Showing papers by "Eugenia Kalnay published in 2001"

The NCEP–NCAR 50-Year Reanalysis: Monthly Means CD-ROM and Documentation

Abstract: The National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) have cooperated in a project (denoted “reanalysis”) to produce a retroactive record of more than 50 years of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involved the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data. These data were then quality controlled and assimilated with a data assimilation system kept unchanged over the reanalysis period. This eliminated perceived climate jumps associated with changes in the operational (real time) data assimilation system, although the reanalysis is still affected by changes in the observing systems. During the earliest decade (1948–57), there were fewer upper-air data observations and they were made 3 h later than the current main synoptic times (e.g., 0300 UTC), and primarily in the Northern Hemisphere, so that the reanalysis is less reliable than for th later 40 years. The reanalysis data assimilation system continues to be used with current data in real time (Climate Data Assimilation System or CDAS), so that its products are available from 1948 to the present. The products include, in addition to the gridded reanalysis fields, 8-day forecasts every 5 days, and the binary universal format representation (BUFR) archive of the atmospheric observations. The products can be obtained from NCAR, NCEP, and from the National Oceanic and Atmospheric Administration/ Climate Diagnostics Center (NOAA/CDC). (Their Web page addresses can be linked to from the Web page of the NCEP–NCAR reanalysis at http:// wesley.wwb.noaa.gov/Reanalysis.html.) This issue of the Bulletin includes a CD-ROM with a documentation of the NCEP–NCAR reanalysis (Kistler et al. 1999). In this paper we present a brief summary and some highlights of the documentation (also available on the Web at http://atmos.umd.edu/ ~ekalnay/). The CD-ROM, similar to the one issued with the March 1996 issue of the Bulletin, contains 41 yr (1958–97) of monthly means of many reanalysis variables and estimates of precipitation derived from satellite and in situ observations (see the appenThe NCEP–NCAR 50-Year Reanalysis: Monthly Means CD-ROM and Documentation

Objective Verification of the SAMEX ’98 Ensemble Forecasts

Abstract: During May 1998, the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma coordinated a multi-institution numerical forecast project known as the Storm and Mesoscale Ensemble Experiment (SAMEX). SAMEX involved, for the first time, the real-time operation of four different ensembles of mesoscale models over the same region of the United States. The main purpose of this paper is the evaluation of the ensemble forecasts, performed at a relatively coarse resolution of 30 km. An additional SAMEX goal not discussed here is to compare the value of the ensemble forecasts against single forecasts made over smaller subregions of the Great Plains at both intermediate (10 km) and high (3 km) resolution. The SAMEX ’98 ensembles consisted of a single 36-h control forecast from the ARPS (at CAPS), the Penn State‐NCAR fifth-generation Mesoscale Model (at NSSL), and the Eta Model and Regional Spectral Model (at NCEP), all with horizontal resolutions of approximately 30 km, and perturbed runs, resulting in a grand ensemble of 25 members. The forecasts of geopotential heights, temperatures, and moisture were verified against the Eta operational analyses, rather than observations. Unlike global ensembles, which tend to be useful in the medium range, the mesoscale SAMEX ensembles provided useful information in the short range. A major result is that the performance of the ensemble of multiple forecast systems is much better than that of each individual ensemble system, probably because it represents more realistically the current uncertainties in both models and initial conditions. A similar advantage from the use of multimodel, multianalysis systems has been observed with global ensembles. The SAMEX results also show that perturbations to model physics parameterizations, as well as the use of consistent perturbations in the boundary conditions, are important for regional ensemble forecasting. Efforts are now under way to compare the ensemble forecasts against those made using higher spatial resolution, and follow-on SAMEX experiments are anticipated in other geographical areas and weather regimes. Although the main results of this paper appear to be very robust, they were based on a small number of cases, and similar experiments carried out during other periods will help to test their significance.

Local low dimensionality of atmospheric dynamics.

Abstract: A statistic, the BV (bred vector) dimension, is introduced to measure the effective local finite-time dimensionality of a spatiotemporally chaotic system. It is shown that the Earth's atmosphere often has low BV dimension, and the implications for improving weather forecasting are discussed.