Thursday Apr. 25, 2013

Today we will be looking at satellite detection and location of lightning.  Here's a list some of the areas where regional or global satellite observations of lightning might be of interest.

magnetospheric and ionospheric research

Earth's electrical circuit
  
atmospheric chemistry
   nitrogen fixation
   ozone reactions and stratospheric chemistry

storm physics
   lightning activity/storm intensity
   ocean/land lightning ratio
   hurricane electrification
   lightning/tornado activity
   lightning/precipitation

We'll just look at a couple of recent satellite sensors, the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS),  that detect the optical emissions of lightning during the day and at night from low earth orbit.  Both were designed and built by researchers at the NASA Marshall Space Flight Center.  A new and improved lightning mapping sensor will be on the GOES-R satellite scheduled to be launched in 2015.

Christian et al. (1989) contains a detailed discussion of the factors that must be considered in the design of a satellite lightning sensor.  Prior to designing the OTD and LIS, the NASA Marshall group also made extensive measurements of lightning optical emissions from a platform on a high-altitude aircraft flying well above the tops of thunderclouds (a U-2 aircraft flying at approximately 20 km altitude).  



This figure provides an idea of the area viewed by a lightning mapper aboard a satellite in geostationary orbit.  We will see that most lightning activity falls between 35 S and 35 N latitude.

An example of a lightning spectrum recorded by the U-2 aircraft is shown below (from Christian et al. 1989).


The U-2 measurements did indeed show that both cloud to ground and intracloud discharges could be detected from above cloud top (though it isn't possible to determine the type of discharge process using the optical signals).  The 777.4 nm and 886.3 nm features (OI and NI) highlighted above each contain 5 to 10% of the optical energy in a lightning flash.  The 777.4 nm OI(1) line was ultimately chosen for the satellite detector.

Now because lightning will be viewed from above the thunderstorm top we need to look at the effects that scattering by cloud particles will have on the light impulses.  In class we first had a quick look at what lightning optical signals look like from the ground.

This short detour is part of a larger lecture on ground based measurements of lightning optical signals.

Examples of recorded fast electric fields (E, shaded blue) and associated optical signals (O, highlighted in yellow) are shown below (from Guo and Krider, 1982).


This was a four stroke cloud-to-ground discharge that occurred at 13 km range.  The first return stroke is shown at the bottom of the figure.  The first 50 μs or so of the record is the stepped leader.  This is followed by an abrupt rise to peak.  Notice that the E field signal is still increasing in ampltitude at the end of the record.  This indicates some of the electrostatic field component is present which is typical of a return stroke field recorded at a range of about 10 km.   These waveforms were photographed on moving film.  The dark black timing marks were from an LED that would flash on and off to code the absolute time onto the film.


And a more recent example (Quick and Krider, 2013)



Signals from a first return stroke, subsequent return strokes (NGC is a channel with a new ground contact point, PEC a pre-existing channel), and a cloud discharge are shown.



A typical return stroke optical signal.  We can use a measurement of the peak optical signal amplitude (in volts) to determine the peak irradiance, Lp (in W/m2).  Then if the range to the discharge is known we can estimate the peak optical power output, P (in Watts) from the return stroke.
 


We treat the lightning discharge as a point source and assume the optical power output during the strike will expand evenly outward in a sphere.  We measure the peak irradiance, Lp, a distance D from the source (W/m2 on the surface of the expanding sphere).  So to estimate P we simply multiply the measured values of Lp by the area of the sphere.


Cumulative distribution of peak optical power estimates.  50% of 1st return strokes have a peak optical power output of about 2 x 109 Watts or more.  Peak power emitted by subsequent strokes is almost a factor of 10 less. 









Just like frosted glass, clouds will blur any image of the lightning channel inside the cloud.  The optical signal risetime and pulse width are each increased by about 150 microseconds.  The following figure helps to understand why this is true.
Photons emitted by a lightning channel are scattered (redirected) multiple times before leaving the cloud.  The path in pink undergoes a relatively low number of scatters.  The green path is scattered more times and takes longer to escape from the cloud.  It is difficult to distinquish between cloud-to-ground and intracloud discharges.

Somewhat surprisingly perhaps, there is very little absorption of the light signal by the cloud.

A lightning flash illuminates about 10 km diameter of the cloud top.  Ideally this would just fill a single pixel on the CCD satellite detector.

