Gedday,

During the past 24 hours we have had what you would have to call a Northwest cloudband sitting across WA and the western Bight.  To some extent I am simply noting it for posterity, as I have  often heard it stated that  the NW cloudbands are responsible for a large percentage of Australian wintertime preciptation.... I am dubious about that... hence this "recording".  There are a couple of noteworthy aspects of it however:

(i)  There is currently an SST anomaly of the order of a degree or more off the northwest Australian coastline.  It has been demonstrated by our mate Nev (Nicholls, Jnl of Climate, Sept '89) that sea surface temperatures in that region are highly correlated with the amplitude of the one of the two dominant rotated EOFs of Australian wintertime rainfall.  This EOF is the result of a rotation of a degenerate leading pair of the unrotated EOF's, the leading pair accounting for 55% of the variance in winter rainfall.  The sign of the correlation is such that warm SST's means increased winter rainfall.

    I also have heard it stated at seminars and conferences that warmer SST's in that region are associated with an increased frequency of winter-time NWCB's.. but the only literature I could find on this is the paper by Kuhnel, International Journal of Climatology, 1989, and also International Journal of Climatology, 1990.  The correlations quoted by Kuhnel are significant but small, accounting for less that 20% of the variance in interannual Cloudband activity.

(ii) The literature (Bill Wrights's various papers in Intnl Jnl of Climatology, 1988, 1997; Tapp and Barrell, Jnl of Climatology 1984,  Mills Mon Wea Rev., 1989), plus casual observations indicate the cloud bands are associated with "feeder convection" in the Indian ocean.  In this context we have a nice cloud cluster sitting at about (10, 100) that has been drifting westward over the  past couple of days.

(iii)  Almost trivially the NWCB's are associated with the upward motion on the eastern side of an upper-level/mid-tropospheric trough.  This is true in the current case, and I have put the current 500 and 250 hPa analyses on my web-page).  There seemed to be an interaction with the front that passed through Melbourne yesterday; though if you look at a sequence of the last 3 nights 500 hPa charts, the upper trough associated with the front has moved on, while a separate lower-latitude cloudband trough has hung back.

It has been documented in the Bill Wright papers that this type of interaction between fronts and cloudbands (as was occurring yesterday before the two troughs separated amicably) is a major contributor to Victorian winter rainfall.  Also there is a cut-off low at 500 hPa developing under the cloudband and the GASP prog has this producing rain over the next couple of days.  Whether the climatologists/NWCB-counters will consider this as cut-off low rainfall or NWCB rainfall remains to be seen.

Bill Wright

Hello all,

        I have been trying to keep a low profile lately, with several work
issues pushing me for time. But since I heard my name mentioned, I will
"take ten" to clarify and elaborate on a couple of John's points re
Northwest Cloudbands

RE John's statement "I have often heard it stated that the NW cloudbands
are responsible for a large percentage of Australian wintertime
precipitation". This statement is true for a large part of western and
central Australia, where over 60% of the April-October rainfall is
contributed by NW cloudbands. Areas as far east as Charleville still receive
some 40% of their cool season precipitation from similar cloudbands, but
most of the productive bands here originate east of 130ºE, and it is
debatable whether these should be called NorthWEST cloudbands at all.

Southern and southeastern Australia don't receive a high proportion of
rainfall from cloudbands (15-30%), but as John flagged, much of this area
does receive a considerable amount (extra 20-35% of Apr-Oct rainfall) from
interactions between frontal systems and cloudbands - Northwest cloudbands,
and also cloudbands originating further east.

In this context, it is worth noting that much of the inland doesn't receive
much winter-spring rain, and good (or bad) seasonal conditions are dependent
on the occurrence (or non-occurrence) of a handful of good events (10mm
plus) in the cool season. Analysis has shown that in inland areas of South
Australia, southwestern NSW and northern Victoria, these occasional good
falls are mostly associated with cloudbands or their interactions.

RE the remark about Victoria: "... it has been documented in the Bill Wright
papers that this type of interaction between fronts and cloudbands.... is a
major contributor to Victorian winter rainfall."  My 1988 paper shows that
in northern Victoria the total contribution from cloudbands and their
interactions is of the order of 50%, but that the proportion drops
substantially in southern Victoria.

RE  "I also have heard it stated at seminars and conferences that warmer
SST's in that region are associated with an increased frequency of
winter-time NWCB's but the only literature I could find on  this is the
paper by Kuhnel... "
My Ph.D work (published Feb 1987) included correlation maps of SSTs versus
cloudband frequency and intensity in Victoria (results for northern Victoria
would be true of much of inland southeastern Australia). The correlation
maps showed the now-classic SST dipole pattern of positive/negative
correlations over the east Indian Ocean. Highly active cloudband years
coincided with negative SSTAs over the subtropical Indian Ocean, and
positive SSTAs over the tropical Indian Ocean and over oceans north and
northeast of Australia. By far the strongest contribution to the signal was
made by the TROPICAL SSTs (high correlations with cloudband activity), with
a weaker contribution by the oppositely-signed subtropical SSTs. This
suggests that the warmer SSTs around northern Australia are the primary
influence on cloudband activity, and/or on the synoptic patterns that favor
cloudband activity.

However I am not convinced the relationship directly reflects northwest
cloudband incidence: for one thing, the correlation pattern was strongest in
August-September, when northwest cloudbands start to decrease in frequency
and influence. The relationship probably reflects more a link between SSTs
and cloudbands originating further east, and their interactions, which
become more influential in the late winter-spring months.

Cheers all,
Bill Wright