Mass Conservative Coupling of MM5 with CMAQ

1. Mass Conservative Coupling of MM5 with CMAQ Yongtao Hu, M. Talat Odman and Armistead (Ted) Russell October 19th, 2004 Georgia Institute of Technology

2. Mass conservation errors in CMAQ due to inconsistency of meteorological data from MM5 • AQMs are expected to maintain a uniform mass mixing ratio field for an inert tracer after transport with the winds produced by MMs. This expectation could only be realized if the two models used the same discretization, however: • They may have different grid structure, • Finite difference forms are usually different, • MMs’ outputs are stored less frequently than AQMs time step… • This inconsistency will perturb uniform mass mixing fields, and the perturbation may grow and generate instabilities. • Ideally, there should be no mass conservation error in AQMs that establish source receptor relationships for the design of emission control strategies . Georgia Institute of Technology

3. Solutions to mass conservation errors in CMAQ • AQMs usually do renormalizing with the concentration of the species based on the perturbation of a uniform field. The mass conservation errors introduced by this method was found to be very large. • Lu et al. (1997) prefer adjusting the density field to satisfy the discrete continuity equation, while Odman and Russell (2000) adjust either the vertical wind component or the vertical flux. • The inverse donor cell method described in Odman and Russell (2000) was applied in CMAQ to satisfy the discrete continuity equation by adjusting the vertical winds. Georgia Institute of Technology

4. Apply horizontal winds to the air density field, , to obtain an intermediate field , then calculate a new value for vertical velocity that would yield the desired , which is the density after the vertical advection is applied to . • The vertical velocities computed from Equation (2) must be used in Equation (1) to advect all other pollutant species . Adjusting vertical winds: Inverse donor cell method (1) (2) Georgia Institute of Technology

5. Testing with CMAQ • Runs conducted with: • CMAQ release version 4.3 • Bug-fixed CMAQ from U of Houston (original code of CMAQ inadvertently assumes uniform grid, but vertical layer spacing is non-uniform) • Mass conservative CMAQ (adjusting the vertical winds) • Tracer 1,2,3,4 was released (24kg each) at 4 locations in the TN valley area. • The total mass within the entire domain of each tracer was examined at each timestep for all three different runs to check the mass conservation. Georgia Institute of Technology

6. Tracer2 Tracer4 Tracer1 Tracer3 Tracer release locations on Fall line Air Quality Study 12-km grid Georgia Institute of Technology

7. CMAQ release 4.3 vs. mass-conservative CMAQ : Domain-wide total mass of tracers during the simulation time Tracer1 Tracer2 Tracer3 Tracer4 Georgia Institute of Technology

8. Bug-fixed CMAQ vs. mass-conservative CMAQ : Domain-wide total mass of tracers during the simulation time Tracer1 Tracer2 Tracer3 Tracer4 Georgia Institute of Technology

9. Surface ozone difference at the peak hour between the runs masscons - orginal masscons – bug-fixed Georgia Institute of Technology

10. Conclusions • Any CMAQ run prior to correcting the coding error in vertical advection may be subject to severe mass conservation errors over complex terrain. • The correction reduces the mass conservation errors but it does not completely eliminate them. • Adjusting vertical winds proved to be an effective method to conserve mass in CMAQ. We recommend the use of vertical wind adjustment in future versions of CMAQ. Georgia Institute of Technology