Report No, AO85-1 November, 1989 Prepared for Minerals Management Service U.S. Department of the Interior 381 Elden Street Herndon, VA 22070-4817 Prepared Under Contract No. 14-12-0001-30396 OCD: THE OFFSHORE AND COASTAL DISPERSION MODEL VOLUME I: USER’S GUIDE Prepared by Donald C. DiCristofaro Steven R. Hanna ABSTRACT The Offshore and Coastal Dispersion (OCD) model has been developed to simulate the effect of offshore emissions from point, area, or line sources on the air quality of coastal regions. The OCD model was adapted from the EPA guideline model MPTER (EPA, 19801. Modifications were made to incorporate overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. Hourly meteorological data are needed from both overwater and overland locations. The overwater measurements include wind direction and speed, mixing height, overwater air temperature and relative humidity, and the sea surface temperature. Overland data include the standard EPA UNAMAP model requirements. Overwater and overland turbulence intensities are used by the model but are not mandatory. For overwater dispersion, the turbulence intensities are parameterized from boundary layer similarity relationships if they are not measured. Specifications of emission characteristics and receptor locations are similar to the standard EPA UNAMAP models. Hourly emission rate, exit velocity, and stack gas temperature may also be specified. Up to 250 point sources, 5 area sources, or one line source and 180 receptors may be used. Plume reflection off elevated terrain is calculated following the method proposed in the EPA TUPOS model (Turner et al., 1986). Plume inpaction on elevated terrain is calculated following procedures in the EPA RTDM (Rough Terrain Diffusion Model) (ERT, :1982). That is, if the plume is below the critical dividing streamline height (Hc), the plume impacts the terrain, and if the plume is above Hc, the plume flows up over the terrain. A revised platform downwash algorithm based on laboratory experiments is incorporated in OCD. Partial plume penetration into elevated inversions is treated using Briggs' model. A virtual source technique is used to change the rate of plume growth as the overwater plume intercepts the thermal internal boundary layer (TIBL) at the shoreline. The TIBL is assumed to be terrain following. ii The revised DCD model (Version 4) and the previous version of OCD (Version 3) are tested with measurements from four offshore tracer experiments. Considering the overall performance of the models, the OCD (Version 4) model is shown to be an improvement over the OCD (Version 3) model. iii EXECUTIVE SUMMARY The Offshore and Coastal Dispersion (OCD) model was developed to simulate plume dispersion and transport from offshore point, area, or line sources to receptors on land or water. The OCD model is an hour-by-hour steady state Gaussian model with enhancements that consider the differences between overwater and overland dispersion characteristics, the sea-land interface, and platform aerodynamic effects. Categories of overland turbulence levels have been successfully parameterized as a function of solar radiation and wind speed only. This approach can be used over land without considering surface temperature or humidity because the surface temperature responds rapidly to changes in solar radiation, and sensible heat fluxes dominate latent heat fluxes in the boundary layer. This is not the case for the boundary layer over water surfaces where diurnal temperature changes are quite small, response times long, and latent heat fluxes important. Therefore, the traditional methods of determining stability category and thus atmospheric turbulence characteristics are not applicable for overwater sources. Overwater turbulence levels are largely governed by the air-water temperature difference, overwater wind speed, and the specific humidity. If overwater turbulence levels are not measured directly, they must be estimated from boundary layer theory using bulk aerodynamics. The OCD model requires both overwater and overland meteorological data. The overwater data include the following parameters: . overwater wind direction, . overwater wind speed, . overwater mixing height, . overwater air temperature, . water surface temperature, . overwater relative humidity, . overwater wind direction shear in the vertical, . overwater vertical potential temperature gradient, . overwater turbulence intensities (y and z components), and . overland turbulence intensities (y and z components). The overland meteorological data required by the OCD model are identical to those required by the standard EPA UNAMAP model. Missing overwater turbulence intensities are parameterized using bulk aerodynamic wind and temperature profile