A Demanding Model
“When we started using the MODIS polar wind products, we saw about a 40 percent increase in the number of upper-atmosphere wind observations that we used to generate the polar wind analysis over the area north of 60 degrees North,” says Bormann. Such a large increase in real-world data on polar winds would be certain to improve forecasts, wouldn’t it?
“There are a lot of factors you have to consider,” explains Bormann. “Over the Northern Hemisphere, we have a lot of conventional observations—weather stations, aircraft, et cetera—and traditionally, it is more difficult to get improvements in the models with satellite data because of that.”
In some situations, the accuracy of the forecasts is more limited by the model’s ability to correctly represent atmospheric processes in mathematical terms than it is by lack of real-world observations for the state-of-the-atmosphere analysis. In addition, wind information derived from satellites just isn’t as accurate as wind information gathered from weather balloons or aircraft. In the case of the MODIS winds, the modelers knew it was possible that they would find that it was better to have fewer, but more accurate observations than more, but questionably accurate, observations.
“When the CIMSS group asked us whether we would be interested in their polar satellite winds,” says Bormann, “we were very keen to try these observations in our system. However, there was also some skepticism, as these observations are fairly indirect, and based on many assumptions. They are based on tracking clouds across orbits, assuming that cloud motions represent the wind, and assuming that the appropriate height for this wind can be estimated from the cloud information in the satellite imagery. Such indirect measurements have errors and biases that are difficult to account for in the analysis systems, as we had seen before with wind observations from geostationary satellites. We certainly did not scoff at the idea that these satellite winds could be helpful, but I think we also did not expect quite such a positive impact.”
Re-predicting the Weather
To test the impact of the MODIS polar winds, Thépaut and Bormann did a ‘what-if?’ experiment. They went back into their data “warehouse” of old forecasts and asked the question: would the forecasts made over a particular period have been better on average if they had included MODIS polar winds data?
The two scientists re-created 58 ten-day forecasts, this time using MODIS winds along with all the other atmospheric observations that were used in the original analysis. For each day of the 10-day forecasts, they compared what the model predicted for that day to the analysis of actual atmospheric conditions on that day. They did the same comparison for the original forecasts, made without the MODIS winds.
Over the first five days of the Northern Hemisphere forecast, the average quality of the forecasts made with MODIS winds and those without wasn’t very different. But somewhere around day 4 or 5 of the forecast, that began to change. The old forecast, the one made without the MODIS winds, began to miss the mark faster than the ones made with the MODIS winds: the old forecasts began to deviate from the actual atmospheric conditions sooner. By day 7 or 8, the MODIS winds had added another three hours to the useful “shelf-life” of weather forecasts for Europe and the North Atlantic.
In the world of weather modeling, a 3-hour extension to the time period over which a forecast is still useful is a pretty big improvement. “In conceptual terms,” says Thépaut, “think of it like this. Typically the quality of a 5-day forecast in the Northern Hemisphere today is about the same quality as the 3-day forecast was in 1981. So, in 22 years time we have improved the forecast by 2 days. [Or about 2 hours a year.] And that is the result of all the people working here at the center, the improvement of the observing and modeling systems, the increase in computing power, et cetera. The improvements from using MODIS winds are about 3 hours or so, which, to a very rough estimate, corresponds to about a year of progress here at the center.”
Of course, he stresses, that is a very approximate kind of estimate, but it goes to show how pleased they are in the improvements the MODIS winds have made. “I think the biggest surprise was just how quickly and easily the whole thing worked and how good the results were—which is probably why other centers have been more eager to try them,” says Thépaut.
The newest member of ECMWF’s satellite products branch, Lueder von Bremen, agrees. “I just got back from the International Winds Workshop in Helsinki, Finland, which is a conference on techniques for getting wind data for weather models, and there were several other NWP centers using the MODIS winds. Nearly all of them were having the same good results as we are.”
Five numerical weather prediction centers are already using the MODIS winds in their operational forecasting system: ECMWF, NASA’s Global Modeling and Assimilation Office, Japan’s Meteorological Agency, the Canadian Meteorological Centre, and the U.S. Navy’s Fleet Numerical Meteorology and Oceanography Center. A sixth center, the United Kingdom Meteorological Office, is using the winds experimentally.
Key and his group at CIMSS will keep refining the MODIS winds products, increasing their accuracy and reducing the delivery time. They are going to make a single MODIS winds product that combines all available Terra and Aqua MODIS observations, to cover the widest possible area in the shortest possible time. They are also looking into getting data from a handful of MODIS “Direct Broadcast” stations—facilities that have their own antenna to receive the MODIS digital data stream—rather than just relying on the NOAA-NASA data feed.
Thépaut will welcome any improvements. “Especially if they could get the delivery time down to an hour,” he says. But he laughs because he knows that’s not very likely given the time between orbits and all the other steps that have to happen. Thépaut is a little uneasy about the future of products like the MODIS winds, however. The upcoming NOAA-NASA-Department of Defense mission called the National Polar-orbiting Operational Environmental Satellite System, or NPOESS for short, is intended to continue collecting many of the types of Earth observation data sets currently being collected by Terra and Aqua.
Unfortunately, says Thépaut, “the planned NPOESS visible/infrared imagers are lacking the spectral channels to detect water vapor features, at least the first two instruments in the series anyway.” Key and his group use water vapor observations along with clouds to detect which way the wind is blowing, and NPOESS will not have that capability. “This is a big concern in the meteorological community. These bands weren’t part of the instrument design because at the time, no one was thinking we would be able to do this with MODIS. We discovered it too late. We know that some efforts are being undertaken in the United States to change the situation for the third satellite in the series, and we can just be hopeful that this will happen. Otherwise, what will we do when there is no more MODIS? We certainly don’t want to lose what we have gained.”