A new system that resolves the stratosphere was implemented for operational medium-range weather forecasts at the Canadian Meteorological Centre. The model lid was raised from 10 to 0.1 hPa, parameterization schemes for nonorographic gravity wave tendencies and methane oxidation were introduced, and a new radiation scheme was implemented. Because of the higher lid height of 0.1 hPa, new measurements between 10 and 0.1 hPa were also added. This new high-top system resulted not only in dramatically improved forecasts of the stratosphere, but also in large improvements in medium-range tropospheric forecast skill. Pairs of assimilation experiments reveal that most of the stratospheric and tropospheric forecast improvement is obtained without the extra observations in the upper stratosphere. However, these observations further improve forecasts in the winter hemisphere but not in the summer hemisphere. Pairs of forecast experiments were run in which initial conditions were the same for each experiment but the forecast model differed. The large improvements in stratospheric forecast skill are found to be due to the higher lid height of the new model. The new radiation scheme helps to improve tropospheric forecasts. However, the degree of improvement seen in tropospheric forecast skill could not be entirely explained with these purely forecast experiments. It is hypothesized that the cycling of a better model and assimilation provide improved initial conditions, which result in improved forecasts.

We have obtained spectra of the yellow giant S1040 in the open cluster M67 using the Goddard High-Resolution Spectrograph (GHRS) and the Faint Object Spectrograph on the Hubble Space Telescope. S1040 is a single-lined spectroscopic binary with a 42.8d period that occupies a "red straggler" position in the M67 color-magnitude diagram (CMD), 0.2 mag blueward of the giant branch. A detection of S1040 at 1620Å with the Ultraviolet Imaging Telescope provided evidence that the secondary is a hot white dwarf, and thus that the anomalous location of S1040 in the CMD is likely due to a prior episode of mass-transfer. Our GHRS spectrum shows a broad Lyα absorption profile that confirms the white dwarf identification of the S1040 secondary. A model atmosphere fit to the GHRS spectrum yields T eff = 16,160 K, log g = 6.7, and a mass of ∼ 0.22 M ⊙ , for an assumed cluster distance of 820 pc and reddening of E(B-V) = 0.02. The unusually low mass derived for the white dwarf implies that it must have a helium core, and that a mass-transfer episode must have begun while the progenitor was on the lower giant branch. We construct a plausible mass-transfer history for S1040 in which it originated as a short (∼ 2d) period binary, and evolved through a blue straggler phase to reach its current state.

Half-hourly GPS zenith tropospheric delay (ZTD) and collocated surface weather observations of pressure, temperature, and relative humidity are available in near-real time from the NOAA Global Systems Division (GSD) research GPS receiver network. These observations, located primarily over the continental United States, are assimilated in a research version of the Environment Canada (EC) regional (North America) analysis and forecast system. The impact of the assimilation on regional analyses and 0-48-h forecasts is evaluated for two periods: summer 2004 and winter 2004/05. Forecasts are verified against radiosonde, rain gauge, and NOAA GPS network observations.The impacts of GPS ZTD and collocated surface weather observations for the summer period are generally positive, and include reductions in forecast errors for precipitable water, surface pressure, and geopotential height. It is shown that the ZTD data are primarily responsible for these forecast error reductions. The impact on precipitation forecasts is mixed, but more positive than negative, especially for the central U.S. region and for forecasts of larger precipitation amounts. Assimilation of the collocated surface weather data with ZTD contributes to the positive impact on precipitation forecasts for the central U.S. region. The NOAA GPS network data also have a positive impact on tropical storm system forecasts over the southeast United States, in terms of both storm track and precipitation. Impacts for the winter case are generally smaller because of the lower precipitable water (PW) over North America, but some positive impacts are observed for precipitation forecasts. The greatest regional impacts in the winter case are observed for the southeast U.S. (the Gulf) region where average PW is highest.

The data assimilation system of Environment Canada (EC) is adapted to accept GPS radio occultation (GPSRO) data. Observations of this type are available with extensive coverage from several satellites. In this study, experiments are performed to compare the skill of EC’s three-dimensional variational data assimilation (3DVAR) system (including all data normally assimilated operationally), with and without the addition of radio occultation refractivity data from the Challenging Minisatellite Payload for Geophysical Research (CHAMP). These data were not available at the time studied as near-real-time (NRT) observations. However, data from this and other radio occultation missions are now available as NRT data, and the conditions (latency, reliability) are improving. It is expected that NRT GPSRO data from a number of satellite missions will continue to be available through the following years. The results of the assimilation tests are evaluated against the following three data types: radiosondes (temperature and dewpoint depression), satellite brightness temperatures (from the Advanced Microwave Sounding Unit-A), and GPS radio occultation refractivity profiles. For the 6-h forecasts, the differences between GPSRO observations and forecasts (O − F) are significantly reduced in the experiment that assimilates the GPSRO data. This reduction increases as the experiment proceeds in time, and stabilizes after a transient period of approximately 2 weeks, suggesting that the addition of GPSRO data to the assimilation system has a beneficial, persisting, and cumulative effect. This effect is more pronounced in the stratosphere than in the troposphere. In the stratosphere, the standard deviation of GPSRO (O − F) of the experiment that assimilates GPSRO decreases after the initial transient period by approximately 10%. This improvement can best be observed in the southern stratosphere where reductions of the order of 30% are common. This shows that, as a globally distributed and vertically well-resolved source of data, the GPSRO observations are not only useful for assimilation, but also as a tool to quantify the forecast skill of the assimilation system. Comparisons with radiometer and radiosonde data confirm the positive impact in these geographical areas. Longer-range forecasts (up to 6 days) also show a positive impact with similar geographical and altitude distribution.

We present a simple method for the calculation of generalized Fermi functions. It is based on a straight integration using quadratures, with an adequate interval subdivision and variable choice. The result is a fast and accurate algorithm, which is compared with other existing methods in accuracy, versatility, and speed.