Background
This page describes the implementation of the Tidal Peak Correction transformation. This transformation is a correction on the first next peak of the astronomical tide. It was implemented to correct the NEMO tide prediction provided by the UKCMF. A memo attached to this page describes the empirical method of tidal correction as a means of reducing the small inherent error in astronomical tidal predictions. Correction is made by applying a correction factor that reflects differences between observations and predictions for a few recent high water levels. To exclude the influence of diurnal inequality of high waters from the correction process, the adjustment is made using predictions and observations for alternate high waters, in accordance with the following correction algorithm:
where HWN is the observed high water level for tide number N, AN is the astronomical prediction for the same tide and SN is the storm surge component derived from the operational CS3x model for that tide. C1, C2, C3 and C4 are coefficients. The tidal peak correction transformation, calculates the part highlighted in red, which are the components with a C1..4 coefficient. THe figure below shows which peaks are selected. In order to correct the next peak N, peak N-2, N-4 and N-6 are used to calculate the different components of the correction.
Configuration
The transformation uses three timeseries as input:
- An astro peaks (highs) timeseries, which contains all the previous peaks of the astronomical tide
- An observed peaks timeseres, which contains all the peaks of the observed waterlevel
- A surge forecast timeseries, which contains a number of forecasts.
For each peak, a value is selected to be used for the calculation of the correction factor. So for peak N = -6, a value of the astro timeseries is selected, a value for the observed water level (within 2 hours)nd a value of the surge forecast. The value of the surge forecast is selected at the time of the observed peak from the nearest surge forecast in time. This is best illustrated with the following example:
A surge forecast is run daily every 6 hours, e.g. at 00:00, 06:00, 12:00. 18:00. The -6 peak occures at 7:15, it will use the surge forecast at 06:00, which is the closest forecast and use the value at 7:15 in the tidal correction transformation.
- Peak 8 is first astro peak after T0 (index 7)
- Peak 7 is first astro peak before T0 (index 6) - Peak nummer -1
- Peak 6 ---
- Input Peak selection variable is astro water level
- Input time selection variable is observed water level
- Find peak -2 from Astro
- Find peak -2 from observed closest to astro peak
- Check if time difference is less than 2 hours, then take time of observed peak
- If no closest observed peak found, then return missing and do not calculate peak correction.
The tidal correction transformation, the XML configuration is also attached to this page. The output of the transformation is a single value, which has the timestamp of the first next peak.
The coefficients are configured as location attributes in a csv file, which is added to the LocationSets:
Coastal_ComputeTidalCorrection_NEMO.xml
