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In situ Air Pressure Tide (IAP) Data Description
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Author: Michael Schindelegger
Department of Geodesy and Geoinformation, Vienna University of Technology
Contact: michael.schindelegger@tuwien.ac.at
Last modification: 4 December 2014
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------------------------------- General information ------------------------------
The available data are mean annual harmonic coefficients of diurnal (S1) and
semidiurnal (S2) surface pressure tides as deduced from barometric in situ
observations. Pressure data at land and marine platforms from the International
Surface Pressure Databank (ISPDv2, Compo et al., 2010) have been processed for a
principal analysis window from 1990.0 to 2010.12; see Schindelegger and Ray (2014)
for details. Seasonal variations of the tides are currently not considered.
--------- 'Version 1': IAP.1 ---------
IAP.1 corresponds to the data presented in Schindelegger and Ray (2014), featuring
3970 land stations and 2924 ocean boxes (2-degree x 2-degree).
Files 'IAP.1S' and 'IAP.1O' under ./STATION contain point-wise tidal estimates for
land stations (suffix 'S') and ocean boxes (suffix 'O'). Their globally gridded
extensions obtained by multiquadric interpolation to a regular 1-degree grid are
available under ./GRID as netcdf-file 'IAP.1G.nc'.
--------- 'Version 2': IAP.2 ---------
IAP.2, comprising 4187 land stations and 1125 ocean boxes, is an upgrade of IAP.1
with improved coverage of the Southern hemispheric oceans achieved through binning
marine pressure observations to cells of size 4-degree in latitude and longitude.
These oceanic data are apparently more coherent than their precusors from IAP.1.
Moreover, the station tide network in Africa, South America, Antarctica, and the
United States has been re-analyzed and augmented by about 200 additional sites.
Users are recommended to use these data for further analysis. File names:
- 'IAP.2S' and 'IAP.2O': station and marine tide data
- 'IAP.2G': S1/S2 climatologies on a regular 1-degree grid
--------------------------------- Implementation ---------------------------------
Cosine- and sine-amplitudes (a_n, b_n) at each location can be combined to yield
the tidal signal (S_n) of form
S_n = a_n*cos(n*t)+b_n*sin(n*t) = A_n*cos(n*t-phi_n), n = 1,2,
where t signifies UTC in radians, A_n = sqrt(a_n**2+b_n**2), and the Greenwich
phase lag phi_n follows from atan(b_n/a_n).
------------------ File structure for STATION data ('S' and 'O') -----------------
Column no.: Description:
1 Running Index
2 Latitude [deg]
3 Longitude [deg]
4 Height of platform [m]
5 NCEP observation type code:
181: Station observation
183: Station observation, sea level pressure only
180: Marine observation
6 S1 Cosine-amplitude [Pa]
7 S1 Sine-amplitude [Pa]
8 S2 Cosine-amplitude [Pa]
9 S2 Sine-amplitude [Pa]
10 Length of time series stacked to a daily mean composite [yr].
11 Number of epochs in daily mean composite. Divide by 24 to get the
approximate sampling interval in hours.
12 Quality flag to distinguish between highly reliable (1), fairly
reliable (2), and mediocre/low-quality (3) tidal estimates.
The categorization into three quality classes has been accomplished by combining
different statistical parameters of the harmonically analyzed time series at each
location, e.g. record length, number of composite epochs, a posteriori variance
factor of the fit, or the observation type. For example, 'Class 1' land stations
were required to
- reside below 1000 m
- record non-sea level pressures
- have a sampling interval of more than 3 hours
- cover at least 5 full years
- yield a standard error of unit weight <= 8% of the local S1+S2 amplitude
1022 of 3790 land stations in IAP.1 meet these requirements. Note however that a
host of stations at high latitudes were examined with considerable care but have
been tagged as 'Class 2/3' due to their short temporal coverage of 3 or 4 years.
For further information, such as location maps and provisory S1/S2 amplitude grids
for various data class combinations, see files 'IAP.1_QClasses.pdf' and
'IAP.2_QClasses.pdf'.
----------------------------------- References -----------------------------------
Schindelegger M., R.D. Ray. Surface pressure tide climatologies deduced from a
quality-controlled network of barometric observations, Mon. Wea. Rev.,
142(12), 4872-4889, doi: 10.1175/MWR-D14-00217.1, 2014.
Compo G. and Coauthors. International Surface Pressure Databank (ISPDv2) 1768 to
2010. Research Data Archive at the National Center for Atmospheric Research,
Computational and Information Systems Laboratory, Boulder, CO.,
http://dx.doi.org/10.5065/D6SQ8XDW, 2010.