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From greve at lowtem.hokudai.ac.jp  Tue Jan 28 10:03:45 2014
From: greve at lowtem.hokudai.ac.jp (Ralf Greve)
Date: Tue, 28 Jan 2014 18:03:45 +0900
Subject: [SICOPOLIS:28] Numerical stability when using the polythermal mode
Message-ID: <52E77271.3070205@lowtem.hokudai.ac.jp>

Dear colleagues,

while working on the enthalpy method (CALCMOD==2) in SICOPOLIS, Heinz 
Blatter and I also had a closer look on the polythermal mode 
(CALCMOD==1) that has been implemented in SICOPOLIS since the very 
beginning.

The polythermal mode is sometimes problematic for numerical stability 
because, in certain situations, it produces a rapid thickening of the 
temperate-ice layer ("skyrocketing CTS") [CTS = cold-temperate 
transition surface] in some small areas that causes the simulation to 
crash. With revisions 461 and 463 I implemented a new option that allows 
to ignore freezing conditions at the CTS and always assume melting 
conditions (new parameter CTS_MELTING_FREEZING==2). While this is a bit 
cheated, freezing conditions do not occur very often anyway (see Fig. 4 
in Greve 1997, J. Climate 10, 901-918), so that it does not make so much 
difference, and it is _very_ beneficial for numerical stability: The 
"skyrocketing CTS" problem is much less severe and simulations therefore 
much more stable.

Highly recommended! If you want to use this new option, please check out 
a new version of the code (revision 463 or newer). The header files in 
the repository have all been changed to the new setting. If you wish to 
modify existing header files, simply add

#define CTS_MELTING_FREEZING 2
!                    Treatment of the transition conditions at the CTS
!                    (only for CALCMOD==1):
!                    1 : Melting and freezing conditions distinguished
!                    2 : Always melting conditions assumed

after the definition of CALCMOD.

Of course, if you come across any issues, please let me know.

Best regards

Ralf

-- 
Dr. Ralf Greve
Professor, Glacier and Ice Sheet Research
Institute of Low Temperature Science
Hokkaido University
Kita-19, Nishi-8, Kita-ku
Sapporo 060-0819, Japan
Phone: (+81)-(0)11-706-6891
greve at lowtem.hokudai.ac.jp
http://wwwice.lowtem.hokudai.ac.jp/~greve/

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