FIG. 3. Schematic data flow through the GOES I-M system. GOES-Tap and GINI will be broadcast by NESDIS from Washington, D.C.
For real-time weather forecasting, GOES I-M data must be transmitted nearly instantaneously to field users. A new ground data processing system enables efficient dissemination of data and products to users. This is accomplished using a completely redesigned GOES I-M Variable (GVAR) format for direct-receive users and existing GOES-Tap services for others. The GVAR format allows direct-receive users to acquire all data from the imager and sounder in real time. Transmission of GOES I-M images also occurs in either the GOES projection over the existing GOES-Tap system or remapped imagery by means of a new mode known as GOES-I NOAA-PORT Interface (GINI).
Archiving of the GOES data, started in 1978, will continue with the GOES I-M series. The GVAR data stream will be captured on the NESDIS videocassette archive housed at the University of Wisconsin‹Madison. User access to the GOES archive is coordinated through the NESDIS Satellite Data Services Division (see section 9).
Facilities with direct-receive capabilities such as NWS centers [NSSFC, the National Meteorological Center (NMC), and NHC] and the NOAA cooperative institutes CIMSS and CIRA (Cooperative Institute for Meteorological Satellite Studies and Cooperative Institute for Research in the Atmosphere) can receive OGE-calibrated and navigated data directly through the GVAR data stream. Because of the difference in sampling frequency and resolution of GOES I-M versus GOES-VAS (see Table 2), direct receive imagery from GOES I-M appears stretched in the east-west direction with respect to GOES-7.
Figures 4a,b illustrate the difference in aspect ratio for a GOES-7 versus GOES-I visible image that will be noticed at direct-receive sites. The aspect ratio of the sampled subpoint resolution (ExW versus NxS) for GOES-7 visible imagery is O.87:1 ,while for GOES-I it is 0.57:1. When data are displayed as 1:1 picture elements, the result is a GOES-I image that appears stretched east-west with respect to a GOES-7 image.
FIG. 4. (a) GOES-7 and (b) simulated GOES-I visible images, from 1941 UTC 26 April 1992, illustrating the difference in aspect ratio.
Composite imagery for AWIPS will be multiband or multisatellite. In the AWIPS era, imagery from GOES-East and GOES-West are to be combined into one polar stereographic image covering much of the Northern Hemisphere. The images will be produced in three separate bands; visible, infrared window, and water vapor. Each image pixel will consist of an average brightness over an 8-km x 8-km area.
Two types of multiband composite images are planned: 1) a visible/infrared (VIS/IR), and 2) water vapor/infrared (WV/IR) combinations. The maximum resolution of each will be retained. VIS/IR images combine information from both types of imagery: the temperature structure of cold, precipitating cloud systems from the infrared and high-resolution depiction of mesoscale features such as low-level outflow and cloud line mergers from the visible. They also provide a smooth transition from daytime to nighttime monitoring of convection. VIS/IR images are already widely used by the NWS and are planned for day 1 distribution. The WV/IR composite is planned as a day 2 product. Important upper-air features such as jet streaks, troughs, and vorticity maxima are often well defined only in water vapor images.
Initially, NOAA plans to support two basic modes of operation: routine and warning. In the routine mode, one full-disk image is taken every 3 h, and images covering the contiguous United States (CONUS) are taken every 15 min. in the intervening times. The warning mode will be enacted when the onset of severe weather is imminent. In that mode, the CONUS is scanned eight times every hour for monitoring rapidly developing storms. Figures 5a,b show the anticipated coverage in the routine and warning modes of operation for GOES-I located at 90 degrees W. NOAA will be initiating this new scheduling capability with GOES-I; however, it is subject to change pending satellite location decisions and a review of alternate scanning strategies.
FIG. 5a. Coverage using the GOES-I imager in the routine mode. Extended Northern Hemisphere frame (covering down to 20 degrees S) is followed by CONUS frame; remaining Southern Hemisphere frame concludes the half hour. Full-disk coverage occurs every 3 h.
FIG. 5b. Coverage using the GOES-I imager in the warning mode. Half-hour sequence proceeds as follows: Northern Hemisphere frame(covering down to the equator),CONUS frame, small Southern Hemisphere frame, CONUS frame, and CONUS frame. Small Southern Hemisphere frame could be defined for any one of several geographic areas.
