Articles, Training, Operations, Technology
Posted on Sep 1, 2019
by Scott Dennstaedt
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A lot. Is it better than having nothing? Yes and no. But you need to drill down to the product level to see the value, or lack thereof.
In August 2018, the Federal Aviation Administration (FAA) began to broadcast six new weather products which included lightning, cloud top height, icing, turbulence, center weather advisories (CWA) and graphical AIRMETs (G-AIRMETs). This was fantastic news to many pilots. What's the downside to the Flight Information System – Broadcast (FIS-B) weather? Let’s look at some of the newly broadcast products and discuss their limitations. Note, these are presented in no particular order and by no means are a complete list of the cons of this service.
Graphical AIRMETs or G-AIRMETs are issued by aviation meteorologists at the Aviation Weather Center (AWC) in Kansas City, Missouri. The legacy textual AIRMET Zulu encapsulated both a forecast for icing as well as a forecast for the freezing level. Now the freezing level is captured in its own G-AIRMET.
The freezing level G-AIRMET has a poor spatial resolution. That’s not the fault of the FIS-B broadcast; it’s just the nature of the freezing level G-AIRMET. At a 4,000-foot resolution, it’s hardly useful when there isn’t a big change in the freezing level over a wide region as depicted in the G-AIRMET shown below. The freezing level contours of 12,000 feet to 16,000 feet MSL span the entire north to south distance along the West Coast of the United States. What’s the freezing level in northwestern Nevada? In mountainous regions, a freezing level difference of 2,000 feet can mean the difference between a safe passage or one fraught with icing concerns. For comparison, the SiriusXM broadcast data has a resolution down to 100 feet.
Freezing level G-AIRMET produced by forecasters at the AWC.
Lightning comes in two distinct flavors — cloud-to-ground and intracloud lightning (sometimes referred to as cloudto- cloud lightning). The lightning broadcast by FIS-B only contains one of those – cloud-to-ground lightning. That’s because FIS-B broadcasts the data from the National Lightning Detection Network (NLDN) that cannot detect intracloud lightning. While this doesn’t seem like a major problem, it really depends on a number of factors. In fact, lightning can often tell you more about the severity of the thunderstorm than simply looking at the colors on the current datalink radar mosaic. For comparison, SiriusXM broadcasts both flavors of lightning.
As they say in the real estate business, it’s all about location; same is true of lightning. A team of researchers from NASA looked at lightning strikes from the Optical Transient Detector, which records the locations of all lightning flashes in clouds beneath the satellite, along with data from the NLDN that only detects where lightning actually strikes the ground. They found that in some locations in the central Plains, for example, for every single cloud-to-ground strike there were 10 intracloud discharges. This kind of energy dissipation in a storm due to intracloud lightning is a direct indication of strong vertical motion in the cell, hence severe to extreme turbulence.
The map below, courtesy of NASA/Marshall, depicts the intracloud to cloud-to-ground lightning ratio. Red areas are close to a 10 to 1 ratio as can be seen in the central Plains. Violet is more representative of a 1 to 1 ratio. If you have an area that’s low in ground-based sensors, you may artificially create a belief that intracloud (IC) is much greater, but it is due to the lack of ground-based sensors. The area in Oregon and northern California, for example, is probably such an area. In many locations throughout the conterminous United States, the ratio of IC strikes to cloud-to-ground (CG) strikes is 10:1. That means for every 10 intracloud strikes there’s a single cloud-to-ground strike.
Intracloud to cloud-to-ground lightning strike ratio.
Research has shown that thunderstorms with high lightning flash rates are typically severe and demand a greater respect when circumnavigating around them. Flash rate is simply a measure of the number of strikes over a given amount of time, usually one minute. A typical non-severe thunderstorm will have a flash rate of 10 per minute. Many severe storms have a rate of 15 and often greater than 30 per minute. Oddly, these severe storms often have very few cloud-to-ground strikes, often less than one per minute. However, they tend to have a high IC:CG strike ratio, where this ratio can become infinite for brief periods in some severe storms.
The other critical component to highlight is that during the early stages of vertical development of a thunderstorm, intracloud lightning dominates over cloud-to-ground lightning. Coupled with the inherent delay in the radar mosaic may lead the pilot down the primrose path of stumbling into a developing storm if cloud-to-ground lightning is absent.
The mean sea level height of cloud tops are likely the Holy Grail of aviation weather, especially during the winter months. Having the knowledge of where the tops are located along your route allows you to fly on top of the cloud deck in visual meteorological conditions (VMC). More importantly, during the cold season it will help you find an altitude that allows you to remain above a potential icing layer. Hence, it makes perfect sense for this to be one of the weather products recently added to the FIS-B broadcast.
