Answered P3D 5.2 HF1 + ASP3D: rain displayed with external -30 ° C.

[RobPol471]

I just finished a LIMC - LEBL flight and at a cruising 28,000 feet I encountered a strong TS for about 80 miles with lightning and thunder all the time all very nice but I found it very strange that with an outside temperature of -30 C ° heavy rain was displayed which continued up to 7000 feet approaching LEBL where the outside temperature
it had risen to + 10 C °.

 

[damian]

We adhere to the surface condition and aloft condition data temps, and if precip is indicated in the METAR, we also depict it accordingly. With +TSRA for example it is common to see heavy rain under and in cloud, with hail possible under the anvil section depending on tstorm intensity and height, other moisture and stability conditions parameters, and some randomness.

Did you expect snow or hail?

Rain doesn't necessarily freeze at 0C. 0C is more of a melting point vs. a freezing point when it comes to icing/freezing of visible moisture. Freezing requires nuclei (small particles, ice crystals, etc.) and when it doesn't freeze you have supercooled water droplets, water not frozen but below 0C. The most dangerous kind is called SLD (supercooled large droplets) and is very common within thunderstorms. This non-frozen water begins to freeze quickly when it contacts the airplane (now having something to freeze to) causing severe icing that runs back over the wing and control surfaces.

It's not until below -40C that supercooled water droplets will certainly freeze without surrfaces/external nuclei. Below -15C there begins to be more frozen ice crystals vs. supercooled water droplets but the unfrozen water still exists.

tl;dr: This is normal and expected, and realistic and dangerous aspect of aviation weather that should be avoided. See https://www.skybrary.aero/index.php/Supercooled_Water_Droplets

 

[RobPol471]

Hi Damian,

thank you for your prompt reply.

Regarding the METAR and here I don't know if it depends on P3D but I had the feeling that if there is a TS + RAIN is not considered the external temperature which obviously varies in relation to the altitude and I say why the rain that was displayed to me from 28000 feet with -30 ° C up to about 9000 feet when the outside temperature began to exceed zero degrees, in terms of the size of the drops that were displayed and that "ran on the glass" has "always been the same" and I'm sorry but in my many "real" flights that began in the distant ... 1970, I have never seen
"rain" and the drops run on the windows when the plane was at high altitude and with outside temperatures of tens of degrees below zero.

Regarding your question if I was expecting snow or hail, considering the 28000 feet I was expecting possibly of see... ice formation while hail obviously possible, I've never seen it displayed on any P3D except hear only the sound.

Thanks again,

Roberto.

 

[BAWV12]

If I remember correctly, super cooled droplets blocked all Pitots on AF447 and this effect in combination with badly trained crew started the chain of the events ended with crash.

 

[RobPol471]

After a satellite search with the help of various METARs I was able to identify a severe TS + RA situation above the apt LGKR (Corfu).

Well, as the photo shows, the rain continued, in this case up to 22500 feet with an outside temperature of 0 ° F which corresponds to -17 ° C.

http://imgur.com/YHK5YuZ

 

[RobPol471]

From further testing it appears that this situation is generated by P3D 5.2 HF1.

The test that I have done, and that everyone can do, is to use only the default weather of P3D, for example with the default situation MAJOR THUNDERSTORM in the WHETER of P3D but by raising the height of the clouds and the maximum that P3D me he accepted it was 30,000 feet and this time instead of stopping the rain at zero degrees it continued down to -1 ° F which correspond at -18 ° C at a height of about 20,000 feet.

So it seems that P3D 5.2 HF1 stops the rain when it reaches 0 ° C but only if the height of the clouds is not very high while it continues to rain even with temperatures far below freezing if the clouds reach high heights.

 

[damian]

As mentioned the rain drops themselves (in the air) are normal when below freezing, until you are WELL below freezing. -15C and unfrozen water is not entirely uncommon. -20C is probably a good standard limit but there can still be unfrozen water until about -40C. There are several factors at play with ASP3D precip and temperature depictions and you will likely see rain possible until below -20C with some other rare cases down to -30C.

Based on your response I think the issue you are describing here is that P3D (by itself) doesn't visually depict icing, e.g. whereas IRL the water droplet hits the windshield and then freezes instantly, you only see the rain drop/windshield effects in sim. It does simulate icing and ice accrual along with degraded performance and lift, with increased drag, although we feel the internal effects here could use some "buffing". There may be some add-on aircraft that have provided visual icing windshield effects. Since the potential visual depictions require close integration with specific aircraft, this is not something that a wx add-on can do alone, although we have been looking at some potential ideas moving forward.