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Post by greysrigging on Jul 13, 2021 20:42:47 GMT -5
I must admit I struggle with how it works 'out in the field'. I'm familiar with the Aussie 'rule of thumb' of 1c per 100m altitude ( or thereabouts ).
What local conditions/features does the lapse rates fail to take into account ? Ie the extreme heat at altitude in the high deserts of SW USA, the high veldt of South Africa, even the modest altitudes of Central Australia have seen some extreme temps ie a 49c at Leonora, WA ( alt 376m ).
I've often heard AA mention the BOM fails with the lapse rates in South Eastern NSW and North Eastern Victoria, and I think he's likely right. Here are the next 7 days BOM max temp forecasts for select towns/sites in the above mentioned regions. I've listed the sites by altitude in meters above sea level. Be interesting to see how close they are to the actuals.....Forecasts as of 14th July.
Albury, NSW ( 165m ) - 12c, 15c, 11c, 9c, 12c, 11c, 11c.
Wagga, NSW ( 180m ) -13c, 16c, 12c, 10c, 11c, 11c, 11c.
Tumut, NSW ( 305m ) - 13c, 15c, 12c, 9c, 11c, 12c, 11c.
Corryong, VIC ( 314m ) - 11c, 13c, 11c, 10c, 11c, 12c, 10c.
Bright, VIC ( 319m ) - 11c, 12c, 10c, 8c, 11c, 11c, 10c.
Khancoban, NSW ( 337m ) 10c, 11c, 8c, 7c, 9c, 9c, 9c
Mt Beauty VIC ( 366m ) - 10c, 10c, 8c, 6c, 8c, 8c, 8c.
Strathbogie VIC ( 502m ) - 10c, 12c, 9c, 7c, 10c, 9c, 9c.
Beechworth VIC ( 560m ) - 10c, 13c, 9c, 8c, 11c, 10c, 10c.
Tumbarumba NSW ( 645m ) - 10c, 12c, 9c, 7c, 9c, 10c, 8c.
Omeo, VIC ( 685m ) - 10c, 12c, 9c, 7c, 9c, 10c, 9c.
Stanley VIC ( 800m ) - 8c, 9c, 7c, 6c, 8c, 7c, 7c.
Cooma, NSW ( 800m - 12c, 15c, 12c, 11c, 13c, 12c, 13c.
Jindabyne NSW ( 915m ) - 9c, 11c, 8c, 6c, 8c, 9c, 8c.
Hunters Hill VIC ( 932m ) - 8c, 8c, 6c, 3c, 6c, 5c, 5c.
Cabramurra NSW ( 1488m ) 4c, 4c, 2c, 0c, 1c, 2c, 1c.
Falls Creek, VIC ( 1765m ) 1c, 2c, 0c, -2c, 0c, 0c, -1c.
Mt Hotham, VIC ( 1849m ) 1c, 1c, -1c, -3c, -1c, -1c, -1c.
Thredbo Top, NSW ( 1957m ) 0c, -1c, -1c, -2c, -2c, -1c, -1c.
I'm sorta basically familiar with the lapse rates in the Pilbara, even the altitude of Tom Price at 700m offers some relief from the heat of the coastal lowlands.
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Post by edmountain on Jul 14, 2021 0:04:24 GMT -5
The DABL has nothing to do with the BOM. It's a physical quantity that's calculated from thermodynamics. It doesn't work "in the field", it's a non-negotiable constant (modulus the assumptions that go into the calculation--but they're not really that negotiable either) whose value is always 9.8 °C/1000 m everywhere.
Perhaps you're thinking of the environmental lapse rate which, by definition, changes with the environment and strictly speaking only applies to the atmosphere directly overhead a particular location at a particular time.
With respect to temperatures in different locations at different elevations, they are only only very loosely approximated by the DABL because the energy exchange between the ground and the air violates the assumption of adiabaticity. Roughly speaking (no pun intended) the degree of violation depends on topography with smooth topography allowing more energy exchange (less adiabatic). That is how locations in the smooth great plains of the USA can get so hot but once you climb up the Rockies the lapse rate increases significantly.
I would speculate the same phenomenon happens in Australia: relative low lapse rates along the smooth Murray basin but higher lapse rates in the Snowies.
My prediction: the BOM forecasts will be pretty good, especially for the first few days.
