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Temperature calibration

Tue, 03/18/2003 - 8:40am — JBlount
I am having a difficult time calibrating the development of a thermocline in the lake I am studying without varying the WSC coefficient substantially over the spring summer and early Fall. The epilimnion is restricted to the upper 7-9 meters of the lake and unless I reduce the WSC to .32 to .45 during the hottest Months (July, Early Aug) the warm temps migrate too deep. I am using a vertical viscosity coeff of 0.001. It appears to me that in reality, the vertical viscosity coefficient is varying substantially as the epilimnion warms up and the temp gradient increases. Is it legitimate to use a lower visc coefficient than 0.001 and would this tend to keep the heat from migrating downward into the hypolimnion as quickly? Is there a way to vary this coeff over time? Jon [addsig]
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Hydrodynamics
Wed, 04/02/2003 - 7:52am — Tom

Jim, Wind sheltering does change over time, so the fact that you have to vary it is not necessarily wrong. However, the values seem too low, so I suspect something else is the culprit. The vertical eddy viscosity value of 0.001 is the maximum value for the vertical eddy viscosity. The model computes this value internally and it changes over space and time as a function of internal shear, wind shear, etc., so you don't need to muck around with this. As you can see from the temperature results documented in the manual, there is no need to worry about this algorithm. Here's a couple of things to try 1. Check to make certain that your volume- elevation relationship does not underpredict the volumes compared to the existing relationship. 2. Make absolutely certain the the outlet elevation is correct and that nothing is restricting the movement of water in the withdrawal zone such as a submerged cofferdam still in place or trash buildup over time. This can affect both the location of the thermocline and hypolimnetic temperatures. 3. Going from a POINT to a LINE sink does give more control over the selective withdrawal algorithm. With a LINE sink, you can adjust the length of the sink. As you increase the length, you narrow the withdrawal zone. Let me know if any of this helps. Tom[addsig]