National Aeronautics and Space Administration

Two papers in the MERRA special issue of J. Climate address performance in high latitudes. Atmospheric energy and moisture budgets provide rigorous tests for any reanalysis, because they not only require data assimilation but also an adequate model representation to characterize surface and top-of-atmosphere fluxes. High latitudes are a particular challenge for assimilating sparse observations and for model physics over variable land, ocean, and ice surfaces and multi-phased moisture processes. These budgets are the topics of the special issue papers. In general, the surface moisture budget appears to be reasonable in comparison to other reanalyses and previous studies. In the figure below, the averaged moisture convergence field for the Arctic reproduces the essential components, with a net evaporation over a limited area near Svalbard, small amounts over the central Arctic Ocean, and largest values occurring along the Gulf of Alaska, the Norwegian coastline, and coastal Greenland.

For Greenland, comparisons indicate that MERRA tends to agree more closely with recent regional modeling studies, which distribute larger amounts along the southeastern coast. The moisture transport vectors clearly represent the wrapping effects of the Icelandic Low along the Greenland coast. For the atmospheric energy budget, MERRA suffers from an over-simplification of surface properties over glaciers and sea ice. The latter has an adverse effect on the springtime surface energy balance. Nevertheless, MERRA compares favorably to other studies in representing energy tendency and convergence. Shown below is the mean convergence of moist static energy in high latitudes which, combined with net input from the surface, must be balanced by the losses at the top of the atmosphere. Turning the globe around a bit, we can see that the transport of energy largely reflects the longwave patterns in the mid-troposphere, with poleward transport in western Alaska transitioning to equatorward over Hudson Bay, and then back to poleward in the North Atlantic.

State and dynamical variables incorporated into MERRA appear to be well represented, and this, along with the high spatial and temporal (hourly) resolution should provide an improved capability for examining high latitude synoptic-scale dynamics in more detail.

Left: average atmospheric moisture convergence [cm yr^-1] and moisture transport. Right: convergence of moist static energy [W m²] and transport