Cloud-drift wind fields are derived using a sequence of three half-hourly images. The winds are calculated by a three-step objective procedure. The initial step selects targets, the second step assigns pressure altitude, and the third step derives motion. Target selection involves searching for regions of maximum brightness and temperature gradients, with the horizontal density of the search controlled by an input parameter (usually about 150-km spacing). Initial cloud height assignments for the selected targets are made using the H2O intercept method that is currently used with Meteosat (Nieman et al. 1993). This is a departure from the GOES-VAS approach, where height assignments are made using the CO2 slicing technique (Menzel et al. 1983); although the latter has been shown to be the preferred approach, there is no CO2 13.3-µm band on the GOES-I imager, and H2O intercept heights are of comparable quality (Nieman et al. 1993). Both height algorithms involve multispectral radiative transfer calculations in the environment of the target that account for the differing radiative attenuation as a function of cloud height. An initial-guess motion, based on NMC wind forecasts at the estimated cloud level, is used to steer the pattern recognition algorithm that locates the "target area" in one image within a "search area" in the second image and again in the third image using cross-correlation techniques. The first-guess motion, the consistency of the two winds, the precision of the cloud height assignment, and the pattern recognition feedback are all used to assign a quality flag to the "vector" (which is actually the average of two vectors). The initial height assignments are quality controlled, and some are adjusted through comparison with ancillary data (e.g., the 6-h model forecast and aircraft wind reports). Winds from moisture imagery (6.7µm) are derived by the same methods used with cloud-drift imagery; heights are assigned from the water vapor brightness temperature. Winds derived using cloud-drift and moisture-drift techniques are complementary in areal coverage. Cloud-drift winds are produced primarily in cloudy and partly cloudy areas, while moisture-drift winds are generated in mostly clear areas. The improved signal in the GOES-I water vapor images is expected to enhance the quality and utility of the moisture-drift winds appreciably. Satellite cloud-drift winds are input into NMC numerical models and are transmitted worldwide on the Global Telecommunications System (GTS).
Total precipitable water vapor (PW) and lifted indices (LI) are produced as derived product images, using the longwave split windows, the shortwave window (at night), and the 6.7-µm water vapor bands. Derived product imagery is formed from pixel-by-pixel retrievals of atmospheric temperature and moisture profiles wherever the atmosphere is quasi-clear. The images appear as the derived product with the cloud cover superimposed (see Hayden and Schmit 1991). Examples of the PW product are shown in Fig.1Oa,b. These products are made available hourly by NESDIS for use by the NWS national centers. Precipitable water vapor and LI are discussed in more detail in section 5d, which addresses sounder products.
GOES imagery is also used to produce precipitation estimates for heavy rainfall events. These estimates are made at the NMC in Washington, D.C., sing an interactive flash flood analysis (IFFA) system. The IFFA estimates are based on a modification of the Scofield-Oliver technique (Scofield 1987) and depend on a number of factors such as infrared temperature, cold cloud area growth rate, cloud shield pattern, merging cloud tops, and overshooting tops. The 6.7-µm water vapor imagery is used to detect tropical water vapor plumes that are often associated with extreme rainfall events (Thiao 1993). The GOES-I version of this technique takes advantage of the higher spatial and temporal resolution multispectral imagery.
In the routine mode, starting at 0000 UTC, NOAA schedules 1-h regional scans (50-25 degrees N and 70-120 degrees W) for the first 5 h. During the winter season (December-May), the sixth hour is dedicated to a Southern Hemisphere 1-h regional scan for the generation of soundings for input to forecast models. During the summer season (June-November), the sixth hour has a 45-in limited regional scan over the CONUS (45-30 degrees N and roughly 70-120 degrees W), followed by a 15-in mesoscale scan (15 degrees latitude by 15 degrees longitude) over the location of a tropical disturbance. This 6-h schedule is repeated four times each 24 h.
The warning mode is enacted when the onset of severe weather is imminent; the location of the mesoscale coverage is adjusted as the weather situation dictates. In the warning mode, 15-in mesoscale scans are scheduled four times an hour over the area of severe weather for the first 2 h. During the third hour, one 15-in mesoscale scan is followed by a 45-in limited regional scan over the CONUS. This 3-h schedule is repeated as long as the warning mode persists.
