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The lightning total flash
density grid and lightning ground flash density grid are based on data from the
Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) for the
Australian region (10ºS - 45ºS, 105ºE - 165ºE). It is important to note that the
analysis is based on eight years of combined OTD and LIS data.
The grid point resolution
of the data is 0.5° (approximately 50 km).
The data are acquired from polar orbiting satellites and
consequently undersampling may influence the estimates of lightning activity in
some areas (Kuleshov et al. 2006). The satellite data were calibrated against the data
derived from observations obtained by ground-based lighting flash counters and
then adjusted applying an
empirical polynomial in a form
RLNP(λ) = A - B
λ2 + CλN ,
with λ in
magnitude of degrees, A = 1.2339, B = 2.8108E-3, C = 1.3661E-5, and N = 3.5.
The
adjusted NtSAT values obtained by multiplying each NtSAT
value by RLNP(λ) were used to produce the total flash density map for
Australia (Kuleshov et al 2006).
Applying the assumption that the value of cloud
flash-to-ground flash ratio,
Z, over the range of latitude for the
Australian continent is uniform (Z = 2), the Ng values were derived
from the average annual
total flash density values, Nt , by
using the equation
Ng = Nt / ( 1
+ Z ) .
OTD and LIS estimates of
lightning ground flash density (Ng) were compared with, and evaluated against,
estimates of Ng derived from a thunder day analysis as well as from direct
measurements obtained by ground-based lightning flash counters (Kuleshov et al
2006).
Ng is conventionally
estimated from Td using an equation of the form Ng = aTd
b, in which a and b are empirically derived
constants that are influenced by the meteorological conditions at a given
location. In this study, Kuleshov and Jayaratne's formula with derived values of
a = 0.012 and b = 1.4 was used (Kuleshov and Jayaratne 2004).
References:
Kuleshov, Y. and Jayaratne, E. R. 2004. Estimates of
lightning ground flash density in Australia and its relationship to
thunder-days. Aust. Met. Mag. 53, 189-196.
Kuleshov, Y., Mackerras, D., and Darveniza, M. 2006.
Spatial distribution and frequency of lightning activity and lightning flash
density maps for Australia. J.Geophys.Res., Vol. 111, D19105,
doi:10.1029/2005JD006982 |
