How Do The Variations In Orbital Parameters And Sampling Strategies Of The NASA Terra And Aqua Satellites Impact The Accuracy Of MODIS-derived Cloud Fraction Estimates Over Mid-latitude Regions, Particularly During The Spring-fall Transitional Seasons When Cloud Cover Is Most Heterogeneous?

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The orbital parameters and sampling strategies of NASA's Terra and Aqua satellites, both equipped with MODIS instruments, significantly influence the accuracy of cloud fraction estimates, particularly in mid-latitude regions during spring and fall transitional seasons. Here's a structured summary of the key factors and their impacts:

  1. Orbital Timing and Diurnal Cycle:

    • Terra and Aqua have different equator crossing times (morning for Terra, early afternoon for Aqua), capturing cloud conditions at different stages of the diurnal cycle. This timing affects the types of clouds observed, as morning may have more fog while afternoon may have cumulus or thunderstorms.
    • Each satellite provides a single daily snapshot, potentially missing peak or minimum cloud cover, especially in heterogeneous conditions.

  2. Swath Width and Coverage:

    • MODIS's wide swath allows extensive coverage, but gaps can occur, particularly over oceans. In mid-latitudes, coverage is better, though cloud heterogeneity remains a challenge.
    • The combination of both satellites offers two daily observations, enhancing data capture but still potentially missing variability due to rapid cloud changes.

  3. Resolution and Viewing Geometry:

    • Coarser resolution (1km) may miss small-scale cloud features, misrepresenting fractional cloud cover in variable regions.
    • Viewing geometry and sun angle variations in transitional seasons can affect cloud detection accuracy, potentially misclassifying surfaces.

  4. Algorithm Limitations:

    • MODIS algorithms may have biases tied to observation time, with visible channels performing differently than infrared under varying conditions.

  5. Combined Satellite Data:

    • While combining Terra and Aqua data provides a more comprehensive view, inherent differences in cloud conditions observed at different times may persist, challenging consistent estimates.

Conclusion: The orbital and sampling strategies provide valuable but incomplete snapshots of cloud conditions. During transitional seasons, the high cloud variability in mid-latitudes may lead to inaccuracies in MODIS-derived cloud fraction estimates, despite the advantages of dual satellite observations.