Diurnal development of the sea-breeze front in Yemen.
22 October 2020
26 May 2012
by Jochen Kerkmann and HansPeter Roesli (EUMETSAT)
MSG has two water vapour (WV6.2 and WV7.3), one ozone (IR9.7) and one carbon dioxide channel (IR13.4) to observe air mass characteristics such as moisture content, ozone content, tropopause height and static stability. In addition, MSG has three window channels (IR8.7, IR10.8 and IR12.0), which allow one to retrieve parameters such as total precipitable water and lower-level humidity.
Because water vapour absorbs slightly more at the IR12.0 than at IR10.8 window channel, the brightness temperature differences (BTD) IR10.8-IR12.0 is directly related to the humidity content, which is dominated by the low-level humidity (assuming a normal (non inverted) temperature profile and neglecting surface emissivity effects). This means, the larger the BTD (e.g. IR10.8-IR12.0) the higher the low-level absolute humidity. However, this only works under standard atmospheric conditions when the temperature, especially in the lowest layers, decreases with height. In the case of low-level isothermal conditions or deep temperature inversions the BTD will be very small (or even negative) and not related to the humidity content.
Among the recommended RGB composites for MSG, the Dust RGB uses both BTDs: the BTD IR12.0–IR10.8 on the red colour beam (range: from -4K to +2 K) and the BTD IR10.8-IR8.7 on the green colour beam (range: from 0 K to +15 K). In addition, it uses the IR10.8 channel on the blue colour beam (range: from 261 to 289 K). Thus, besides monitoring dust and ash clouds and tracking of thin, ice clouds, it is also useful for the detection of low-level moisture boundaries.
During daytime (neglecting again spatial variability in surface emissivity), moist air will have a less reddish colour than dry air because of a more negative BTD IR12.0-IR10.8 (typically around -3 to -4 K for moist air and -1 to 0 K for dry air). During night-time, the detection of moisture boundaries is reduced because of low-level temperature inversions.
A classic example of a moisture boundary is the sea-breeze front. According to Wikipedia, a sea-breeze (or onshore breeze) is a wind from the sea that develops over land near coasts. It is formed by increasing temperature differences between the land and water; these create a pressure minimum over the land due to its relative warmth, and forces higher pressure, cooler air from the sea to move inland.
The sea-breeze front is a weather front (convergence zone) created by the sea-breeze. The cold/moist air from the sea meets the warmer/drier air from the land and creates a boundary like a shallow cold front. Depending on its strength, this front may create cumulus clouds, and if the air is humid and unstable the front can sometimes trigger thunderstorms. At night, the sea-breeze usually changes to a land breeze, due to a reversal of the same mechanisms.
The case below shows the diurnal development of the sea-breeze front in Yemen on 26 May 2012, as seen in the Meteosat-9 Dust RGB product. Starting at around 6:00 UTC, the cold, moist air from the sea (in blue along coastline on lower left) moves inland where it meets the warmer, dry air from the land (in more reddish colour). At around 13:00 UTC, the sea-breeze front reaches its maximum penetration inland. The colour difference between the two airmasses comes mainly from the red component of the RGB product: moist air has much less red than dry air.
In this particular case, the colour contrast is stronger than normal because of a very strong moisture contrast due to a large dust outbreak that occurred on the previous day (25 May 2012, see hourly animation (25 May/00 UTC–29 May/11 UTC)). The dry, dusty air coming from the north (transported by Shamal winds) meets the cool, moist air from the Red Sea and the Arabian Sea (transported inland by the sea breeze).
The development of the sea-breeze front can be nicely monitored in the hourly animation. Note that, as discussed above (and helped by the strong heating of high mountain areas in Yemen), several thunderstorms develop along the sea-breeze front on all days of the animation (25 to 29 May 2012). The earliest and strongest convective development often occurs in the area where both sea breezes (the one from the Red Sea and the one from the Arabian Sea) converge, as shown in the slides .
Animation 1 (large area) (25 May/00 UTC–29 May/11 UTC)
Animation 2 (zoom) (25 May/00 UTC–29 May/11 UTC)
Convergence of seabreeze fronts over the Salento Peninsula, Italy (12 June 2007)
Moisture boundary across the Arabian Peninsula (15 March 2006)
Monitoring airmass/moisture boundaries with MSG (4 October 2005)
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