GOES-West GLM data recently became available to several NWS offices. This, of course, benefits western US NWS WFOs and offices with Pacific forecasting duties. Recall that GLM gridded products are reformatted to the 2 km ABI fixed grid, so share the same parallax as the ABI imagery (shifted away from nadir). With GOES-East at 75.2W, and GOES-West at 137W, the “cutoff” for “better” GLM data (less parallax, better detection efficiency) is at 106W (equal distance from both satellite subpoints). Offices west of 106W should use GOES-West GLM, and offices east of 106W should use GOES-East GLM (see Fig 1).

A comparison between GOES-East and -West GLM FED and ABI VIS data over eastern Montana (near 106W) using ENI point data as a constant (with assumed very little parallax) shows a similar degree of parallax between the two satellites (Fig 2). Of course, from the East point of view, the GLM and ABI data are displaced to the north and west, while that from the West point of view are displaced to the north and East.

Another example from northeast California (120.5W), well west of 106W and closer to GOES-West subpoint, shows similar parallax between the two satellites meridionally (Fig 3). Zonally, however, there is significantly less parallax displacement in the GOES-West data (minor shift to the east) than in the GOES-East data (bigger shift to the west). Additionally, the nearer GOES-West is able to detect more flashes, and the GOES-East pixels are stretched further away from the satellite sub-point.

Analyzing GOES-East GLM data over the northeast US (near 75W and 38N), we observe no parallax in the east-west direction, but significant displacement to the north (Fig 4).

Now looking further south (~15N) but still near 75W, northern displacement is much less, with ENI and GLM detentions very similar in location (Fig 5). The west-east parallax remains negligible.

One final example, this time from GOES-West, shows parallax with lightning associated with Hurricane Barbara remnants just south of Hawaii. Being south of 20N but between 150W and 160W (well west of the 135W subpoint), the parallax appears negligible to the north, but a shift to the west is apparent. (Fig 6). Both ENI and GLD lightning data are used in this case to ensure data quality in this remote area.

Currently, only GOES-West GLM data within the GOES-West CONUS ABI sector are sent to WFO AWIPS (covers the western 1/3 of CONUS). Similarly, only GOES-East GLM data within the GOES-East CONUS sector is sent to WFO AWIPS (covers the full CONUS). Figures 7 and 8 show GLM coverage over the CONUS from both satellites, respectively for the same time period on 3 July 2019.

Therefore, while all CONUS offices are covered by GOES-East GLM data, only offices in the western 1/3 of the CONUS receive GOES-West GLM over their area of responsibility. If GOES-East GLM suffers a data outage, the eastern 2/3 of the US will be left without any GLM data. If GOES-West GLM suffers an outage, all of the CONUS will still have access to GLM data from GOES-East.

Fortunately, there are plans to extend GOES-West GLM coverage in AWIPS eastward to near Kentucky in the coming weeks.

Bill Line, NWS