. (2007) **

** ****Africa**** **

**OMI Cloud-Slicing: Slope******** Mixing Ratio; Intercept ******** Stratospheric Column**

** ****Pacific**** **

**Provides indirect validation of retrieved cloud-pressures**

- Joiner, Aura Sci Team Meeting

- 10/04/07

## A-train cloud synergy

**(micro-wave)**
**AMSR**
**AMSU**

**(IR)**
**MODIS**
**AIRS**

**(UV/VIS)**
**OMI**
**POLDER**
**MODIS**

**Passive Sensors**

**Active Sensors**

**Cloudsat**

**CALIPSO**

**MLS**

- Joiner, Aura Sci Team Meeting

- 10/04/07

## Summary

- OMI DOAS retrievals use cloud pressures from O2-O2
- OMI TOMS currently uses cloud climatology from thermal IR (e.g. can have significant errors over bright clouds); next version (and TOMS reprocessing) will use OMI-derived climatology
- Can estimate radiative cloud fraction from UV/VIS reflectances or radiance ratio, cloud pressures from Raman scattering, O2-O2, or O2 absorption.
- Radiative cloud pressure is distinct from (IR) cloud-top; Significant photon penetration inside clouds
- Can use these concepts e.g. to retrieve in-cloud mixing ratios
- A-train provides unique opportunity to combine cloud information to derive information about cloud vertical extent from different passive sensors and validate with active sensors.

- Joiner, Aura Sci Team Meeting

- 10/04/07