Application and Evaluation of MODIS LAI, FPAR, and Albedo Products in the WRF/CMAQ System

Abstract

MODIS vegetation and albedo products provide a more realistic representation of surface conditions for input to the WRF/CMAQ modeling system. However, the initial evaluation of ingesting MODIS data into the system showed mixed results, with increased bias and error for 2-m temperature and reduced bias and error for 2-m mixing ratio. Recently, the WRF/CMAQ land surface and boundary layer processes have been updated. In this study, MODIS vegetation and albedo data are input to the updated WRF/CMAQ meteorology and air quality simulations for 2006 over a North American (NA) 12-km domain. The evaluation of the simulation results shows that the updated WRF/CMAQ system improves 2-m temperature estimates over the pre-update base modeling system estimates. The MODIS vegetation input produces a realistic spring green-up that progresses through time from the south to north. Overall, MODIS input reduces 2-m mixing ratio bias during the growing season. The NA west shows larger positive O₃ bias during the growing season because of reduced gas phase deposition resulting from lower O₃ deposition velocities driven by reduced vegetation cover. The O₃ bias increase associated with the realistic vegetation representation indicates that further improvement may be needed in the WRF/CMAQ system.

Question and Answer

Questioner: D.G. Steyn.

Question: In many of your maps, you present variables (O3 & MLD) with strong diurnal variation at a fixed time (2000 UTC). Surely, it would be better to present these quantities at the same local time.

Answer: For the large scale modeling over the conterminous US domain which crosses four time zones, it is often questionable to select just one UTC time across the domain for displaying surface fluxes and deposition. Often, surface fluxes show big changes around peak temperature time which varies seasonally from around 2–3 pm LT in mid-winter to about 4–5 pm LT in mid-summer. Yes, it could be better to show the quantities at the same local summer time. We evaluated O₃ and surface fluxes changes over different hours. It appears that O₃ shows big difference around 20Z which corresponds to 1 and 2 pm for the Pacific and Mountain Daylight summer time zones. Thus, we selected 20Z to display the whole domain flux and O₃ differences because most vegetation change from the use of MODIS input is in the US western drylands.