Warm rain (redux)

This I had to post: one of my buddies – Droopy – saw my post on Warm rain, and sent me a mail that one of his friends sent him. This one about warm rain in Mecca, on June 5th.

Droopy’s friends are oceanographers and climate scientists, so it isn’t surprising that the mail he forwarded was full of explanation for the warmth of the rain.

First, the news article that reported this:

June 7, 2012 – SAUDI ARABIA – Pilgrims to the holy city of Mekkah (Mecca), Saudi Arabia must have been astonished on Tuesday afternoon, when the weather transformed from widespread dust with a temperature of 113°F (45°C) to a thunderstorm with rain. Remarkably, the air temperature during the thunderstorm was a sizzling 109°F (43°C), and the relative humidity a scant 18%. It is exceedingly rare to get rain when the temperature rises above 100°F, since those kind of temperatures usually require a high pressure system with sinking air that discourages rainfall. However, on June 4, a sea breeze formed along the shores of the Red Sea, and pushed inland 45 miles (71 km) to Mekkah by mid-afternoon. Moist air flowing eastwards from the Red Sea hit the boundary of the sea breeze and was forced upwards, creating rain-bearing thunderstorms. According to weather records researcher Maximiliano Herrera, this is the highest known temperature that rain has fallen at, anywhere in the world. He knows of one other case where rain occurred at 109°F (43°C): in Marrakech, Morocco on July 10, 2010. A thunderstorm that began at 5 pm local time brought rain at a remarkably low humidity of 14%, cooling the temperature down to 91°F within an hour. Thunderstorms often produce big drops of cold rain, since these raindrops form several thousand meters high in the atmosphere, where temperatures are much cooler than near the surface. Some drops even get their start as snow or ice particles, which melt on the way to the surface. Additional cooling of the drops occurs due to evaporation on the way down. However, in the case of the June 4, 2012 Mekkah storm, I think the rain was probably more like a hot shower. Thus, the thunderstorms’ raindrops would have been subjected to 100 seconds of some very hot air on the way to the surface, likely warming them above 100°F by the time they hit the ground. With the air temperature a sizzling 109°F (43°C) at the time of the June 4 thunderstorm in Mekkah, the raindrops could easily have been heated to a temperature of over 105°F (41°C) by the time they reached the surface! On Saturday, June 2, the temperature in Mekkah hit 51.4°C (124.5°F), a new record for the city, and just 1.1°F (0.6°C) below the all-time hottest temperature record for Saudi Arabia (125.6°F, or 52°C, recorded at Jeddah on June 22, 2010.) I expect that 20 – 40 years from now, we’ll begin seeing occasional cases where rain falls at a temperature above 117°F (47°C) in the desert regions of North Africa and the Middle East.

The following email was written by Lance Bosart, my friend’s friend:

Life has been a bit tough in Mecca, Saudi Arabia, of late. First they had the hell (51.4 C) on 2 June 2012, a new record for the city, and then they had the “hot” water three days later (5 June) in the form of a thunderstorm (try not to cringe too much at some of the purported physical explanations such as “high” surface pressure over an intensely heated desert). Motivated by the story, I pulled up the metar observations (appended below) for Mecca (OEMK) and Jedda (OEJN). OEJN is located on the coast of the Red Sea and OEMK is situated ~70 km inland.

I had to look up metar. It’s apparently a format for reporting weather information. More info at http://en.wikipedia.org/wiki/METAR. But to continue:

The OEJN metar observations show typically hot and humid conditions with a climatological expected WNW wind down the Red Sea. Inland at OEMK, a temperature/dew point of 47/12 C was reported at 1100 UTC 5 June with a wind from 2204 (the wind was calm in previous hours. At 1200 and 1300 UTC, the temperature/dew point was 45/17 C with continued light WSW of 4-5 kt. The observed 5 C increase in dew point is suggestive that modified coastal moisture reached far inland. The attached SLP, 1000-500 hPa thickness, and precipitable water (PW) analysis (courtesy of Heather Archambault) shows PW values between 22-30 mm over the Red Sea and thickness values near 588 dam over most of Saudi Arabia.

I assume ‘dam’ means decameter. But I may be mistaken. SLP is sea-level pressure. Here is the SLP diagram:

Lance continues:

A 1200 UTC 5 June, an attached sounding taken from Al-Madinah (40430), Saudi Arabia, (located between Jedda and Mecca), courtesy of the University of Wyoming, shows a surface-based mixed layer extending to 550 hPa and an airmass devoid of CAPE and shear. Of interest is the shallow layer of midlevel moisture between 550-500 hPa. If a surface-based air parcel could reach 500 hPa then a high-based thunderstorm could be possible. Modifying the Al-Madinah sounding to reflect the observed surface dew point increase of 5 C to 17 C at Mecca, shows that mid-level CAPE would be present. At issue, is whether the the observed surface moistening and (presumed) weak convergence with the implied sea-breeze passage at Mecca was sufficient to allow air parcels to reach their LFC? Given the reported thunderstorm and rain, and the “cooling” and drying to 40/12 C in the 1400 and 1500 UTC meter observations, the question arises as to how much rain must have fallen to allow the observed cooling. The absence of any reported strong winds during the time a thunderstorm and rain was reported suggests that the rain was scant and likely evaporated well above the surface. I don’t have a good explanation as to why the surface dew point decreased back to 12 C under these conditions.

This is, unfortunately, a bit dense for me. I tried googling CAPE and I think it stands for ‘convective available potential energy’. But I couldn’t really understand what it means in this context. CAPE – http://en.wikipedia.org/wiki/Convective_available_potential_energy

Similarly, I don’t really understand what a ‘high-based thunderstorm’ means. LFC means ‘level of free convection’. Again, all I can do is to point you to the Wikipedia page: http://en.wikipedia.org/wiki/Level_of_free_convection

So if I understand this right – and most probably I don’t – sea breeze met moist air in an area of high pressure, and this caused the dew point (at which water vapor becomes rain) to become rather high, causing rain.

This is real science, folks! If anyone can explain it to me, I would be really grateful.

Here is another curve that Lance attached. It is called a Skew-T plot, and it maps pressure to specific volume. It’s a thermodynamic thing – I know nothing more.



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