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Post by greysrigging on Jul 14, 2021 0:15:51 GMT -5
The DABL has nothing to do with the BOM. It's a physical quantity that's calculated from thermodynamics. It doesn't work "in the field", it's a non-negotiable constant (modulus the assumptions that go into the calculation--but they're not really that negotiable either) whose value is always 9.8 °C/1000 m everywhere. Perhaps you're thinking of the environmental lapse rate which, by definition, changes with the environment and strictly speaking only applies to the atmosphere directly overhead a particular location at a particular time. With respect to temperatures in different locations at different elevations, they are only only very loosely approximated by the DABL because the energy exchange between the ground and the air violates the assumption of adiabaticity. Roughly speaking (no pun intended) the degree of violation depends on topography with smooth topography allowing more energy exchange (less adiabatic). That is how locations in the smooth great plains of the USA can get so hot but once you climb up the Rockies the lapse rate increases significantly. I would speculate the same phenomenon happens in Australia: relative low lapse rates along the smooth Murray basin but higher lapse rates in the Snowies. My prediction: the BOM forecasts will be pretty good, especially for the first few days. No, no, I understand what they are, and have read ( if barely understood ) the thermodynamics of it etc. Was looking into historical and current temp anomalies and yes, topography/location obviously plays a large part. The 47.2c at Las Vegas the other day has prompted my curiosity, which I thought a rather high reading for the altitude. And yes, like national forecasters everywhere, the first 3 or 4 days are generally OK, 4 to 7 days out is a lottery re accuracy. |
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Post by knot on Jul 14, 2021 0:35:42 GMT -5
As Edmountain already said, it's largely dependant on geography. I will add however that clouds are naturally attracted to exposed high-points like hills and slopes, and thus maxima will be markedly cooler relative to altitude—this can be seen easily just by looking at a station's RH% during a weather event; the higher the RH%, the heavier the cloud cover.
Assuming a 1 km LCL and both on the windward side: an exposed hill/slope of just 450 m, can end up with a cooler maximum than a flat plateau/valley of 800 m. This is because the 450 m hill/slope will remain at 99% RH for much of the event (upslope fog), but the plateau would struggle to even pass 90% RH and may even catch a sunny break. I like to refer to valleys/plateaus as "sun holes" for this reason.
The reason why BoM often gets the forecasts wrong on the South West Slopes (my region), is because they erroneously apply the dry adiabatic lapse rate, instead of the more accurate wet adiabatic lapse rate. This can be seen with a recent forecast example featuring Cabramurra (1,482 m) and Hunters Hill (981 m)—both are exposed stations, but on the post-frontal days Cabramurra's maxima are shown to be 4° C (!) cooler than those at Hunters (1° C / 125 m). But in reality, there should only be a 2° C difference in maxima between Hunters and Cabra in a W/NW flow (closer to 1° C / 200 m): so if Cabra maxes at –1° C, Hunters should max at 1° C. TWC does a much better job at this, which is why I always use it for W/NW frontal events.
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Post by edmountain on Jul 14, 2021 9:16:55 GMT -5
No, no, I understand what they are, and have read ( if barely understood ) the thermodynamics of it etc. Was looking into historical and current temp anomalies and yes, topography/location obviously plays a large part. I mean when you create a thread that is literally titled "Who Can Explain in Layman's Terms?" what else did you expect other than an explanation?  The 47.2c at Las Vegas the other day has prompted my curiosity, which I thought a rather high reading for the altitude. And yes, like national forecasters everywhere, the first 3 or 4 days are generally OK, 4 to 7 days out is a lottery re accuracy. Partial list of maximum temperatures in °C for July 14 compared to the forecasts you posted: Albury 11.4 (12) Wagga Wagga 10.9 (13) Khancoban 9.6 (10) Omeo 9.1 (10) Cooma 11.6 (12) Hunters Hill 5.8 (8) Cabramurra 3.2 (4) Mount Hotham 0.4 (1) Thredbo Top 0.4 (0) |
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Post by edmountain on Jul 14, 2021 11:32:58 GMT -5
The reason why BoM often gets the forecasts wrong on the South West Slopes (my region), is because they erroneously apply the dry adiabatic lapse rate, instead of the more accurate wet adiabatic lapse rate. This can be seen with a recent forecast example featuring Cabramurra (1,482 m) and Hunters Hill (981 m)—both are exposed stations, but on the post-frontal days Cabramurra's maxima are shown to be 4° C (!) cooler than those at Hunters (1° C / 125 m). But in reality, there should only be a 2° C difference in maxima between Hunters and Cabra in a W/NW flow (closer to 1° C / 200 m): so if Cabra maxes at –1° C, Hunters should max at 1° C. TWC does a much better job at this, which is why I always use it for W/NW frontal events. That's not the reason; modern weather prediction is far more sophisticated than that. Adiabatic heating and cooling are just a small part of the physical processes incorporated into the models. These physical processes are described by the primitive equations shown below; the adiabatic heating and cooling part is the term highlighted in yellow.  Source: cpb-us-w2.wpmucdn.com/sites.uwm.edu/dist/8/663/files/2019/09/02-ModelFormulation-1.pdfIf there's a bias in the BOM forecasts it's probably because of limitations in spatial resolution of the model or errors in parameterization of sub-grid-scale processes not captured in the equations such as snow cover, snow melt, evaporation, vegetation, soil properties, incoming solar radiation, condensation, turbulence, reflection, scattering by aerosols, deep convection, or sensible heat flux. |
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Post by edmountain on Jul 15, 2021 9:08:27 GMT -5
Maximum temperatures July 15 in degrees Celsius compared with the forecasts issued the morning of July 14:
Albury 15.5 (15)
Wagga Wagga 16.5 (16)
Khancoban 13.6 (11)
Omeo 12.6 (12)
Cooma 13.2 (15)
Hunters Hill 7.7 (8)
Cabramurra 4.6 (4)
Mount Hotham 1.2 (1)
Thredbo Top 0.9 (-1)
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