Marine Ecosystem Dynamics Modeling Laboratory

Thermal Plume

Thermal Plume and Its Impacts on Water Temperature

One of the challenges for the MHB modeling is to simulate the Power Plant discharge induced thermal plume. The temporal and spatial structures of this plume were detected in the remote-sensing aircraft photos taken in September 1998. The photos taken during one tidal cycle clearly showed that the plume looked like a narrow jet and varied significantly with tidal motion. Due to the limitation in model horizontal resolution, the size of this plume was over-predicted by previous modeling efforts. For example, the plume water predicted by the Applied Science Associate, Inc (ASA) curvilinear coordinate model spread almost the entire MHB area, no jet structure was resolved. As a result, the impact of this plume on the heat increasing in MHB was exaggerated in their results.

The unstructured grid used in FVCOM allows us to increase the horizontal resolution in the plume region without significant modification of the boundary forcing. An effort was made to use FVCOM to simulate the Power Plant discharge induced plume and compared it with the aircraft photos. Our experiments show that the size of the thermal plume is sensitive to the model horizontal resolution. When the horizontal resolution reaches 50 m, the model-predicted thermal plume is like the aircraft-observed plume (see Fig. 1 for an example of the comparison during the ebb tidal period and click the picture will show the animation for a tidal cycle), however the width of this plume is still much larger than that observed in the high-resolution CTD measurements made in August 2005.

After the field measurement was made, we conducted a model experiment to examine the sensitivity of the model-simulated plume to the model resolution. For the given same initial conditions of the water temperature and salinity, the thermal flux from the Power Plant, and tides, we ran FVCOM for four cases with horizontal resolutions of 50, 25, 10 and 5 m in the plume regions. The experiment was made for the August 2005 during which the field survey was conducted. The examples of the comparison results are shown in Fig. 2 (horizontal) and Fig. 3 (vertical). The comparison results show that the width of the thermal plume becomes smaller as the horizontal resolution increases. The numerical convergence is reached when the horizontal resolution is up to 10 m or less. Although the model simulation was made for the 2005 summer condition, the result should apply for the other seasons and years. This suggests that previous modeling results obtained from a model run with a horizontal resolution of 200 m tends to exaggerate the size of the thermal plume and its impact on the heat flux in the bay.

The impact of the thermal plume on the water temperature in MHB for August, hottest period of a year in this year, was estimated by running the FVCOM for the cases with and without inclusion of the thermal plume. For a case with the horizontal resolution of 50 m, the model suggests that during the summer season (a season during which the ocean gain heat from atmosphere) the maximum value of the water temperature increased due to the Power Plant thermal plume is about 0.7oC. As we learned from the sensitivity analysis, the 50-m resolution model tends to overestimate the size of the plume and thus underestimate the vertical and lateral thermal dissipations. Taking these factors into account, the water temperature increase resulted from the thermal plume should be less than 0.7oC during the summer. This value is much smaller than the model results shown in previous low-resolution models.

We also used the heat flux measurement data recorded at the Power Plant to estimate the heat impact on the water temperature increase in the MHB. In August 2005, the net heat flux is -2.6 W/m2. This suggests that a significant heat from the thermal plume water is fluxed into the air. Consider an idealized case with no inclusion of the water exchange between MHB and NB and also turbulent thermal dissipation, we can easily estimate the maximum water temperature increase by

and the result shows that

It should be noted here that the estimation was made for an idealized case with no inclusion of the thermal dissipation and the heat exchange between MHB and NB. Considering all these factors, the maximum heat impact of the thermal plume during the summer time must be less than 0.75oC. This also suggests that the water temperature increase due to this thermal plume in the other month should be significantly less than this value, especially during winter-spring in which the ocean tends to loss the heat to atmosphere

Posted on January 16, 2014