# Presentation: 063884

## Ice cloud retrievals using the 1.38-μm water vapor band

- Kerry Meyer, Steve Platnick
- NASA Goddard Space Flight Center
- Ping Yang
- Texas A&M University
- Bo-Cai Gao
- Naval Research Laboratory

## Terra MODIS: 10-21-2007

- RGB (0.47-, 1.64-, 2.11-μm))

- MOD06 Cloud Mask

- MOD06 Ice Cloud Optical Thickness

- MOD06 Cirrus Reflectance Flag

## Thin Cirrus

- MOD06 cloud retrievals “miss” thin cirrus clouds (failed retrievals, no cloud mask).
- Sensitivity to thin cirrus at 1.38-μm (MODIS band 26).
- Strong water vapor absorption band.
- Due to high altitudes, 1.38-μm radiation not reflected by ice clouds is presumably completely absorbed in the atmosphere below  “screens” the surface.
- Caveats:
- Up to 10% of atmospheric water vapor is located above ice clouds  non-negligible attenuation at 1.38-μm.
- Arid atmospheres (polar regions, deserts, high-altitude locations, etc.) have insufficient below-cloud absorption at 1.38-μm  surface and low-cloud contamination.
- Cloud microphysics affects reflectance!
- Nevertheless, 1.38-μm should be a useful compliment to the operational retrievals for thin cirrus (optical thickness).

## Optical Thickness Retrievals

- Empirical approaches to estimating in/above cloud absorption:
- Cirrus reflectance (Gao et al., 2003).
- MOD06 operational product.
- Use 1.38-μm (band 26) reflectance to remove the contribution of the surface and low clouds from 0.66-μm reflectance.
- Can convert cirrus reflectance to cloud optical thickness (Meyer et al., 2007).
- Can also provide an estimate of above-cloud two-way transmittance at 1.38-μm.
- Use 0.86- and 1.24-μm as proxies for 0.66-μm.
- Pixel-level approaches:
- Calculate above-cloud 1.38-μm two-way transmittance on a pixel-by-pixel basis.
- Requires cloud top pressure (from MOD06) and atmospheric profile (from NCEP analyses).
- Use correlated-k approach to estimate two-way transmittance.
- Use MODTRAN-generated transmittance look-up library.

## r1.38 = rc,0.66

- r1.38 = rc,0.66

- Aqua MODIS: 4-22-2004

**Notes:**
Gamma (slope) is derived for envelope of the leading edge of pixels (typically corresponding to the highest clouds in the sub-granule).  This has consequences later on - introduces a bias in the retrieved optical thickness.

## MOD06 Ice Cloud Optical Thickness

- Ice Cloud Optical Thickness
- Empirical: 0.66-μm Cirrus Reflectance

- Ice Cloud Optical Thickness
- Empirical: Corrected 1.38-μm Reflectance

## Slide 7

## Assessment

- Forward radiative transfer calculations with DISORT.
- ECMWF “cloudy” atmospheres (Gala Wind) - tropical ocean case.
- Rayleigh scattering scheme (Bodhaine et al., 1999).
- Ice cloud bulk scattering properties (MODIS Collection 5).
- Three cloudy conditions: Single layer ice clouds, single layer water clouds, and multilayer (ice over water) clouds.
- Populate test granules.
- Two retrieval tests:
- Current method - assume 30 μm Re.
- Known Re - use exact Re to select look-up table.
- Compare retrieved values to actual values input into DISORT.

## Assessment

- Scenario 1:
- Clear sky (20%).
- Single layer ice cloud (80%).
- CTH between 12 and 15 km, optical thickness between 0.1 and 20, effective radius between 10 and 50 μm.
- Scenario 2:
- Clear sky (20%).
- Single layer ice cloud (40%).
- CTH between 12 and 15 km, optical thickness between 0.1 and 20, effective radius between 10 and 50 μm.
- Single layer liquid (low-level) cloud (30%).
- CTH between 3 and 6 km, optical thickness between 2 and 20, effective radius between 4 and 20 μm.
- Multilayer cloud (ice over liquid) (10%).
- Sun/satellite configuration:
- Solar zenith angles between 20° and 25°, sensor zenith angles between 0° and 25°, relative azimuth equal to 155°.

## Modeled Scenario 2

- Aqua MODIS: 4-22-2004

## Look-up Tables for Re= 30 μm

- Look-up Tables for Re= 30 μm

## Look-up Tables with Exact Re

- Look-up Tables with Exact Re

## What Next?

- Evaluate confidence in “real world” retrievals.
- Continue forward modeling.
- Determine 1.38-μm contamination.
- Beta scheme for a contamination “flag.”
- Include an uncertainty analysis.
- Determine the situations and band combinations that give the “best” result.
- Incorporate thin cirrus retrieval into MOD06 Collection 6.

## MOD06 Ice Cloud Bulk Scattering Models

## Slide 15

**Notes:**
Also mention 0.94 (Multilayer detection) and the 1.88 (MAS and AVIRIS)

## MOD06 Cloud Mask

- MOD06 Cirrus Reflectance Flag