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L. Cucurull
J. C. Derber


The next generation of NCEP’s Global Data Assimilation System became operational on 1 May 2007. This system incorporates the assimilation of global positioning system (GPS) radio occultation (RO) profiles from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) mission launched in April 2006. Roughly 1 yr after the launch of COSMIC, NCEP has begun operational use of this new dataset.

A preliminary assessment of this observation type was performed with an earlier version of NCEP’s analysis at a lower resolution. These experiments showed positive impact when GPS RO soundings from the Challenging Minisatellite Payload (CHAMP) mission were assimilated into the system in non–real time. In these earlier studies, two different forward operators for the GPS RO profiles were evaluated: one for refractivity and another one for bending angle.

In this paper, the data assimilation experiments with COSMIC observations that led NOAA/NCEP to assimilate COSMIC data into operations are described. The experiments were conducted with the current operational version of the code and at full operational resolution. Based on the results of the experiments analyzed here, profiles of refractivity were selected as the type of GPS RO observation to be assimilated. Further enhancement to the assimilation of bending angles is currently being evaluated at NCEP.

The results show a significant improvement of the anomaly correlation skill and a global reduction of the NCEP model bias and root-mean-square errors when COSMIC observations are assimilated into the system. The improvement is found for the temperature, geopotential heights, and moisture variables. Larger benefits are found in the Southern Hemisphere extratropics, although a significant positive impact is also found in the Northern Hemisphere extratropics and the tropics. Even if GPS RO observations cannot produce direct impact on the wind field through the adjoint of the forward operator, a slight benefit is found in the wind components.

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John C. Derber
David F. Parrish
, and
Stephen J. Lord


At the National Meteorological Center (NMC), a new analysis system was implemented into the operational Global Data Assimilation System on 25 June 1991. This analysis system is referred to as Spectral Statistical Interpolation (SSI) because the spectral coefficients used in the NMC spectral model are analyzed directly using the same basic equations as statistical (optimum) interpolation. The major differences between the SSI analysis system and the conventional optimum interpolation (OI) analysis system previously used operationally at NMC are:

  • –The analysis variables are closely related to the coefficients of the NMC spectral model.

  • –Temperature observations are used, not heights as in the previous procedure. As a result, aircraft temperatures are being used for the first time at NMC.

  • –Nonstandard observations, such as satellite estimates of total precipitable water and ocean-surface wind speeds, can be easily included.

  • –No data selection is necessary. All observations are used simultaneously.

  • –The dynamical constraint between the wind and mass fields is more realistic and applied globally.

  • –Model initialization has been eliminated. The analysis is used directly as the forecast model initial condition.

Extensive pre-implementation testing demonstrated that the SSI consistently produced superior analyses and forecasts when compared to the previous OI system. Improvement in skill is shown not only for the 3–5-day forecasts, but also in one-day aviation forecasts.

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