Fair Weather Doesn’t Mean Boring
It may be tempting to look out on a pleasant, synoptically quiescent day, and think that there’s not much going on. However, one of the great things about being a meteorologist is that we always seem to be able to find things worth looking at even in the most mundane of environments. For example, let’s look at this broad field of fair-weather cumulus across the southeastern continental United States, as captured by GOES-19’s (GOES West) true color imagery. At first glance, it’s just a typical warm weather afternoon, with scattered cumulus over a broad area.
But a closer look reveals that there’s a lot going on. Let’s start by zooming in over Houston, the fourth most populated city and fifth most populated metro area in the United States. Houston lies in a somewhat unusual position: it’s kind of, but not really, coastal. This is the legacy of the great 1900 Galveston hurricane that destroyed its namesake city and caused those residents and businesses to relocate further inland to avoid the wrath of future storms. Today, downtown Houston is over forty miles from the shores of the Gulf but is located much closer to a series of bays. By zooming in, we see a number of interesting processes taking place. Due to the potential of severe weather in western Texas, the GOES-19 mesoscale scan was out and just happened to collect Houston in the far southeastern corner. Here is the highest spatial resolution band (Band 2 at 0.5 km) at the highest temporal resolution (1 min).
There are quite a few things worth watching in this image. First, notice the presence of the sea breeze. The southeastern edge of the clouds parallels the Gulf shore and is pushing inland. This, of course, is due to the temperature difference between the relatively cool water and the relatively warm land. Sea breezes bring stabilizing air ashore and kill convective plumes, and thus they cause clearing as they move inland. Second, notice how both the city of Houston and the bays to its east are both inhibiting the development of cumulus clouds as the locations downstream of those places tend to have clearer skies. In the case of the bay-influenced air, this is likely due to the cool waters killing convective plumes; we recently discussed this in the context of cold Minnesota lakes. However, why is Houston also inhibitiing the development of convective clouds. After all, cities are known to be warmer than the surrounding countryside. In fact, we can load up the Band 13 (10.3 micron infrared window) to see just how the temperatures vary across the region.
Note how mow much warmer Houston is over the surrounding rural areas, and how those areas themselves are warmer than the Gulf and the bays. That latter temperature imbalance is what is inducing the sea breeze. However, why are there fewer clouds forming over Houston even though it’s warmer? After all, a warmer surface should lead to more convection. However, it also takes moisture to produce clouds, and while there’s plenty of evaporation and transpiration surrounding Houston, there’s comparatively few moisture sources within the developed area. Existing moist plumes may get enhanced by the warm temperatures as they move over the city, but new ones struggle to form.
Let’s now turn our attention to an area further to the north along the Texas/Louisiana border. Once again, here’s the true color loop.
Again, it looks like a typical cumulus field, and we can once again see the influence of a cool lake on reducing cumulus development. However, look at the wind shear present here: the shallowest clouds appear to be moving from the south to the north. However, the tops of the deeper convection are moving from the north to the south. And if you go even higher up in the atmosphere, the cirrus clouds are clearly streaking from the west to the east. In the end, there are three clearly different wind directions that are easily resolvable as a function of different heights. We don’t need radiosondes or specialized multidimensional retrievals to tell us something useful about the wind; we can just look at a simple satellite loop.
Finally, let’s look at Louisiana’s bayous and Gulf coast, again with the true color view.
Look at all the brown that is in the water. This is silt that has been carried into the Gulf by the Atchafalaya River and other waterways. That water provides a marked contrast to the blue of the ocean. If you look carefully, you can even see how the currents in that silty discharge evolve over the 80 minutes covered by this animation.
Let’s take a look at how that brown area has evolved over just a few days. Here is a slider that compares this day to one just a few days earlier. It’s easy to see that in the interim three days, the silt has penetrated much further into the Gulf.
Why did this happen? Simple: there was a lot of rain on May 27th, as can be seen in this radar loop. With significant rainfall over the land, that water collected dirt and silt, flowed downstream, and out into the Gulf.
It’s clear that just because there are no active hazardous weather situations, it doesn’t mean the satellites aren’t worth looking at. There’s always something interesting to see.