A Unified Approach to Interpolating Satellite-Derived Land-Surface Fields

for NCEP’s Global and Regional Models

George Gayno (JCSDA/SAIC),

John Derber and Ken Mitchell (NCEP/EMC)

4th JCSDA Workshop May 31 – June 1, 2006

Introduction

• History– NCEP/EMC global and regional branches wrote their

own codes; limited communication.– Code development often done in an ad hoc manner.– Interpolation methods poorly chosen or implemented.

• Recent unification initiative– Ensure consistent treatment of land fields in EMC.– Use a single package of programs for all models.– Increasing sophistication of land surface physics and data

assimilation methods require better interpolation algorithms.

Design

• Collection of FORTRAN programs or modules to work with regional and global models.– Stand-alone programs.– Modules that may be called from other programs.– GRIB format for most I/O.

• Smarter interpolation methods.– Careful treatment of coastlines.– Nearest neighbor interpolation of discreet fields.– Additional methods for continuous fields.– Preserve total water volume of snow pack.

Interpolation of Continuous Fields

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Ignore source points that are water (*)

Area averaging

Accounts for grid stagger.

Method based on resolution of source and model grids.

Four point

Interpolation Methods - Tiling

• Vegetation and soil type are interpolated using a “tiling” approach.

• Allows for more than one category at a grid point.• Categories are placed into broad groups.

– Ex: Vegetation type -> tall, short, etc.– Groupings set in a namelist.

• Tiles are the dominate category from each group.– Categories may be ignored if their grid box % is below

a user-specified threshold.

Tiling Example

GrasslandGroup - 60%

ForestGroup - 40%

Cropland -34%

Pasture - 26%

NeedleleafTrees - 4%

DeciduousTrees - 36%

Dominate category not deciduous trees.

Interpolating Fixed and Climatological Fields

• Stand-alone program to create fixed and climatological land fields on model grid.– Land mask, terrain, soil type, etc.

• Input fields in GRIB format.– Map projection and bitmap information.

• Only run when model grid or input land fields change.– More efficient for high-resolution datasets and more

sophisticated interpolation algorithms.

Interpolating Snow, Sea Ice and SST

• Stand-alone programs interpolate analyses to the model grid.

• WRF-NMM to use NCEP/MAB 1/12-degree SSTs.– JCSDA-developed physical retrieval.

• NMM and GFS to use 4 km NESDIS IMS snow/sea ice data.– Italy test case shows big impact in GFS.

Snow extends into Po valley

No snow is more realistic

April 7, 2005Test case

Water equivalent in mm

Improvement due to:-Area averaging.-Single interpolation.-4 vs. 23 km IMS data.

Updating Surface Fields

• Module that updates fields for each analysis cycle.– Applies snow, sea ice, SST created from previous

programs.– Time interpolates climatological fields such as

greenness fraction, snow-free albedo.– Performs consistency checks between fields.

• Ex: Lower thermal boundary temperature at sea ice.

– May be run as part of stand-alone program.• Create unique wrapper for model specific I/O.

– May be called from model.• Climate runs.

Changing Resolution

• Module that interpolates surface fields from one model grid to another model grid.– Accounts for input and target grid land mask and terrain.

• Default nearest neighbor interpolation of all land-surface fields.– Studies show land fields require long spin-up time.

• Other interpolation methods allowed for continuous fields.– NOT: Soil/vegetation type, soil moisture/temp., canopy water.– Used for going to a lower resolution grid.

• Option to read in fields already on target grid.– Used for going to a higher resolution grid.– Perform any needed conversions. Ex: soil moisture rescaling.

QUESTIONS

EXTRA SLIDES