bulk parameter –A parameter tracked as a surrogate for actually following individual parcels of water. Temperature is the primary parameter used since it has a low diffusivity, meaning it will be transported with the parcel of fluid (advected) rather then moving between parcels (diffused)
day of year –Starts with January 1 as day 0 and December 31 being day 364 (or 365 in a leap year). The decimal fraction (e.g. .64) is simply what fraction of a day has passed. This can easily be converted to a time if needed.
density –The amount of mass per unit volume of a substance. Temperature, salinity, and pressure (or depth) all affect the density of water. Fresh water reaches it’s maximum density at 4°C, increasing to either side of this temperature. In Cayuga Lake and other bodies of water, small changes in density can play major roles in the physics of stratification, gravity currents and internal waves.
fetch –The length of water a wind can blow over. The longer the fetch, the more influence the wind will have on the body of water. In Cayuga Lake the fetch is about 60 km in the N-S direction and 3 in E-W direction.
flux –Generically; A flow through a unit area (i.e. W/m2).
free surface –In Cayuga Lake (and most fluid mechanics problems) the interface between water and the atmosphere where pressure is taken to be zero in the fluid.
gravity current –The flow of a heavier denser fluid underneath a lighter less dense fluid. In Cayuga Lake these occur in the late fall as surface water cools, becoming more dense then the surrounding water and sinks. The southern shelf provides a hill for the cool water mass to flow down until encountering equal density fluid.
internal wave – Internal waves occur on the interface between two layers of fluid. They travel along this interface in the same manor as surface waves, but at much slower speeds because the effects of gravity are reduced. In Cayuga Lake internal waves of sufficient amplitude will break on the southern shelf resulting in turbulent mixing. They can often be seen in summer months on the water profile plot after a strong wind event.
monomictic –A term used to describe lakes which undergo one period of complete mixing during the year separated by one period of thermal stratification. Monomictic lakes also relatively deep and do not freeze over completely in the winter. Cayuga Lake mixes completely from late fall to late spring and becomes stratified early summer through late fall.
nephelometry –A technique used to measure turbidity by measuring the scatter of radiation (in this case light) passing through a sample. Higher scattering indicates higher turbidity.
net heat flux –The summation of shortwave radiation (direct solar radiation, what we measure), longwave radiation (reflected from clouds), sensible heat flux (heat energy transferred from the earth's surface to the atmosphere) and latent heat flux (heat flux due to evaporation). By convention, negative fluxes are into the body of water and positive are out of the water.
photosynthesis –A chemical process carried out by plants, some algae, bacteria and protistans which converts energy from the sun into sugar and oxygen
stratification –The separation of a fluid into two or more layers of uniform, but different densities.
surface waves –A wave traveling on the interface between the air and water, visible to us as changes in the free surface elevation.
thermocline –The simplest definition is the area of maximum temperature change (gradient) in the water column. This definition can fail however depending on the vertical resolution of the temperature profile (the distance between samples). A better definition is the area of maximum temperature change over a length scale suitable for the layer depths (e.g. in Cayuga Lake gradients over a meter rather then a millimeter).
Total Suspended Solids –The amount of solids that would be retained on a filter if a water sample were passed through it.
upwelling – Warmer surface water is moved and replaced with cooler deeper water due to wind action. In Cayuga Lake we will see upwelling when strong winds from the south push warm water off the shelf and it is replaced by cooler deeper water. The opposite will happen at the northern end of the lake as warm surface water is pushed into shore and the underlying water is pushed down and into the deeper sections of the lake. Upwelling is important because the deeper waters of lakes generally contain more nutrients to support biological populations and it is the major means of bringing these to the surface.