Simulation of Ship Emission Transport and Dispersion in Coastal Southern California Using the COAMPSŪ Model

Introduction

This presentation documents the results of a trial study to assess the potential for using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPSŪ) model to simulate ship emission transport and dispersion. Commercial ship location data were acquired for the Southern California coastal region for the month of October 2013 and were used to create emission source locations that varied in time and space. COAMPS was run for the entire month of October 2013 with its passive tracer module activated to transport and disperse the emissions released from the source points. The results show that COAMPS is capable of simulating the transport and dispersion of ship emissions with limitations.

Methods

The COAMPS model is a mesoscale grid point simulation model of both the atmosphere and the ocean developed by the Naval Research Laboratory. The model also includes an embedded aerosol module that if activated can simulate the transport, dispersion and transformation of aerosols. This module can be configured to run in a passive tracer mode to simulate the transport and dispersion of an inert gas. For this study COAMPS was configured with 4 nested domains of 40 vertical levels centered at 33.25° N latitude 119.5° W longitude and the following horizontal dimensions and grid resolutions: 64x64 at 54 km, 73x73 at 18 km, 100x100 at 6 km and 181x181 at 2 km.

The model was run for 6 hour sequential forecast periods, with each forecast period being initialized using the previous run's 6 hour forecast as the first guess then blending in observations using the Naval Research Laboratory Variational Data Assimilation System (NAVDAS). The boundary conditions were provided by analyses from the Navy Global Environmental Model (NAVGEM).

Marine vessel positions collected by the Automatic Identification System (AIS) message data service were acquired for the region bounded by 31-36° N latitude and 116-124° W longitude for the month of October 2013. The ships' position data was not reported at regular time intervals, but ranged from intervals of a second or less to hours or in some cases even days. The positions for each ship were interpolated to 5 min intervals, graphed, inspected and edited to remove obvious errors. Ship positions reported while in port were also deleted.

The COAMPS aerosol module has a hardcoded limit of 500 emission source points for a given forecast run. The 5 minute interval ship position data far exceeded this limit for most of the 6 hour forecast periods. In order to stay within this limit the ship position data was first thinned by removing ship tracks that were furthest from the coast. The positions were then binned into the discrete model grid point locations. Although this modification resulted in far fewer source points, many forecast periods still exceeded the 500 point limit. So the binning algorithm was modified to stop producing source points once the 500 point limit was met. This modification caused some of the ship's tracks to be abruptly terminated and for others to not be used at all. Each emitting grid point was assigned an emission value of 1 kg/s for every ship positioned within its boundaries for the 5 min interval.

There are three other limitations in using the COAMPS model for simulating ship emissions. The first is that ships are point sources with emitting stacks with a radius on the order of 1-10 m. The model is only able to resolve the emission sources to the resolution of a model grid volume, which in this case was 2 km in the horizontal and 10 m in the vertical. The second limitation is that source points were only created for the innermost domain, so that when a ship left the domain the emissions from that ship ceased to be simulated. In reality the emissions would have existed and in many cases been carried back into the innermost domain by the wind. This limitation could have been overcome by creating source points in the outer domains, but this would have created more source points beyond 500 point limit. The third limitation is that emissions transported out of the innermost domain were not carried into the next outer domain where they could have been carried by the wind back into the inner domain at a later time. This limitation could have been overcome as COAMPS has the option for transporting emissions between grids, but it was decided for this trial study to not enable that capability.

Results

[Plots and animations of ship positions, emission concentrations and winds at 10 m height as well as emission concentrations at 10 m height at San Nicolas Island will be presented.]

Conclusions

This trial study shows that COAMPS is capable of simulating the transport and dispersion of ship emissions varying in time and space with the following limitations: the maximum number of emissions source points during a forecast run is 500; the spatial size of the emission source is the size of a grid cell and is thus overestimated for a point source. In spite of these limitations the model could be useful in simulating cases where it's desired to know where ship emissions are transported to on the regional scale and where a rough estimate of their concentrations is sufficient. More exact simulations of actual plume size and concentrations would require a plume in grid style model and a model that includes chemical processes and deposition.