| NAVGEM 2.0 |
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The NAVy Global Environmental Model (NAVGEM) was recently upgraded to version 2.0 and became operational on 29 April 2020 at the 12Z run. This page will not focus on physics or data assimilation changes between successive NAVGEM versions, but will simply guide the end-user of its output. The main difference is an increase in the horizontal resolution. NAVGEM 1.3/1.4 ran at T425L50 whereas NAVGEM 2.0 runs at T681L60. The array size is now 2048 x 1024 and its effective resolution is 0.17578125°. However, it is distributed on a Gaussian grid, i.e. the grid spacing is constant in longitude, but varies in latitude. The HYCOM interpolation scripts can deal with this grid and in the *.D files, DYIN = 0.0 is a marker to tell the scripts that the forcing is on a Gaussian grid. With this version of NAVGEM, a long reanalysis of model output was not provided prior to receiving operational output. The model output is 3-hourly and so the diurnal cycle of shortwave radiation is not properly resolved. Thus an anti-alias correction is applied to the shortwave component. Note that HYCOM provides an option to apply an analytic diurnal cycle to shortwave (dswflg=1), however, this should only be turned on if using daily filtered shortwave. As with previous versions of NAVGEM, the user needs to be aware that there can be differences in the surface winds and heat fluxes between successive versions. The good news is that the surface momentum and heat fluxes are very similar between NAVGEM 1.4 and 2.0. As with other NAVGEM fields, 'sec' versions are made, where the 'c' stands for corrected. The zonal and meridional winds are calibrated against satellite scatterometer observations and the downward shortwave and longwave fields are calibrated against NASA CERES satellite data. Use 'sec' versions if they exist, otherwise use the ‘sea' versions (that differ from the 'std' versions) that mitigate coastline mismatches between the atmospheric model's land-sea mask and HYCOM's land-sea mask. navgem_0.176deg_computagrid_mask_2048x1024.D is the land-sea mask needed for NAVGEM 2.0.
The 3-hourly files are generally updated once per week (Thursday or Friday) on hycom.org.
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