R. Manasseh Papers
R. Manasseh Home
Distributions were also calculated by combining the operations-relevant gust
bin data for seven days when thunderstorms occurred. These were thunderstorms
that created significant horizontal wind shear on at least one occasion during
the day. It can be seen from table 5
that the distributions have lower variances than those from the compilation of
gusty wind days. This means that strong gusts are rarer on thunderstorm days,
when gusts are calculated according to
(1-2). This should not be
surprising, since thunderstorms can unpredictably create very strong winds
over timescales of a few minutes, but over the 20 second time used here for the
mean there are simply fewer strong gusts above the mean, even if the mean
represents an unpredictable change since the last mean and causes severe wind
shear. Furthermore, thunderstorms are rarely sustained for more than a few
hours. In comparison, days of consistently gusty winds caused by synoptic
conditions give a greater risk of sudden gusts for a given operations
direction. As an illustration, compare the data from a day of sustained strong
winds in table 2 with that from
a typical thunderstorm day, 17 January 1994, in
table 6. At about 1540
Australian Eastern Standard Time on 17 January 1994 a thunderstorm gust front
created wind shear of about 15 
(30 knots) as it moved across the
airfield; nevertheless, over 20 second timescales, there were far fewer strong
gusts than on the day of sustained strong winds. As another example, a set of
thunderstorm-generated atmospheric waves
created a
low-altitude wind shear hazard but was relatively gust-free, the variation
being over 15 minute periods.
Table 5: Standard deviations and probabilities of gusts greater than
5 for a compilation of 7 days on which thunderstorms causing
significant wind shear at Sydney Airport occurred. As
table 4.
From the details in table 5, it is
again the case that the along-track gusts are worse than the cross-track gusts.
This is probably for the same reason as outlined above for the gusty
synoptically-generated winds; the orientation of the turbulent rolls in the
boundary layer over land. Again, direction 16 shows a reversal of this trend,
although landing in direction 25 on the thunderstorm days also carries a higher
risk of cross-track gusts. Although there are fewer days in this analysis of
thunderstorm days than in the analysis of strong synoptic-wind days, the
numbers of records are still large (
), ensuring that the variances
remain significantly different. Even the 
and 
results for along- and cross-track landing gusts in direction 25 are
significantly different; the statistical hypothesis that they are different
would be rejected only at a confidence level of 99.999990%.
As with the strong synoptic winds, direction 07 poses the greatest risk of gusts but now direction 16 gives the least risk, not 34. However, these differences are not so marked on the thunderstorm days; the ratio of the highest to lowest variance is about 2.3, while for the days of strong synoptic wind the equivalent ratio is about 4.2. This probably reflects a greater variability in the direction of strong thunderstorm winds, which results in the strong gusts being spread more widely over the operations directions. An illustration of the distributions with greatest and least variance is in figure 4.
Figure 4: Probability distributions of along-track gusts for take-offs in
direction 07 (lower, broader curve) and along-track gusts for take-offs in
direction 16 (taller, narrower curve), for
a compilation of 7 days on which thunderstorms causing
significant wind shear at Sydney Airport occurred. These are the distributions
with greatest and least variance respectively.
As figure 2.
Table 6: Operations-relevant gusts at Sydney Airport on 17 January 1994:
Landing gusts. As
table 2.
R. Manasseh Papers
R. Manasseh Home
