Bench Talk

When I was young, my family was transferred to Darwin, Australia, where two-meter long iguanas occasionally dawdled down the street in front of our house, which was built on stilts as was common at the time. Although the raised construction kept us above the iguanas, snakes, and wet-season waters, the stilt homes were no match for Cyclone Tracy, which struck on December 24, 1974 and left 80 percent of the city in ruins (Figure 1). We had returned to the U.S. less than a month before, thankfully; however, photographs and neighbors’ accountings—clinging to the cyclone fence for hours while being pummeled by torrential rains and winds that exceeded 185km/h—are permanently etched in my mind even 43 years later.

Figure 1: Moil, a northern suburb of Darwin, was devastated by Cyclone Tracy on Christmas Eve, 1974. The author’s former home is in the upper right of this photo. (Source: Wikipedia)

Here in the U.S., as Hurricane Harvey crept toward the Texas coastline over the past week or so, I watched anxiously as the storm gained impressive strength in a short amount of time, surprising even seasoned weather forecasters. Videos taken from within aircraft cockpits are perhaps most impressive—albeit visually anticlimactic—as the idea of being aboard an aircraft flying through wind speeds of 250km/h is unfathomable to me. Yet until drone technology can withstand and function in such harsh environmental conditions, these manned flights are necessary for hurricane forecasting.

Meanwhile, the U.S. National Hurricane Center (NHC) is just one organization that uses reconnaissance aircraft equipped with instruments to gauge hurricane conditions and predict their behavior. One instrument is the dropsonde—a pod-like device that includes a GPS receiver, radio transmitter, microprocessor, battery, and sensors that measure temperature, pressure, humidity, and wind speed and direction (Figure 2). As many as 1500 sondes might be dropped over the course of a single training mission or hurricane event.

Figure 2: About the size of a paper towel cardboard tube, a dropsonde uses sensors to measure temperature, pressure, humidity, and wind speed and direction. (Source: Wikipedia)

Created by the National Center for Atmospheric Research (NCAR), the device—formally called an airborne vertical atmospheric profiling system, or AVAPS—is about the size of a paper towel cardboard tube and weighs less than 0.5kg. A small square-cone parachute slows its decent and enables the device to transmit sensor data several times during its decent, which ranges between 38–58km/h. On its way down, the sonde’s GPS chip transmits its position; by comparing positions every few seconds, computers can calculate wind speed and direction—two data points that are key in hurricane forecasting.

Hurricane hunting aircraft are also equipped with stepped frequency microwave radiometers (SFMRs, also called “smurfs”), which are installed on the underside of the aircraft’s wing. The SFMR is a compact radiometer that measures radiative emission of the sea surface, expressed as brightness temperature in six frequency bands ranging from 4.5–7GHz. Emissivity is a function of sea foam created by breaking waves and has a strong correlation to surface wind speed. Using sensor data about surface conditions, computers can determine wind speeds based on the amount of microwave radiation emitted from sea foam.

A downside of SFMRs is the very limited data provided by nadir measurements, which measure single points calculated from the aircraft; however, a newer technology—hurricane imaging radiometer (HIRad)—maps brightness temperature like the SFMR, but does so over a ±45° swath, which provides more data points for calculation.

Although we can’t stop Mother Nature, we can sense her many data points using dropsonde, SFMR, and HIRad technologies. Hopefully, with the help of these and other technologies, future hurricanes will become record-breakers for their size and strength, not their toll on human life or city destruction. Meanwhile, our thoughts go out to those in the path of Hurricane Harvey.