# Presentation: 467451

## A Comparison of Satellite-Based Cloud Observations to GLOBE Cloud Observations using the MODIS Cloud Product

**Matt Rogers and Graeme Stephens**
**Colorado State University**
**28 July 2004**

## Introduction

- Purpose: Intercomparison of surface- and satellite-based observations of cloud type.
- Satellite-derived cloud type is determined from MODIS observations of cloud-top pressure and cloud optical depth using the ISSCP cloud classification matrix (Rossow and Schiffer, 1991.)
- Surface-observed cloud type is from the GLOBE dataset during the fall of 2003.
- Co-located (in time and space) observations between the two datasets are compared.

## The GLOBE Atmosphere Protocol

- GLOBE student observations made daily within one hour of solar noon
- Cloud types categorized by height (low, middle, and high cloud)
- Broken into cumuliform and stratiform subcategories, identified from cloud chart
- Similar to other surface-based  observations of cloud-type

- Allowed GLOBE observations: cirrus, cirrostratus, cirrocumulus, altostratus, altocumulus, stratus, stratocumulus, cumulus, nimbostratus, cumulonimbus.
- Also allowed – clear sky and sky obscured (haze, etc.)
- Identification from GLOBE cloud chart
- GLOBE also estimates cloud areal coverage

## GLOBE Observations

- Representative observation from the GLOBE dataset
- Contains Julian date (no time), latitude and longitude, school ID, and observation

## EOS/PM and MODIS

- EOS/PM (renamed ‘Aqua’) launched 4 May 2002
- Polar-orbiting, sun-synchronous with an equator-crossing ascending node at 1:30 pm local solar time
- MODIS instrument (Moderate Resolution Imaging Spectroradiometer) onboard Aqua satellite provides 36 spectral bands between 620 nm and 14.385 m
- Used to generate the MODIS Level-2 Cloud Product dataset containing observations of cloud-top pressure and cloud optical depth
- Global coverage between 1-2 days

## ISCCP Cloud Classification

- International Satellite Cloud Climatology Project (ISCCP) designed to use satellite observations in infrared and visible produce a global cloud dataset
- Cloud classification scheme based on 8 years of satellite observations – uses radiative transfer model with cloud detection algorithm to determine likely cloud type
- Computations and observations determine classification scheme shown at right – cloud type may be inferred from location of a given pressure/optical depth pair

## MODIS Observation of Cloud Type

## Co-location of observations

- First, find GLOBE and MODIS observations on the same date/time
- MODIS observations tend to be around 1:30-2:00pm LST, GLOBE observations between 11:00am-1:00pm LST
- Three-hour window sufficiently close except for rapidly evolving weather systems
- Generate 55x42km box around school’s latitude/longitude, fill with satellite pixels
- Reject incomplete boxes
- Determine MODIS cloud type from satellite box using previous method
- Compare to GLOBE observation of cloud type

- Schools in the survey
-   o Superior Elementary School, Superior, CO
-   o GLOBE Charter School, Colorado Springs, CO
-   o Colorado State University, Fort Collins, CO
-   o Goddard Middle School, Littleton, CO
-   o Holmes Middle School, Colorado Springs, CO
-   o Olander Elementary School, Fort Collins, CO
-   o Eads High School, Eads, CO
-   o Gilpin County School RE-1, Black Hawk, CO
-   o Alexander Dawson School, Lafayette, CO

## Data – September 2003

| Julian Date | MODIS 
observation | GLOBE observation | Agreement | Possible cause of discrepency |
| --- | --- | --- | --- | --- |
| 2003 245 | AlCu/AlSt | Ci, Cu | Partial | Height estimation error |
| 2003 246 | Ns | Ns, St | Yes | None |
| 2003 247 | AlCu/AlSt | Cu | Partial | Height estimation error |
| 2003 248 | Ci | CiSt, Cu | Partial | Cirrus misidentification |
| 2003 252 | CiCu/CiSt | Sc | No | Unknown |
| 2003 252 | Ci | Cu | No | Unknown |
| 2003 252 | CiCu/CiSt | No Clouds | Partial | Subvisual cirrus  |
| 2003 254 | St | Cu, Sc, Cb | Partial | St/Sc misidentification |
| 2003 259 | Cu/Sc | Ci, Cu | Yes | None |
| 2003 259 | CiCu/CiSt | Cu, Cb | Partial | Height estimation error (Cu) |
| 2003 259 | CiCu/CiSt | AlCu, Cb | Partial | Height estimation error (AlCu) |
| 2003 259 | Ci | Ci, Cu | Yes | None |
| 2003 259 | Cu/Sc | Cu | Yes | None |
| 2003 261 | Cu/Sc | CiCu, CiSt, Cu, Sc | Yes | None |
| 2003 273 | St | Cu, St | Yes | None |
| 2003 273 | Cu/Sc | AlCu | Partial | Height estimation error |
| 2003 273 | Cu/Sc | Ci, Cu | Partial | Height estimation error |

- 17 observations from September 2003
- 6 matches, 9 partial matches, 2 mismatch

## Cloud Height Uncertainties

- Uncertainties in cloud height dominate partial-match category:
-       o  Difficulty in determining cloud base from ground *and* satellite
-       o  Certain amount of subjectivity between low-level and mid-level
-            cloud designation, although important for radiative properties

## Other Uncertainties

**Ambiguity between cloud types**
- Similarity between stratus and stratocumulus
- Stratiform clouds versus nimbostratus (was rain observed by ground?  Is ISCCP accurate with precipitating clouds based solely on optical depth?)
- Similarity between cirrus and cirrostratus
- Nimbostratus versus cumulonimbus
**Satellite-based uncertainties**
- If thin cirrus exists, satellite will return cirrus cloud type, while ground observer will see clear sky
- Case Example: 8 Sept. 2003: MODIS returned a cirriform cloud structure with optical depth < 1.3, visible only in IR satellite analysis, but clear in visible.  GLOBE school reported ‘No Clouds’
- Could be useful to identify cases of sub-visual cirrus?

## Preliminary Data – Fall 2003

**Expanded dataset covering Sept. – Dec. 2003**
**214** total GLOBE/MODIS comparison points (preliminary)
**100** matches, **77 **partial matches, **37 **mismatches
- Similar errors – generally height discrepancies, some cloud misidentification, possible sub-visual cirrus cases
- Error increases towards Christmas vacation, similar to results of Ross et. al
**Further analysis for all of 2003 forthcoming**

## Concluding Remarks

- General agreement between GLOBE observations and MODIS observations
- Particularly in type of cloud (cumuliform/stratiform)
- Height estimation errors addressed by focusing on estimation of cloud height in GLOBE protocol, lessening error in satellite retrieval
- Improvements to the Atmosphere Protocol
- Including time-stamp of ground observation
- Add training methods for determining cloud-base height
- Utility of global cloud-type dataset of great benefit to science
- Integration of observations into climate models (e.g. Ross et al.)
- Verification of satellite observations of cloud type
- Potential comparison with CloudSat observations
- Further Research
- Blend in areal cloud cover amount into analysis
- Expand area of interest to cover other regions

## Thank you for your time

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