relationships as well as the overwater stability category (defined in terms of the Monin-Obukhov length). Missing overland turbulence intensity measurements are replaced by the rural Briggs (19731 defaults. Several options available in the standard EPA UNAMAP models are included in the model: . terrain adjustments, . stack-tip downwash, . gradual plume rise, . buoyancy-induced dispersion, and . pollutant decay (monthly daytime transformation rates are user-specified). The OCD model has incorporated several other features: . Complex terrain is treated as in COMPLEX I/II (EPA, 19861, except for the consideration of partial reflection and an improved method to calculate deflection around or over terrain. Plume reflection off elevated terrain is treated using a method proposed in the EPA TUPO!S model (Turner et al., 1986). Building downwash due to platform influence on the plume is treated using a revised platform downwash algorithm based on laboratory experiments, dispersl’on-coefficients are enhanced and final plume rise is reduced as a result of downwash effects. The effective mixing depth at the shoreline includes mixing that is effectively unlimited if the plume is in a stable layer. The default turbulence intensity is inversely proportional to the wind speed for all stabilities. The Thermal Internal Boundary Layer (TIBL) is terrain following. Point, area, or line sources may be modeled. Partial penetration of elevated inversions is accounted for. Stacks can be oriented at any angle relative to the vertical to V -accommodate a variety of oil platform sources. The land/sea interface need not be a straight line; a rectangular grid system is used to accommodate any complex coastline. A virtual source technique is used to change the rate of plume growth as the overwater plume intercepts the overland internal boundary layer. Continuous shoreline fumigation (stable overwater and unstable overland conditions1 is parameterized using the Turner method where complete vertical mixing through the TIBL occurs as soon as the plume intercepts the TIBL. Hourly source emission rate, exit velocity, and stack gas temperature can be specified. The OCD model can provide estimates of pollutant concentrations at a maximum of 180 receptors from a maximum of 250 point sources, 5 area sources, or one line source. Summary tables generated by OCD may be used to determine the peak modeled concentrations. Alternatively, modeled concentrations can be written to an output tape or disk file for subsequent postprocessing by the ANALYSIS program. The postprocessor can provide several statistical summaries: . the top N concentrations for each receptor for averaging periods up to 24 hours in length; . cumulative frequency distributions of concentrations for each receptor; and ,--y . identification of periods for which threshold concentrations are exceeded at any receptor. - In addition, the ANALYSIS post$rocessor can create new concentration files which can be used as input to the processes described above: . a file of running averages (up to 24 hours in length), and . a file that is the sum of concentrations from up to five separate files. (Concentrations from each file summed are first multiplied by a user-specified scale factor.) ,.-x7 A performance evaluation of the OCD (Version 4) model along with the OCD (Version 3) model was conducted with measurements from four different -., Vi offshore tracer experiments. The four experiments included 17 hours of data from the MMS sporikored experiment at Ventura, CA, 31 hours from the MM!S experiment at Pismo Beach, CA, 26 hours of data collected at Cameron, LA in an experiment sponsored by the American Petroleum Institute (API), and 36 hours from the API experiment at Carpinteria, CA. The uncertainties associated with the OCD model (Versions 3 and 4) are examined using a blocked bootstrap or jackknife resampling method to estimate whether there are significant differences in the fractional bias (FBI, normalized mean square error 0#4!5E1, and correlation (RI. 95% confidence limits are calculated using bootstrap resampling for FB and R for each model, and the difference in FB, NMSE, and R between models. An arbitrary scoring scheme is used to combine all the results into a final "score." Considering the overall performance of the models, the OCD (Version 4) model is shown to ,-,- be an improvement over the OCD (Version 3) model. Vii _ OCD: The Offshore and Coastal Dispersion Model Volume I: User's Guide TABLE OF CONTENTS Page ABSTRACT ............................ ii EXECUTIVESUMMARY ........................ iv LISTOFFIGURES.........................xi i LISTOFTABLES.. ..................... ..x v LIST OF SYMBOLS. ........................ xviii ACKNOWLEDGEMENTS........................xxi v