GSI Overview

Background

The GSI system was initially developed by NOAA/NCEP/EMC as a next generation global/regional analysis system based on the then operational Spectral Statistical Interpolation (SSI) analysis system and the then operational grid-space regional analysis system at NOAA/NCEP. It became operational as the core of the North American Data Assimilation System in June 2006 and the Global Data Assimilation System in May 2007 at NOAA. The GSI is also being used in other NCEP operational systems, such as the real-time mesoscale analysis (RTMA) and Hurricane WRF (HWRF). The system was also adopted by NASA/Goddard GMAO as their primary atmospheric analysis system and they have been contributing to the development of the GSI.

Lately, the number of groups involved in GSI development has expanded from the central development group (NCEP and NASA) to include two development groups, NOAA/ESRL/GSD and NCAR/ESSL/MMM, to support the upcoming Rapid Refresh (RR) and Air Force Weather Agency (AFWA) applications.

The Developmental Testbed Center (DTC) added the GSI DA system to the NWP code it supports for the WRF community in 2009. The DTC currently focuses on testing and evaluation of WRF for limited area NWP applications, but transitioning to include global forecast applications is included in the DTC long range planning, so it is in the DTC's interest to support a GSI system that includes the various capabilities developed by all the GSI developers.

Description

The current version of GSI is a three dimensional variational (3D-Var) data assimilation (DA) system. The analysis variables in GSI are

  • stream function
  • unbalanced velocity potential
  • unbalanced virtual temperature
  • unbalanced surface pressure
  • pseudo relative humidity (qoption =1) or normalized relative humidity (qoption=2)
  • ozone mixing ratio (only for global GSI)
  • cloud condensate mixing ratio (only for global GSI)
GSI can assimilate (but not limited to) the following observation types:
  • Conventional observations, e.g., radiosondes, pibal winds, synthetic tropical cyclone winds, wind profilers, conventional aircraft reports, ASDAR aircraft reports, dropsondes, MODIS IR and water vapor winds, GSM, METEOSAT and GOES cloud drift IR and visible winds, GOES water vapor cloud top winds, surface land observations, surface ship and buoy observations, SSM/I wind speeds, QuikScat wind speed and direction, SSM/I precipitable water, SSM/I and TRMM TMI precipitation estimates, GPS precipitable water estimates, GPS radio occultation profiles, SBUV ozone profiles...
  • Radar observations: Doppler radial velocities, VAD (NEXRAD) winds ...
  • Satellite observations: GOES 11 and 12 sounders, AMSU-A, AMSU-B/MHS, HIRS, AIRS...
The observation operator is used to simulate observations from model variables. It can be a simply interpolation procedure from model space to observation space or a more complex procedure depending on the assimilated observation types. For satellite radiance data, the GSI uses the Community Radiative Transfer Model (CRTM) developed by the Joint Center for Satellite Data Assimilation (JCSDA).

The background and observation errors are critical components for a variational data assimilation system since they form weights for the background and observation terms of the cost function. While they are up to the users'definition, the released GSI tar file provides their estimations computed from NCEP/EMC as a start applicable to both global and regional applications.

For more information, please refer to the GSI User's Guide.