The NCEP and NCAR are cooperating in a project (denoted "reanalysis") to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957-96. This eliminates perceived climate jumps associated with changes in the data assimilation system.The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a "quick look" CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CD-ROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, "C" variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems.The 40 years of reanalysis should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data.NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation ...

No abstract

No abstract

This note describes changes that have been made to the National Centers for Environmental Prediction (NCEP) operational ''early'' eta model. The changes are 1) an decrease in horizontal grid spacing from 80 to 48 km, 2) incorporation of a cloud prediction scheme, 3) replacement of the original static analysis system with a 12-h intermittent data assimilation system using the eta model, and 4) the use of satellite-sensed total column water data in the eta optimum interpolation analysis. When tested separately, each of the four changes improved model performance. A quantitative and subjective evaluation of the full upgrade package during March and April 1995 indicated that the 48-km eta model was more skillful than the operational 80-km model in predicting the intensity and movement of large-scale weather systems. In addition, the 48-km eta model was more skillful in predicting severe mesoscale precipitation events than either the 80-km eta model, the nested grid model, or the NCEP global spectral model during the March-April 1995 period. The implementation of this new version of the operational early eta system was performed in October 1995.

No abstract

No abstract

The inertial subrange in the energy spectrum appears to be the most consistent feature of atmospheric turbulence at all altitudes. The implications of the fact that amplitudes are related to the 1/3 power of the wavelength are imposed upon the probabilistic structure of the turbulence by B. Mandelbrot's concept of self‐similarity. A process with random variable x is self‐similar if the variable xh obtained by magnification of the wavelength by h can be represented as a suitable magnification of the amplitude of x so that both x and xh have the same probabilistic structure. If this is the case, and the spectra have power‐law behavior S(k) ∼ k−v, then the distribution functions for x and xh obey P(x) = Ph(xh) = Ph[h(v − 1)/2x]. The implications for the distribution of increments yT = x(t) − x(t + T) are examined. Observational data from the 150‐meter tower at Cape Kennedy and from airborne measurements in the troposphere and stratosphere show that self‐similarity gives a remarkably accurate prediction of the behavior of the empirical data in all three cases. The distribution of the increments also reveals an intermittent structure for the turbulence. It is concluded that an attempt to construct a theoretical explanation for the apparently self‐similar behavior of atmospheric turbulence may be revealing.

No abstract