Vertical temperature profiles from sounder radiance measurements are to be produced at 40 pressure levels from 1000 to 0.1 mb using a physical retrieval algorithm (Hayden 1988) that solves for surface skin temperature, atmospheric temperature, and atmospheric moisture simultaneously. Also, estimates of surface emissivity, cloud-top pressure, and cloud amount are obtained as by-products. The retrieval begins with a first-guess temperature profile that is obtained from a space-time interpolation of fields provided by NWS forecast models. Hourly surface observations and sea surface temperature from AVHRR help provide surface boundary information. Soundings are produced from a 5 x 5 array of FOVs whenever nine or more FOVs are determined to be either clear or contaminated by "low cloud."
Vertical moisture (mixing ratio‹hence, specific humidity) profiles are obtained in the simultaneous retrieval and are provided at the same levels as temperature up to 300 mb. Since the radiance measurements respond to the total integrated moisture above a particular pressure level, the specific humidity is a differentiated quantity rather than an absolute retrieval. Layer means of either temperature or moisture can also be derived. Layered precipitable water can be integrated from retrievals of specific humidity; three layers (1000-900 mb, 900-700 mb, and 700300 mb) and the total atmospheric column precipitable water are provided as output products and are put into the standard archive.
Lifted index, an estimate of atmospheric stability, is derived for each retrieval. It represents the buoyancy that an air parcel would experience if mechanically lifted from a mixed boundary layer to the 500-mb level . The lifted index expresses the difference in temperature between the ambient 500-mb temperature and the temperature of the lifted boundary-layer parcel. Negative values (parcels warmer than the environment) represent positive buoyancy, with large negative values indicating the potential for severe storms; positive values denote stability. The formulation used to derive the lifted index is a thermodynamic relationship requiring the 500-mb temperature as well as a mean pressure, temperature, and moisture for the boundary layer. These quantities are all available from the retrieved profile.
Geopotential height profiles are derived from the full-resolution temperature and moisture profiles. The geopotential height of a pressure level is derived from a 1000-mb height analysis (from the NMC forecast supplemented with hourly data), a topography obtained from a library (with 10-in latitude-longitude resolution), and the retrieved temperature and moisture profile. Thickness can be calculated from this profile.
Thermal gradient winds, derived indirectly from the soundings, are provided with each profile. These are derived from objective analyses of the geopotential profiles calculated with each retrieval. The analyses are performed on a 1 degree latitude-longitude grid. Gradient winds are calculated using finite-difference operators that involve surface fitting over 5 x 5 grid points centered at the grid point closest to each retrieval. Wind estimates are provided from 700 to 400 mb. Verification studies with observed winds have shown that this product from VAS consistently depicts the temperature gradient more accurately than the 12-h NMC forecast. The GOES-I product should be an improvement and will have expanded geographical coverage. Cloud-drift and moisture-drift winds from the imager combined with thermal gradient winds from the sounder have been found to be of value in models for determining hurricane trajectories (Velden et al. 1992). These deep layer mean wind fields are produced with a pressure weighting of the winds at all levels.
The GOES-I sounder is also providing an hourly cloud product to supplement the ASOS. This is required by the NWS introduction of the ASOS nationwide. ASOS is designed to support weather forecast activities and aviation operations. ASOS uses automated equipment to provide near-continuous observations of surface weather data including cloud height and amount that are currently obtained by NWS and Federal Aviation Administration observers. The cloud information from the ASOS equipment is limited to altitudes below 12000 ft, and GOES-I provides supplemental information about cloud cover above 12000 ft at each ASOS site. The combined ASOS/satellite (ASOS/SAT) system depicts cloud conditions at all levels to 25000 ft. Because observations are required every hour, the satellite cloud product can be derived only from the geostationary spacecraft data. The satellite cloud information is derived using sounder data with the CO2 slicing technique, which calculates both cloud-top pressure and effective cloud amount from radiative transfer principles. It also reliably separates transmissive clouds that are partially transparent to terrestrial radiation from opaque clouds in the statistics of cloud cover. For a given ground observation site, the algorithm uses radiation measurements from an area of roughly 50 km x 50 km centered on the site. Further information can be found in Schreiner et al. (1993).