Detecting and nowcasting Aviation Icing.

by Bodo Zeschke

Comments and questions

Posted by bodoz on
Dear Colleagues

Thank you for your contribution at the Aviation Icing presentation of 'Science Week'.

Here are some thoughts about how to incorporate aviation icing into tropical forecasts. Also a query about the issuing of a SIGMET for embedded storms.

One of the challenging questions pertains to forecasting aviation icing for tropical regions.
In particular, when to issue a SIGMET for this and what form this SIGMET should take.
That is why I included the Brunei - Jakarta flight scenario in my Science Week presentation.

From my own experience of forecasting at the Darwin Regional Forecasting Centre (RFC), it is very difficult to find an region of tropical cloud for which an icing SIGMET is appropriate.
That is because the atmosphere is so very convective.
Even an active monsoon flow will have embedded thunderstorms at some time during the diurnal cycle.
So large areas of cloud and convection are best defined by issuing a SIGMET for embedded Cb.

Ronik Kumar from our National Meteorological and Oceanographic Centre (NMOC) has done a lot of forecasting for tropical regions and he agrees with this.

Sooo...when do you issue a SIGMET for embedded Cb ?.
The Aviation Services Handbook is not very clear about this.
For example, one of the conditions for issuing a SIGMET for thunderstorms is that they are embedded within cloud layers and cannot be readily recognised. (ASH chapter 10 section 3.2.2)
But this says nothing about the size of the area which makes up the embedded storms, nor about their longevity.

Ronik Kumar (NMOC) mentioned that the area of embedded storms should be of dimension to warrant significant diversion of flights.
Also, storms that are purely diurnally driven are not likely to require a SIGMET.

But what about Mesoscale Convective Systems (large squall lines and Mesoscale Convective Complexes) ?.
I recall annotating "Embedded Cb" for large scale and long lived squall lines (gulflines) on the SIGWX charts when I was working at the Darwin RFC some years ago, and this was approved by the shift supervisor and NMOC.

Large scale Mesoscale Convective Complexes which form along a convergence zone also seem to satisfy this criterion.
These large complexes, 200-300 km in diameter can persist for 12 hours or more.
I think the west Kalimantan storm complex I presented was one such case.

So I think a SIGMET for embedded Cb is appropriate for long lived and large squall lines. But also for long lived and large scale slow moving storm complexes spawned within convergent zones.

What does everybody else think ?. Do you agree with the final sentence or not. If not, please give a reason.

Thank you for your input to this discussion.
Posted by bodoz on
Timothy Birch (Northern Territory Regional Office) replied with the following (7 August):

Hi Bodo,

The ASH states in 3.2 SIGMET Phenomena that SQL TS is a valid phenomenon in its own right and go on to describe a squall as an a line of TS about 100nm or more in length with little or seperation between the clouds (I presume the boffins are talking about AC/AS and CB). Are you suggesting we tend towards an EMBD TS SIGMET instead of a SQL TS for squall lines?

I have issued a SQL TS SIGMET previously, and reserve EMBD TS for MCS. Do we think this is an acceptable practice? I'd love to know what other tropical experts think!

Tim
Posted by bodoz on
Bodo Zeschke replied (7 August)

Hi Tim

Yes, I think that SQL TS is of course the most the most appropriate way to describe squall lines in a SIGMET. Thank you for clarifying this.

I also received feedback from Michelle Hollister, the Bureau of Meteorology Aviation Forecaster Competency Coordinator

Michelle mentioned that a SIGMET for EMBD CB should be issued for an extensive area of embedded storms, which would be about 100 nm and therefore a large enough area to require a major detour in the flight path.

Michelle mentioned that longevity (how long the area of embedded storms is maintained) is not as important as the area covered by the embedded storms, as the SIGMET will be issued on the AREA and the VALIDITY TIME will be affected by the lifetime of the embedded storms.

Michelle agrees that a SIGMET for EMBD CB is appropriate for long lived and large scale slow moving storm complexes spawned within convergent zones.

Regards

Bodo
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