Wednesday, 25 January 2012: 10:00 AM
Comparison of High Resolution Cloud Climatologies From MODIS Imagery and Mesoscale Models
Room 257 (New Orleans Convention Center )
To date there have been few attempts to compare the climatology of mesoscale model cloudiness forecasts against high resolution cloudiness data. This study examines the accuracy of the North American Mesoscale Model (NAM) total cloud fraction product in order to evaluate its reliability. To examine the accuracy of the NAM total cloud fraction product, a validation technique was needed. Two products were used, both derived from high-resolution visible imagery from a Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. A simple threshold product, developed at the NSSL, uses various tolerance levels for pixel brightness to determine cloudiness while a more complex, NASA-developed cloud mask product employs a series of visible and infrared threshold and consistency tests to determine if a given pixel is cloudy or not. The months of July, August, and September of 2010 were selected for analysis due to the limited amount of complete, archived NAM data and 15 hour, 18 hour, and 21 hour NAM forecast times were chosen to compare with corresponding MODIS satellite cloud frequency products over the NAM domain. Since the NAM forecasts 84 hours into the future we were able to compare NAM cloud forecasts on the initial day, one day, and two day forecasts. Three-month averages of cloudiness from the NSSL threshold cloudiness product, NASA cloud mask product, and NAM total cloud fraction product were produced and compared against each other. Our methodology was to analyze the difference maps between these cloudiness estimates to identify systematic differences between the NAM forecast cloudiness and the observed average cloudiness from the two MODIS-derived products. The analysis revealed that the NAM total cloud fraction product has issues resolving certain small-scale topography, sea breezes, and various other meteorological features. Comparison of initial day to one day and two day NAM forecasts showed that the model cloudiness varies significantly over time. The composition of the three-month cloudiness map from the NASA cloud mask product revealed that agricultural and urban areas are frequently incorrectly diagnosed to be cloudy. From this, we can conclude that the NAM total cloud fraction product must be improved to better handle certain meteorological features and that the NASA cloud mask product has systematic biases over certain land features.