LNOx Estimates Directly from LIS Data

Abstract:

Nitrogen oxides (NO_x = NO + NO₂) are known to indirectly influence climate since they affect the concentration of both atmospheric ozone (O₃) and hydroxyl radicals (OH). In addition, lightning NO_x (LNOx) is the most important source of NO_x in the upper troposphere (particularly in the tropics). It is difficult to estimate LNOx because it is not easy to make measurements near the lightning channel, and the various NO_x-producing mechanisms within a lightning flash are not fully understood. A variety of methods have been used to estimate LNOx production [e.g., in-situ observations, combined ground-based VHF lightning mapping and VLF/LF lightning locating observations, indirect retrievals using satellite Ozone Monitoring Instrument (OMI) observations, theoretical considerations, laboratory spark measurements, and rocket triggered lightning measurements]. The present study introduces a new approach for estimating LNOx that employs Lightning Imaging Sensor (LIS) data. LIS optical measurements are used to directly estimate the total energy of a flash; the total flash energy is then converted to LNOx production (in moles) by multiplying by a thermo-chemical yield. Hence, LNOx estimates on a flash-by-flash basis are obtained. A Lightning NO_x Indicator (LNI) is computed by summing up the LIS-derived LNOx contributions from a region over a particular analysis period. Larger flash optical areas are consistent with longer channel length and/or more energetic channels, and hence more NO_x production. Brighter flashes are consistent with more energetic channels, and hence more NO_x production. The location of the flash within the thundercloud and the optical scattering characteristics of the thundercloud are complicating factors. LIS data for the years 2003-2013 were analyzed, and geographical plots of the time-evolution of the LNI over the southern tier states (i.e. upto 38^o N) of CONUS were determined. Overall, the LNI trends downward over the 11 yr analysis period. The LNI has been added to the list of indicators presently provided by a sustaining assessment tool developed at the NASA Marshall Space Flight Center (MSFC) for monitoring lightning/climate interactions over the United States, as part of the National Climate Assessment (NCA) program.