Date: Tue, 23 Aug 2005
From: John McBride
To: synoptic_discussion@bom.gov.au
Subject: [synoptic_discussion] More on... "Earth has not anyhting to show more fair"
 

I was talking to Graham Mills about the PVA (or according to John Nairn CVA) max moving
northward.  I said the deep convection was moving with it.

However, Graham proposes the convection moves only a very small distance and new convective
cells pop up as the mid tropospheric destabilizing cold air field moves northward and rotates.

As Graham says, the deep Cb cells probably only have a lifetime of a couple of hours; so they
are left behind as the system moves on.  Thus the convective effect is like a wave, with the
disturbance moving on, and in-situ convection popping up as it goes over.

Does this make sense?

John McB

Still More on "Earth has not anything to show more fair"
Hello again,

In response to my question as to whether the forecast models produced convection in the cold air field corresponding to the PVA maximum, Lawrie  Rikus sent me a link to some of his work.

Lawrie produces routinely synthetic satellite images.  Quoting  from his web page:

"These products are derived from the Bureau of Meteorology's Numerical Weather Prediction models. They represent the satellite image that would be seen by the infrared channel of the GMS-5 geostationary satellite. The forecast loops are available for all the Bureau's numerical weather prediction models and are updated once a day after one of the twice daily analysis/forecast cycles. The comparisons with actual satellite data are a form of validation of the technique and are also updated once a day. The comparison involves the averaging of the satellite brightness temperature field onto the approriate model grid."

On my own web-page I have placed a numbr of Lawrie's synthetic satellite images, from the MesoLAPS run beginning at 1200 UTC on the 16th.  Beside each image I have placed the still from the infrared satellite loop.

You will see the model does a pretty good job, as it produces a white blob, corresponding to cold cloud tops, in about the right place and with the right orientation.  It is difficult to tell from Lawrie's display whetehr it is convective or not.  I assume the effect of convection in the model is to increase both water-vapour and liquid water at upper levels over the entire grid box.  Hence we see a stratiform-type effect in the synthetic satellite image.  To be sure of this, someone would have to dig further into the model output.

          1200 UTC 16 August

 

            1500 UTC 16 August

            1800 UTC 16 August

            0000 UTC 17 August

                1200 UTC 17 August