We'll spent most of the remainder of our discussion on the Optical Transient Detector (OTD).  This is the first of two satellite "lightning mappers" designed by the people at the NASA Marshall Space Flight Center.  You can read more about that research group and some of the activities on the Global Hydrology and Climate Center webpage.

The OTD was a prototype for the Lightning Imaging Sensor.  The OTD was launched on Apr. 3, 1995 and turned off in Mar. 2000, having operated 2 years beyond its expected lifetime.

It was launched into a nearly circular orbit 740 km (~450 miles) high.  The orbit was inclined 70o (relative to the Equator) so the satellite coverage extended from 75o S to 75o N latitude.

The field of view was about 1300 km x 1300 km (about 1/300 th of the earth's surface) and was imaged onto a 128 pixel x 128 pixel CCD sensor array (thus about 10 km x 10 km per pixel which is about the size of the cloud top illuminated by lightning)

Location accuracy (at nadir) was about 8 km.

The orbit precesses about 15 minutes (1/4 of a degree) per day.  It takes about 50 days for the satellite to review a specific location on the earth's surface at the same time of day.

In a year the OTD images most locations for a total time of > 14 hours (400 individual over passes).

The OTD detects lightning during the day and at night.  The typical daylight cloud background (sunlight reflected off the cloud tops) is 50 to 100 time brighter than lightning. 

Several steps must be taken in order to detect lightning signals superimposed on this bright background.
(i)   The pixel size corresponds roughly to the size of the cloud top illuminated by lightning.
(ii)  A 1 nm narrow band interference filter centered on the 777.4 nm OI emission was used.  Much of the cloud background falls at other wavelengths and was blocked.
(iii) CCD signals were integrated for 2 ms - well over the duration of the lightning optical pulse.
(iv) successive frames were subtracted.  This subtracts out much of the slowly time varying background signal

This last process is illustrated below

The leftmost image shows a picture of the scene.  The middle two frames show the CCD data for pictures taken of the scene with and without a lightning.  It's hard to see the lightning until you difference these middle images.  The lightning appears in the 4th picture (the three pixels with values of 1 in the middle of the scene).

There are still sources of noise such as sun glint that create erroneous lightning counts.  The South Atlantic Anamoly is also a source of false triggers.  This is a region where the Van Allen Radiation Belts make their closest approach to the earth's surface.  The inner belt contains mainly high energy protons.  When the satellite passes through the belt, the high energy particles strike the CCD array and produce anamolous triggers.

Here are some images of lightning activity derived from OTD data.



Here's the first image showing all of the lightning detected in 1999. 

Most of the lightning detected is found over land affected by the Intertropical Convergence Zone (ITCZ).  This refers to a feature in the 3-cell model of the earth's global scale circulation pattern.


The figure above shows most of the surface features (high and low pressure belts and winds) predicted by the 3-cell model.  The ITCZ is nominally located near the Equator (also labelled the equatorial low in the figure above).  Surface winds converge and produce rising air motions.  Since the air is usually quite moist you can usually see a band of clouds associated with the ITCZ on global satellites photographs.  The ITCZ will move north of the Equator in summer (northern hemisphere summer) and south of the equator in the winter. 



Here's the lightning detected during the months of December, January, and February 1999.  Note how the activity has shifted into the southern hemisphere.  The activity at higher latitudes in the northern hemisphere might be associated with storms forming along the polar front.


This is the June, July and August map for the same year.  Activity is now primarily in the northerm hemisphere.

78% of the lightning detected (and remember the OTD doesn't distinguish between cloud-to-ground and intracloud discharges) is found between 30 S and 30 N latitude.  88% of the lightning is found over continents, islands, and coastal regions.  There is much less activity over the oceans.

Here is a link to the source of these OTD GIF Images

One interesting result from 5 years of OTD lightning data is a new estimate for the global lightning flashing frequency:
44 flashes/sec
(plus or minus 5 flashes/sec)

This is about half of the 100 flashes/sec value that was long thought to be true.  The 100 flashes/sec value dates back to about 1925.

Just a quick mention of the Lightning Imaging Sensor launched on Nov. 28, 1997 into an orbit with 35 degree inclination.  The LIS mapper is still operating.  You can read more about its design and look at examples of data here.



"The Detection of Lightning From Geostationary Orbit," H.J. Christian, R.J. Blakeslee, S.J. Goodman, J. Geophys. Res., 94, 13329-13337, 1989,