This study investigates how to get the largest improvements to tropical cyclone forecasts from wind profiles obtained from a proposed new instrument, a Doppler Wind Lidar, on polar-orbiting satellites. Wind profiles have the largest impact when the tropical cyclone’s structure was changing rapidly and when the measurements were taken close to the tropical cyclone center.
Accurate forecasts of the future weather depend on observing what is happening now in the atmosphere. There remain many gaps in our current observations, especially over the oceans where most measurements come from satellites. An instrument that could measure the winds at many levels through the atmosphere (called a Doppler Wind Lidar) could be placed on satellites in the future to fill this gap. These wind profiling instruments on satellites that orbit the earth may provide frequent observations of what is happening currently in the atmosphere. This could lead to more accurate forecasts by the numerical models used by forecasters to make their predictions.
A Doppler Wind Lidar currently exists on a satellite that circles the earth (polar-orbiting satellite) and observes a single line of wind profiles. In an effort to study how future Doppler Wind Lidars could operate, we looked at how the increased coverage from a single line to a swath of multiple wind profiles changed hurricane forecasts. One important question is if the instrument were not able to measure near the center of the tropical cyclone because of thick clouds, would this change the forecasts?
To study this future potential version of a Doppler Wind Lidar, we ran experiments in an entirely simulated world, called an Observing System Simulation Experiment (OSSE). This gave us the freedom to test things we cannot do in the real world, for example, including observations of temperature and humidity in addition to wind. We can use a computer simulation of the earth (called a Hurricane Nature Run) to create observations like those that the profilers would make in the real world, and use those in another computer model to test the data impact. In our first set of experiments, we created observations in a uniform grid across the whole forecast area; since this is the largest amount of observations possible, this is likely to have the largest forecast impact, what we call a benchmark. Since the satellites will not be able to observe everything all the time, though, we ran a set of experiments that varied the timing of the polar-orbiting satellite. For the final set of experiments, we removed wind profiles from the swath if they were within a certain range (100, 200, 300, or 500 km) of the center of the hurricane to study how the forecast changed if the instrument were blocked by thick clouds.
- The benchmark experiments showed that the most accurate forecasts were generated when all variable types – wind, temperature, and moisture, were available at the beginning of every forecast in a uniform grid, the most observations possible.
- When only wind observations were available, relatively accurate forecasts were still produced. Note how close the blue and purple lines are in Fig. 1.
- When only a swath of wind profiles was available, it was important to occasionally observe the innermost portion of the hurricane especially during rapid changes in intensity.
- Wind profiles near the center of a hurricane are needed to produce more accurate intensity forecasts and more realistic hurricane structure (Fig. 2).
For more information, contact firstname.lastname@example.org. The paper can be accessed at https://journals.ametsoc.org/view/journals/mwre/149/6/MWR-D-20-0153.1.xml.
This work was supported by NASA Grant NNX13AQ36G and by NOAA’s Quantitative Observing System Assessment Program (QOSAP).