GLM is coming: The instrument

The first beta-release data of the Geostationary Lightning Mapper (GLM) instrument will be out this week. (Update as of 12 June 2017:  GLM beta release has been delayed until July.)  As we get closer to having real-time GLM observations, here is a quick post about the GLM instrument itself.

glm_instrument

Figure 1:  An artist’s image of the GOES-16 satellite with the Geostationary Lightning Mapper (GLM) shown as the zoom out in the upper right.

In the post describing the origin of the GLM (here), it was discussed how the GLM is not the first space-based instrument to observe lightning.  However, it is the first lightning sensor available in geostationary orbit.  Conceptually, the GLM can be thought of as a very large digital camera.  Each pixel of the camera is identifying optical brightness difference from cloud top.  Each pixel is monitoring if any light is observed and if the light observed exceeds a background threshold.  This check is occurring every 2 ms and these observations become the basic GLM “event” observations.  The background field and threshold criteria are designed to reduce false alarms.  The placement of the charge couple device, or CCD pixels, on the instrument designed to help with the instrument’s spatial resolution.  The instrument’s CCD pixels vary in size to help account for the increasing parallax the closer to the edge of the field of view the observations get.  This allows the resolution of the GLM to go from 8 km directly beneath the satellite to only 14 km at the edge of the field of view.

The actual field of view for GLM is shown in Figure 2 for both the GOES-East (eventual location of GOES-16) and -West (future position of GOES-17) positions.  The underlying, normalized annual lightning flash rate comes from observations made by the Optical Transient Detector and Lightning Imaging Sensor from 1995-2005.  Currently, the GLM is in the GOES-16 check-out location (Figure 3).  The total field of view will range from 52 degrees north and south.  Additionally, the GLM does observe total lightning, or the combination of intra-cloud and cloud-to-ground observations.  However, the GLM will not distinguish between the two.  Still, observing total lightning, particularly over such a large domain will aid in warning decision support, lightning safety, as well as situational awareness in data sparse regions.  This will be helpful for detecting flash flooding (noting where is convection) in the inter-mountain west, convection monitoring for aviation, as well as opening up new avenues of research for tropical cyclone forecasting.  Lastly, the GLM was designed to be able to detect 70% of total flashes over the entire field of view over 24 hours.  The false alarm rate was designed to be less than 5%.  Recently, a calibration and validation field campaign had been underway to investigate the GOES-16 instruments.  Early indications are that the GLM will likely exceed the design specifications.  Exact values will be provided later after the field data has been analyzed.

glm_fov_E_and_W

Figure 2:  The field of view for GLM in the GOES-East and -West position.  The normalized, annual lightning flash rate shown is derived from 10 years of Optical Transient Detector and Lightning Imaging Sensor, low-Earth orbiting instrument observations.

goes16_central

Figure 3:  Same as Figure 2, but showing the current GLM field of view through November 2017.

Subsequent posts will start to focus on actual GLM observations once they are made available.

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