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L2 Joint Atmosphere Product
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Modification History
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Product Description

designed so a full day of files fits on a CD The post-launch MODIS Atmosphere Level 2 Joint Product contains a spectrum of key parameters gleaned from the complete set of standard at-launch Level 2 products: Aerosol, Water Vapor, Cloud, Profile, and Cloud Mask. The new Joint Atmosphere product was designed to be small enough to minimize data transfer and storage requirements, yet robust enough to be useful to a significant number of MODIS data users. Scientific data sets (SDS's) contained within the Joint Atmosphere product cover a full set of high-interest parameters produced by the MODIS Atmosphere group, and are stored at 5-km and 10-km (at nadir) spatial resolutions. There are two MODIS L2 Joint Atmosphere data product files: MODATML2, containing data collected from the Terra platform; and MYDATML2, containing data collected from the Aqua platform. Both of these products began production on October 14th 2003. The first available data days are julian days 285 (10/12/2003) for Aqua and 286 (10/13/2003) for Terra.

Research and Application

A few limitations were introduced into the Joint Product to reduce the file size. First, some parameters that were stored at 1-km resolution in their original (source) Level 2 product file were subsampled to 5-km in the Joint product. These include the cloud mask, cloud optical thickness, cloud effective radius, cloud quality assurance (includes cloud phase information), cirrus reflectance, and the precipitable water (near IR) parameters. Second, geolocation arrays that were stored as 4-byte floating-point real numbers in the original (source) product file were compressed to 2-byte scaled integers. This sacrifices 0.001 degrees of geolocation accuracy. Finally, only a limited set of QA (quality assurance) arrays were included; and only for parameters where it's absolutely crucial to use and/or interpret the data. These drawbacks were considered acceptable in the context of the overall goal of this project: To create a small and user-friendly Level 2 HDF file of the "greatest hits" of MODIS Atmosphere such that a full day of data files could be stored on a CD (< 700 MB).

Another complexity introduced by this product relates to the geolocation arrays. The Aerosol (04_L2) parameters are computed and stored at a different resolution (10-km) than the rest of the parameters in the Joint Atmosphere product file (5-km). The 5-km resolution geolocation is copied from center (3, 3) 1-km cell in the 5x5 1-km region. The 10-km resolution geolocation is computed by averaging the four central 1-km cells [(5,5), (5,6), (6,5), (6,6)] in the 10x10 1-km region. Because of this, two sets of geolocation (one at 5-km and one at 10-km) were included in the product. The first (default) geolocation set is for the 5-km resolution parameters; these have the Scientific Data Set (SDS) names: Latitude and Longitude. The second set is for the 10-km resolution parameters; these have the SDS names: Latitude_10km and Longitude_10km. Users should ensure they are using the correct geolocation array when working with various parameters in the file.

A positive feature of the Joint Atmosphere product is enhanced program executable (PGE) tracking capability. Tracking PGE versions used to create various parameters is important to the vast majority of scientific research efforts. Since the Joint Atmosphere product collects Scientific Data Sets (SDS's) from a number of L2 products each created with a unique PGE version, this task is exacerbated. To help solve this problem, a new local attribute "source_info" is added to each SDS. This local attribute contains: 1.) the file name of the source HDF product, 2.) the PGE ID number and PGE version used to create the source file, and 3.) the source SDS name. PGE version information provided can be interpreted using PGE version history pages (found by clicking on the PGE History links on the the MODIS Atmosphere product processing and availability calendar). This new feature allows scientists to more easily monitor the evolution of various parameters by tracking bug fixes and enhancements implemented in the algorithm software.

Additional Information

Coverage: Global

Spatial Characteristics: 5 and 10-km (parameter dependent)

Temporal Characteristics: Aerosol, cloud phase, and cloud optical parameters are daytime only. Water vapor, cloud top, cloud mask, cloud fraction, cirrus reflectance, and profile parameters are available both day and night.

Key Science Applications: Aerosol climatology, biomass-burning aerosols, cloud radiative properties, hydrological cycle climatology, effect on aerosol and clouds, atmospheric correction, characterization of the atmosphere, cloud parameterization, climate modeling & monitoring, precipitable water, cloud determination and screening.

Key Geophysical Parameters: Atmospheric aerosol optical depth, aerosol size distribution over oceans, cloud particle phase, cloud particle size, cloud optical thickness at 0.66 Ám, cirrus reflectance at 0.66 Ám, cloud top temperature, cloud top pressure, clear-sky total-column precipitable water derived from both infrared and near-infrared algorithms, and cloud fraction.

Processing Level: 2

Product Type: Post-launch

Maximum File Size: ~2.95 MB daytime, ~1.5 MB nighttime

File Frequency: 288/day

Primary Data Format: HDF-EOS

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