Perhaps the biggest negative with the FIS-B cloud tops is that these are a forecast and not directly based on observational data. It would have been more accurate to use actual tops (even if only updated once an hour) based on observational satellite imagery, but that would have required Harris (the company that collects and processes the data for the FIS-B broadcast) to combine two data sources, cloud top temperature and a temperature aloft analysis, to calculate the cloud top height. For whatever reason, Harris is restricted from combining two or more data sets.
The High-Resolution Rapid Refresh or HRRR (pronounced “her”) model is the source for the FIS-B cloud tops product. The HRRR model is run hourly and takes about an hour to complete its execution and includes a forecast for cloud top height. At 10 minutes past the hour (the data collection window), if the newest run of the HRRR has completed its forecast, the one-hour forecast will be used. If not, the two-hour forecast from the previous run of the model will be used. Regardless, it will be valid at the top of the most recent hour. This new cloud top height forecast will be broadcast four times over the next hour (every 15 minutes) with the emphasis that it is still valid at the top of the previous hour.
While the HRRR model does a fairly good job modeling the tops of a stratiform cloud deck, modeling the top of a cumuliform cloud deck is a different story. This is especially problematic if the model doesn’t have a good handle on a developing line of convection. It’s essentially “guessing” when and where the convection might develop. Yes, the model does get initialized with observational data, but will it spin up quick enough to show you the tops?
Here’s a common occurrence. The image below is the radar mosaic valid at 14Z. Notice the area of significant radar returns (convection) circled in red in northwest Louisiana moving through Shreveport. The highest tops of these storms were around 35,000 feet.
NEXRAD mosaic valid at 14Z.
If we look at the one-hour forecast for simulated reflectivity (note that this is not a product that broadcasts), you’ll notice that no returns are showing in this region of northwest Louisiana. Therefore, the model has absolutely no clue that this area of deep, moist convection even exists in this immediate area ... keep in mind that this is only a one-hour forecast and it hasn’t caught on. Consequently, there’s no chance it had the cloud tops right in this area either.
A one-hour simulated reflectivity forecast from the HRRR valid at 14Z.
For comparison, the SiriusXM cloud top broadcast uses observational data from using actual cloud tops which are based on infrared satellite imagery. Similar to the FIS-B cloud top height forecast, this is only updated once each hour and it’s always valid in the recent past but will provide a more representative picture regardless of the type of clouds. Moreover, it will ultimately match more closely with the SiriusXM NEXRAD mosaic.
In addition to cloud top height, FIS-B also recently started broadcasting an icing and turbulence forecast. The broadcast process is similar to the cloud top forecast previously mentioned, in that it provides a onehour or two-hour forecast with the data collection window starting at 15 minutes after the hour. They are both updated hourly and broadcast four times each hour.
Here’s the problem. Let’s say you get a fresh update valid at 14Z and you’d stare at that forecast until it’s updated an hour later. Icing and turbulence are highly transient in nature. As a result, it’s not likely that the one-hour forecast that’s valid in the recent past would have much relevance to a flight through an area of icing or turbulence an hour later especially if it’s convective.
If you are a high-altitude flyer, you may still receive a FIS-B broadcast when flying above 24,000 feet, however FIS-B was designed for aircraft flying at or below this altitude. Consequently, they do not broadcast the icing and turbulence forecasts that extend above 24,000 feet. Icing isn’t probably a big concern since outside of deep, moist convection, icing is rare above 30,000 feet. Turbulence, on the other hand, is a significant issue when flying above 24,000 feet.
For now, FIS-B generally stops at the U.S. border. That is, if you are flying outside the conterminous United States you won’t likely receive a FIS-B broadcast signal and most of the weather data such as cloud top height, icing and turbulence are not broadcast over Canada, Mexico and the Caribbean. For reference, a SiriusXM broadcast can be received throughout most of southern Canada, northern Mexico and the northern part of the Caribbean. You can find the SiriusXM coverage map at https://tinyurl.com/ y2gn4shf. More importantly, they include the Canadian Doppler radar as well as cloud tops and any turbulence and icing forecasts that extend into Canada, Mexico and the Caribbean.
On April 23, 2019, the FIS-B radar mosaic was quietly updated and is now broadcast as I discussed in a blog post located at https://tinyurl.com/y27ge5bz. The new Multi-radar/Multi-sensor (MRMS) radar depiction represents the latest technology and is a great new addition to the broadcast. It essentially gives SiriusXM a run for their money (or your money) with no need to change hardware or wait for a software upgrade to get the new radar mosaic. When released at the end of April, it was 100 percent backward compatible and appeared on your in-cockpit display without any pomp and circumstance.
Of course, the biggest plus is that FIS-B weather doesn’t require a monthly subscription. As long as you understand the limitations, FIS-B weather has some redeeming value.
While SiriusXM will require that you pay a subscription to receive their broadcast, it has less limitations and will offer you more choices than you get with the FIS-B broadcast.
Scott Dennstaedt has been a CFI for the past 20 years and is a former National Weather Service meteorologist. He is the co-author of a new book, “Pilot Weather: From Solo to the Airlines.”
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