ASOS has been designed and developed primarily to satisfy the meteorological needs of the aviation community. To meet those needs, ASOS sites are typically configured with one visibility sensor and one ceilometer. ASOS generates meteorological observations by processing weather sensor information through a complex set of algorithms. Users can derive representative reports of visibility and sky condition with single-sensor algorithms at most ASOS sites. At locations that commonly experience weather affecting only a portion of the airport, multi-sensor algorithms provide information about meteorologi-caldiscontinuities in sky condition and visibility. For example, San Francisco International Airport needs a ceilometer a few miles away to detect approaching low stratus coming in from the ocean. The meteorological discontinuity algorithm will provide a remark indicating the existence of the low stratus northwest of the airfield. Multi-sensor algorithms also have been designed to provide redundancy at major airports in the event that the primary ceilometer or visibility sensor should fail.
The backup sky condition algorithm requires two ceilometers. The ceilometers both must view portions of the celestial dome that are representative of the station as a whole. Typically, the sensors are collocated. The primary and backup ceilometer generate an independent sky condition report that is updated each minute. The report generated by the primary sensor is used in all official observations. Should the primary sensor become inoperative for any reason, ASOS uses the backup sky condition report until the primary sensor is restored. The backup visibility algorithm requires two visibility sensors. The sensors are sited so that the output from either is representative of the station. Both the primary and backup sensor generate independent visibility reports that are Both the primary and backup sensor generate independent visibility reports that are updated each minute. The report generated by the primary sensor is controlling and is used in all official observations. If the primary sensor becomes inoperative, the backup visibility report is used in all subsequent observations until the primary sensor is restored to service. Since both the primary and backup sensors are sampling virtually identical portions of the atmosphere, the visibility and sky condition reports generated are similar. When ASOS uses the backup sensor report in place of the primary sensor report, the transition is usually transparent to the user.
The meteorological discontinuity sky-condition algorithm also requires two ceilometers. The primary sensor is sited to provide a sky condition report that is representative of the airport. The meteorological discontinuity sensor is sited to detect operationally significant discontinuities in cloud cover conditions, similar to those described in the introduction. Both the primary and meteorological discontinuity ceilometers generate independent sky condition reports that are updated each minute. The report generated by the primary sensor is the controlling observation provided in the body (as opposed to the remarks) of the observation. Once the system generates the sky condition reports, the ceiling layers of each report are compared for significant differences in height (See Table 1).
CEILING LAYER HEIGHT
|H < 1000 Feet||> 300 Feet|
|1000 < H < 3000 Feet||> 400 Feet|
|3000 < H < 5000 Feet||> 600 Feet|
|5000 < H < 8000 Feet||> 1000 Feet|
|H > 8000 Feet||> 1600 Feet|
|NOAA (1990), Page C-A-I-1-28.|
If the ceiling height reported by the meteorological discontinuity sensor is lower than that of the primary and exceeds the criteria, ASOS generates a remark of the format "CIG VALUE LOC," where "VALUE" and "LOC" are the height of the ceiling and nominal location of the meteorological discontinuity sensor. For example, the remark "CIG 1600 RWY11" would indicate a 1600 foot ceiling in the vicinity of runway 11.
The meteorological discontinuity visibility algorithm also assumes the use of two visibility sensors. The primary sensor is sited to provide a visibility report representative of the airport. The meteorological discontinuity sensor is sited to detect operationally significant discontinuities in visibility. Both the primary and meteorological discontinuity sensors generate an independent visibility report updated each minute. The report generated by the primary sensor is used in the body of all official observations. Once ASOS generates the visibility reports, they are compared for significant differences (See Table 2). If the visibility reported by the meteorological discontinuity sensor is lower than that of the primary sensor and exceeds the criteria, the system generates a remark. The format is "VSBY VALUE LOC," where "VALUE" and "LOC" are the visibility report and nominal location of the meteorological discontinuity sensor. For example, the remark "VIS 2 1/2 RWY26L" would indicate a 2 1/2 mile visibility associated with runway 26 Left.
visibility less than 3 miles
|Primary minus meteorological |
discontinuity visibility greater
than 0.5 miles
|NOAA (1990), Page C-A-I-2-12.|
Backup ceilometers and visibility sensors are configured at selected airports to assure continuity of meteorological observations should the primary sensor fail. The transition from primary sensor report to backup sensor report should be transparent to the user. ASOS uses the backup sensor output as the official sky condition or visibility until the primary sensor is restored. The effect of meteorological discontinuity sensors is more noticeable. In this case, remarks are generated that alert the user of significant differences in sky condition and/or visibility in the vicinity of the airport. ASOS also will generate a remark whenever the meteorological discontinuity sensor is inoperative.
NOAA, ASOS Program Office, 1990: Appendix I to ASOS Statement of Work,Contract No. 50-SANW-1-00050, December 1990. By Malcolm Doug Gifford
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