Abstract
Abstract. Here, we present a mapping method OBLIMAP, which projects and interpolates fields like surface temperature, surface mass balance, and surface height between a geographical based coordinate system of a General Circulation Model (GCM) and a rectangular based Ice Model (IM). We derive an oblique stereographic projection and its inverse, which holds for any area at the Earth's surface, and which can be combined with two different interpolation methods. The first one is suited to interpolate the projected fields of a coarse GCM grid on a fine meshed IM grid. The second one is appropriate for the opposite case. Both grids are allowed to be arbitrary and irregularly spaced. Therefore the OBLIMAP technique is suitable for any GCM-IM combination. After a first scan of the GCM grid coordinates and the specification of the IM grid, fast mapping of various fields is possible. To and fro (GCM-IM-GCM) mapping tests with the Climate Community System Model (CCSM) at T42 resolution (~313 km) and the Regional Atmospheric Climate Model (RACMO) at ~11 km and ~55 km, show average temperature differences of less than 0.1 K with small standard deviations. OBLIMAP, available at GMD, is an accurate, robust and well-documented mapping method for coupling an IM with a GCM or to map state of the art initial and forcing fields available at geographical coordinates to any local IM grid with an optimal centered oblique projection. Currently, the oblique stereographic and the oblique Lambert azimuthal equal-area projections for both the sphere and the ellipsoid are implemented in OBLIMAP.
Highlights
In case of OBLIMAP any grid has the Cartesian orientation relative to the normal vector on the spherical surface, which implies that a continuous collection of projected grid planes over the globe can be obtained including the polar cases
Our direct oblique methodology might be of interest in future applications of high accuracy: to derive an oblique stereographic projection and its inverse in case the earth surface is represented by a geoid, or by a function even closer to the earth topography
Concerning the third important remark of referee and D. Roche (Referee) #1: we can compare the integrated values of the mapped area of both grids, but the involved GCM points which correspond to the edge points of the IM grid will not represent exactly the same area
Summary
As an example we added the specific configuration file for the Northern Hemisphere to OBLIMAP. While testing such extended cases we encountered situations in which our optimal alpha estimation is not longer unambiguous, this is improved by giving an advising message. This γ = 45 degrees for half the globe.
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