The purpose of this evaluation methodology was to determine the availability rate of the ASOS and whether ASOS observations were missing as a result of ASOS equipment failure or longline/communication line failure. When evaluating for availability, it is necessary to determine whether the observation was produced by the ASOS and whether the observation passed through the longline/communication line dissemination system to the end user.
This analysis was performed on five commissioned ASOS sites listed in Table 3.3-1. With the exception of Roswell, NM, all the sites were non-augmented. The sites were selected because they didn’t have a contract weather observer assigned and they had no known communication problems. Figure 3.3-1 shows the ASOS sites included in this study.
The data provided by the NWS reports included the following information:
The name of the site
Number of hours checked for each day
Specific hours not checked
During hours checked, specific times of "missing obs"
For each hour listed under "missing obs", the actual observation (obtained from dialing into the ASOS) was manually checked to determine if the data was actually generated. If the data was generated, the observation was listed as missing due to a communication problem (e.g., the data was generated but was never received). If, on the other hand, there was no data listed for the time of the missing observation, then the discrepancy was listed as an ASOS problem.
The NWS supplied reports were generated by hourly runs of a NWS program named CHKSFCOBS (Check Surface Obs). The reports contain a caveat which states that,
"Some observations will be detected as missing because the start time of CHKSFCOBS is occasionally delayed from its normal start time of ten minutes past each hour. These delayed start times are caused by other scheduled software, which is still executing beyond its allotted time window. It may well be that these observations were available on time."
ASOS raw data was acquired via a modem directly from the selected ASOS systems.
Table 3.3-2 displays the number of hours checked for each of the five stations along with the number of hours where a communication line failure prevented reception of the report followed by the number of hours where an ASOS failure prevented reception of the report. The percentage of communication line problems compared to the total number of observation is included in the last column. The total number of hours checked, total number of communication failures, and total number of ASOS failures are graphed in Figure 3.3-2. It is evident that failure of the communication line was the primary reason for the ASOS report not being received by NWS. For the entire period of the analysis, there were only five hours in April at Childress where the ASOS was responsible for an hourly report not being received.