Island-induced Convection over Guadalcanal
Islands in warm tropical seas can create convection through a combination of factors that work together to foster deep convection. A recent example from the island of Guadalcanal in the Solomon Islands during the day of Monday 20 April 2026 illustrates this. Guadalcanal is famous as the site of a pivotal campaign of land and naval battles during World War II but today is known as a lush island of tropical rainforests, beaches, and the home of the nation’s capital city Honiara.
There are multiple ways that an island can foster convection. First and foremost, during the day, the island heats up faster than the surrounding ocean. Hot air rising, of course, is at the heart of deep moist convection. But this is enhanced by two additional factors: coastal breezes and orographic lift. Coastal breezes are a frequent byproduct of daytime island heating. Intro to Meteorology textbooks and popular media often simplify this as the air over the land gets warm causing it to rise while cooler ocean-based-air rushes in to fill the gap. The truth is more complex but more interesting; if you really want to know more it’s at the end of this post (be warned, though, it’s pretty technical). For now, though, we’ll note that as the island heats up it causes an offshore breeze to form. For a large continental landmass, this breeze can penetrate inland dozens of kilometers. For a small island, the breeze can’t penetrate that far because it will soon collide with the breeze coming from the other side of the island. This creates a low level convergence zone which enhances the upward lift caused by the daytime heating. Finally, the terrain of the island also has an impact. Many of these tropical islands are quite rugged, which means that as the wind rushes onshore it undergoes orographic lifting by the many mountains. This enhances the upward motion even more. This map of the terrain of Guadalcanal (courtesy of Wikipedia) shows that the island is dominated by an east-west ridge. Any flow from the north or the south is going to be enhanced by this vertical lift.
We’ll take a look at Guadalcanal on this day, starting with the true color view from the Himawari-9 Advanced Himawari Imager (AHI). Guadalcanal is the island at the center of this animation. The loop starts at 2130 UTC, which is 8:30 AM local time. The island starts clear, and is a beautiful deep green. For those of you who are mostly familiar with looking at GOES satellite imagery, you might think it is green because of the tropical vegetation that isn’t normally seen over the continental United States. However, it’s important to remember that the AHI has a true green channel unlike the US geostationary satellites and so its colors are going to appear a little more saturated. Shortly after the start of this loop localized convection forms across the island. However, if you watch the animation to the end, howver, you see that the convection starts to get squeezed into the middle of the island by the sea breezes on either side, forming a band of clouds that runs alongside the island’s mountainous spine.
It may be good to evaluate the kind of environment in which this convective development is taking place. Here’s a plot of the NOAA Physical Sciences Laboratory satellite-observed sea surface temperatures (SSTs). The Solomon Islands are circled in yellow, and it’s clear from this map that SSTs are a toasty 29-30 C (84-86 F). As such, there’s a lot of latent heat available to support convection.
Furthermore, the vertical structure of the atmosphere is also supportive of convection. Here is a skew-T plot of a nearby NUCAPS-retrieved vertical profile at 0338 UTC (2:38 PM local time), provided courtesy of the NUCAPS Savvy tool maintained by NASA SPoRT. The profile is generally well-mixed, with a near dry-adiabatic low level and a largely moist-adiabatic environment aloft. The instability is large, however, with CAPE values ranging from over 3200 to nearly 4300 J/kg depending on the values used. If low-level air parcels can ascend to around 1400-1600 m, they’ll breach the level of free convection and ascend on their own.
However, remember that terrain map above? That central mountainous spine has an elevation of around 1000-1200 m, so much of the work is already being done by orographic lift. The convergence and daytime heating doesn’t have to be very strong at all to unlock deep moist convection. Let’s see what happens over the next few hours. This loop is the same True Color RGB product as earlier, but now runs from 0100 to 0500 UTC (noon to 4:00 PM local) and covers the initiation of the deep convection.
Looking at the same period but through the Band 13 (10 micron) infrared channel helps to show how deep this convection was in a way that’s not possible with the true color alone. Here we see a burst of cold brightness temperatures as the convection ascends to the tropopause.
Additional RGB products can provide useful insight as well. The Day Convection RGB shows bright yellow plumes where the vertical growth is the most extensive while the red clouds are where ice particles can be found.
The Day Microphysics RGB also highlights some useful information. The salmon and golden areas indicate thick clouds with small water droplets (salmon) or ice particles (golden) which are commonly associated with deep convection. The green clouds are high-level ice clouds, which show that the cloud top is glaciating and thus the storm is transitioning rapidly to the mature stage.
Despite the strong forcing and favorable thermodynamics, this storm didn’t last very long. One possibility is that the deep convection to the south created an outflow boundary, which can be seen in the satellite loops as lines of clouds propagating northward. This cold, dense air would be kryptonite to any deep convection and cut it off just as it was getting going. At the same time, the day was getting late and the natural solar heating was dying out. Here’s the Band 13 view of the end of this event, from 0500 to 0900 UTC (4:00 PM to 8:00 PM local time). Sunset is around 0715 UTC (6:15 PM local time).
As promised, here is a more detailed description of coastal breeze formation. This post’s author has a special interest in coastal breeze formation (as depicted in this paper published in the Journal of Atmospheric Sciences) so he’s happy to take the time to offer a more precise explanation. As is well-known, the land heats up more readily than the water does due to differences in thermal heat capacity. This means that the air that is adjacent to the land also gets warmer than the air over the water. This creates a density gradient from land to sea, which in turn results in localized baroclinicity (we weren’t kidding when we said this was going to be a technical explanation) as the isobars remain largely static while the isopycnals tilt from parallel to the isobars before the heating to crossing them afterward. According to Kelvin’s Circulation Theorem, circulation in a baroclinic fluid is constant with time, but a barotropic fluid is going to cause a change in the circulation. Therefore, as the environment transitions from baroclinic to barotropic, it goes from zero circulation to a measurable one. That circulation is the coastal breeze.
