850hpa level temperatures

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850hpa level temperatures

Post  droughtbreaker on Sun May 17, 2009 6:32 pm

I'm just wondering what the general rule is for deriving surface (or near surface) temps from 850Ts at different times of the year. I notice in mid summer the figure of add 16C or 17C is often used but what is it at other times of year where the amount of solar radiation fluctuates from season to season (obviously). With the warm period up ahead I'd be interested to know this as I'm a bit confused atm as to how the forecast temps are derived from the model output.

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Re: 850hpa level temperatures

Post  hillybilly on Mon May 18, 2009 6:12 pm

At this time of year it is so variable that it's hard to make a rule. Very often you fail to establish a well mixed layer between the surface and 850hPa which means the dry adiabatic rule of summer of 10C per km falls short (actually a super adiabatic rule is used in summer which gives you ~17C instead of the 13-14C you would get with the adiabatic lapse rate).

As a reasonable guide a 0C 850T in winter usually translates to about 14C in Melbourne (give or take 1C)... but I've seen a differences from around 0C to around 20C. Often the best thing to do is look at the low level model temperatures - LAPS has 1000hPa temperature and add 1-3C (depending on surface pressure) to these to get an idea of a maximum temperature. Today's a good example with the 1000hPa T at 4pm ranging from around 13C in the south to 18C in the north and maximums which went from about 14-15C to near 20C.

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Re: 850hpa level temperatures

Post  droughtbreaker on Mon May 18, 2009 6:26 pm

Thanks for that David. 15C here today which is a fairly warm one for this time of year, 18C in Melbourne.

A week of 20C is a bit ridiculous for this time of year but probably normal now given global warming's effects.

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Re: 850hpa level temperatures

Post  Karl Lijnders on Mon May 18, 2009 7:56 pm

Some of the upper level temp progs could support temps closer to the middle twenties, or spiking to that level with the northerly wind rushing down the slopes into Melbourne (foehn wind perhaps?) and thus giving places like Avalon the chance of cracking into record territory.

Certainly a good time to test the upper temp rule/guide.
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Re: 850hpa level temperatures

Post  hillybilly on Tue May 19, 2009 6:47 am

EC has 850T locally up to 11C on Fri/Sat/Sun. If it goes adiabatic that could seem temperatures around 25C on each of these days. Mildura has low 20s progged right through this sequence.

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Re: 850hpa level temperatures

Post  AUSSKY on Mon Aug 31, 2009 5:22 pm

you can always check the temperature in Trentham at 700m...to give you an idea of the low level lapse rate at the time (knew we were useful for something!)
http://trentham.bsch.au.com/FrogIsla/Current_Vantage_Pro.htm
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Re: 850hpa level temperatures

Post  Karl Lijnders on Wed Sep 02, 2009 5:59 pm

Excellent Jane!! Could be quite useful tomorrow Smile
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