Local Knowledge v. Tidetech Currents: Time Comparisons
NOAA and TT Currents on September 7, 2010 at the San Francisco Bay Entrance PRS, ranging to a max ebb of 5.0 knots. The current predicted by TT is 20% less than NOAA (a 3.7 knot max ebb, and a 3.5 knot max flood, compared to NOAA flood max of 4.2 knots). TT timing is also off, running about a half hour earlier than the NOAA timing.
This is the second page of comparisons of the Local Knowledge and Tidetech ("TT") current models of San Francisco Bay (for related pages see the links at the bottom of the page). Here we focus on the time-dependence of predicted currents compared to the published values for NOAA Primary Reference Stations in the Central Bay. A decade ago the only Primary Reference Station (for which NOAA publishes detailed predictions) for San Franciso Bay was at the San Francisco Bay Entrance (the annual daily predictions reproduced in SF Bay "tidebooks"). In the mid to late 1990's, NOAA operated current sensors at various locations, measuring current magnitude and direction every few minutes. Beginning in 2004, NOAA introduced additional Primary References Stations ("PRS") at the same locations and recalculated the timing of many Secondary Stations relative to the nearest PRS. The graphics below compare the TT and NOAA predictions at the PRS locations in the Central Bay (the LK model is constrained to exactly reproduce NOAA currents at these locations). In each graph the spacing represents 10 minute intervals, beginning at 12:00 am on September 7, 2010. The length of the blue bars are proportional to the current specified by NOAA, and the red bars to the current predicted by TT. Positive values correspond to flood, and negative values to ebb.
NOAA and TT Currents on September 7, 2010 at the Golden Gate Bridge PRS (actually located about 0.5 nm SE of Yellow Bluff, well north of and inside the Bridge) , ranging to a max ebb of 2.1 knots. The current predicted by TT is 20-25% less than NOAA (a 2.8 knot max ebb, and a 3.3 knot max flood, compared to NOAA flood max of 3.9 knots). TT timing here is pretty close to NOAA (and LK) timing.
NOAA and TT Currents on September 7, 2010 at the Oakland PRS (actually located about 0.5 nm E of the southern tip of Treasure Island, north of the Oakland Channel) , ranging to a max ebb of 1.9 knots. The current predicted by TT is 15-30% less than NOAA (a 1.6 knot max ebb, and a 1.3 knot max flood, compared to NOAA flood max of 1.8 knots). TT timing here lags behind NOAA/LK by about 50 minutes. Note that "flood" here occurs at roughly the same time as "ebb" at the SF Entrance, since the current is ebbing out of the South Bay.
TT Model at SF Entrance PRS
NOAA/LK at SF Entrance PRS
The boxes at the right show the TT and LK/NOAA current predictions at the SF Bay Entrance PRS in the early hours of 9/7/2010. Both show a declining ebb current towards (approximately) 250 degrees, although the TT-predicted magnitudes are significantly less and decline much faster (1.36 knots at 0600 compared to the LK/NOAA prediction of 2.67 knots). Oddly, the TT predictions also display rotary current behavior during the approach to minimum current at 0650), with direction clocking counterclockwise from 250 to 161 degrees at minimum, and continuing counterclockwise to approximately 68 degrees during increasing flood. This rotary behavior is very unlikely at a location such as the center of the Golden Gate Bridge, and appears to be an artifact of the TT model. Similar behavior occurs at the next current minimum (before ebb), where the current turns counterclockwise through zero degrees, falling to approximately 255 degrees with increasing ebb.
Detailed LK-TT Comparison at San Francisco Bay Entrance on 9/7/2010
Current Comparisons at SF Entrance