3.2 WRF-Chem Model Development

Mary Barth (ACOM/MMM), Alma Hodzic (ACOM), Young-Hee Ryu (ACOM), Gabriele Pfister (ACOM) and Stacy Walters (ACOM)

ACOM scientists have continued to add improvements to the WRF-Chem model. Two recent developments are an upgrade to the photolysis rate calculations and the addition of trajectories. The photolysis rate calculations now include the data and algorithms used in the most recent Troposphere Ultraviolet-Visible radiation model (TUV v5.3) that was released in summer 2016. The upgraded module includes cloud and aerosol feedbacks on the photolysis rates as well as the most recent data for quantum yields and cross-sections. The new TUV code in WRF-Chem predicts photolysis rates that are similar to observations (Figure 1) and are generally improved compared with predictions by other methods.

Sample template image
Figure 1. Comparison of photolysis rates as observed during the SEAC4RS field campaign on 14 August 2013 (black line), and predicted by the WRF-Chem model using different parameterizations to predict the photolysis rates. Purple line is the previous version of TUV used in WRF-Chem, blue line is the Fast-J parameterization (Wild et al., 2000), green line is fast TUV (Tie et al., 2003), and red line is the new TUV version just implemented. The two photolysis rates are O3 dissociating to O1D + O2 (top panel) and NO2 dissociating to NO + O (bottom panel).

Trajectories have been added to WRF-Chem to monitor meteorological and chemical parameters characterizing an air parcel as it follows the resolved-scale air motions predicted by WRF. With the output of the air parcel characteristics, WRF-Chem researchers can do additional analysis of air parcels as they are transported from urban centers or through thunderstorms. An example is shown below for air parcels that are ingested into a thunderstorm. For this case, 864 trajectories were tracked. The initial locations of these trajectories were place in the inflow region of the storm (panel a of Figure 2) at altitudes ranging from 0.5 to 3.0 km and over a 4-minute period. Analysis of the trajectory output found 251 trajectories were ingested into the updraft and lofted above 7 km altitude. These trajectories that reached the upper troposphere were then analyzed to produce a histogram of the time the air parcel was in contact with cloud water (panel b), showing that most air parcels spend 5-15 minutes in contact with cloud water.

Sample template image
Figure 2. a) WRF-Chem simulated maximum radar reflectivity (gray shading, dBZ) and 10 trajectories that reach altitudes > 7 km. The trajectories are colored by altitude ranging from < 1 km (black) to over 12 km (pink). b) Frequency distribution of time in contact with cloud water for 251 trajectories that reached altitudes > 7 km.

NCAR/ACOM WRF-Chem activities are found at https://www2.acom.ucar.edu/wrf-chem.