# Presentation: 033218

## What is a Cloud(According to MODIS)

- Steve Ackerman
- Rich Frey
- CIMSS/UW-Madison

## What is a cloud?

- “I know one when I see one.”

## What is a cloud?

- “I know one when I see one.”

## GCMs make extensive comparison with satellite  derived cloud amount. Total cloud amount from different satellite algorithms can vary significantly even among accepted standards, as shown below in a comparison of annual zonal mean cloud fraction from CLAVR, ISCCP and UW-HIRS.  Global distributions demonstrate expected patterns but can differ in magnitude by more then 10%.

**GCMs make extensive comparison with satellite  derived cloud amount. Total cloud amount from different satellite algorithms can vary significantly even among accepted standards, as shown below in a comparison of annual zonal mean cloud fraction from CLAVR, ISCCP and UW-HIRS.  Global distributions demonstrate expected patterns but can differ in magnitude by more then 10%.**

## What is a cloud?

**The answer to that question is determined by the application.  What is considered a cloud in some applications may be defined as clear in other applications.  **
**Detection of clouds is also a function of instrument capability and algorithm design.  **
**Cloud detection is a function of contrast between the target (e.g. cloud) and the background. Contrast can be:**
**Spatial: ****	**** Large fov are generally more uniform lowering contrast**
**Temporal: ****	****Clouds can be detected in a sequence of images if the clouds are ****	****	****moving**
**Spectral: ****	****Spectral contrast is determined by the radiative properties of the ****	****	****cloud and surface. **

## What is a cloud?

- “I know one when I see one.”

- Preparing for CALIPSO and MODIS

- Water, Weather

## The total cloud fraction is a function of cloud optical depth, and the cloud fractions when considering the plus and minus sigma values of optical depth from June 2004. Each optical depth time profile has an associated error bar due to the molecular return and the density profile. 

**The ****total cloud fraction is a function of cloud optical depth****, and the cloud fractions when considering the plus and minus sigma values of optical depth from June 2004. Each optical depth time profile has an associated error bar due to the molecular return and the density profile. **

## GLI and MODIS observations were compared to the HSRL site over the University of Wisconsin-Madison.   Comparison indicates passive approach flags a cloudy region as Uncertain Clear, the optical depth is less then approximately 0.3.  

**GLI and MODIS observations were compared to the HSRL site over the University of Wisconsin-Madison.   Comparison indicates passive approach flags a cloudy region as Uncertain Clear, the ****optical depth is less then approximately 0.3****.****  **

## Field of View

**The percent of total observations of clear (blue), high cloud (green) and total cloud (red) as a function of MODIS fov size. ****Smaller FOVs are more likely to be all clear or all cloud cover.**

## Additional spectral observations can improve cloud detection capability.

**Additional spectral observations can improve cloud detection capability.**

**Left: Cloud fraction increase due to addition cloud detected by the MODIS 1.38 micron channel.**

## Top: Zonal mean frequencies of cloudy conditions for October 16,2003, daytime ocean scenes as a function of three cloud detection tests and the combination of all 16 tests from MODIS. Note, in this case a single spectral test does very well.

**Top: Zonal mean frequencies of cloudy conditions for October 16,2003, daytime ocean scenes as a function of three cloud detection tests and the combination of all 16 tests from MODIS. Note, in this case a single spectral test does very well.**

## Zonal mean frequencies of cloudy conditions for October 16,2003, daytime land scenes as a function of three cloud detection tests and the combination of all 16 tests from MODIS.

**Zonal mean frequencies of cloudy conditions for October 16,2003, daytime land scenes as a function of three cloud detection tests and the combination of all 16 tests from MODIS.**

## Thresholds

_**Thresholds**_

**0.86 reflectance (x-axis) versus the percentage of pixels less then that value (e.g. cloud fraction if this reflectance was a threshold ) for ocean scenes solar zenith angles and viewing angle s between 0 - 10 degrees. For different viewing geometries, the cloud detection threshold varies. **
**A small change in the threshold can result in a large change in cloud amount.****  **

## Sensitivity to Input Reflectance Biases and Reflectance Thresholds

**Daytime Terra MODIS Data from April 1, 2003**
**60N to 60S, No Snow/Ice**

|  | Nadir Cloud Amount (~± 0.9 vza) |   Cloud Amount from
     All Pixels |
| --- | --- | --- |
| Collection 5 Cloud Mask | Water                68.9%
Land                  51.1% | Water                72.7%
Land                  54.1% |
| Increase All B1, B2 Reflectances by 5% of Original | Water   69.4% (+0.5%)
Land     51.4% (+0.3%)  | Water   73.3% (+0.6%)  
Land     54.6% (+0.5%) |
| Decrease All B1, B2 Reflectances by 5% of Original | Water    68.4% (-0.5%)
Land      50.7% (-0.4%) | Water    72.2% (-0.5%)
Land      53.6% (-0.5%) |
| Increase VIS/NIR Reflectance Test Thresholds by 1% | Water    67.4% (-1.5%)
Land      50.8% (-0.3%) | Water    70.7% (-2.0%)
Land      53.6% (-0.5%) |
| Decrease VIS/NIR Reflectance Test Thresholds by 1% | Water  72.0% (+3.1%)
Land    51.4% (+0.3%) | Water   75.5% (+2.8%)
Land     54.7% (+0.6%) |

## Summary

**Cloud detection optical depth threshold limit appears to be approximately 0.3 **
**Cloud coverage varies with the spatial and spectral resolution of the instrument****. **
**Cloud detection thresholds vary as a function of viewing geometry, scene illumination and thermal structure of the scene. **
**The dependence of cloud detection on these parameters and the need to monitor with changing instruments and satellites, will likely make it difficult to compare cloud amounts from different approaches and achieve the 1% accuracy needed for long-term monitoring of cloud amount.**** **