[MUSIC] We will see now that with a microwave radiometer, we can also observe sea surface temperature. Why do we want to do this? We saw that infrared is where the temperature can be measured from a satellite. This has a maximum power of emission at infrared frequency. Why can we attempt doing this with microwaves? The reason is very clear. This is due to our old enemy, the cloud cover. Here, we see two maps of the world oceans. The top map is derived from measurements make with the microwave radiometer. The bottom map is made with an infrared radiometer. Here, the colors are indicating the percentage of data that could be recorded during a period of one year and a half. If we look at the microwave radiometer, we see that most of the ocean can be sampled almost near to 100% of the time, except near to the areas that are covered by ice. While if we go to the infrared radiometer, we see that large portions of the ocean can be observed less than 50% of the time and the rest something between 70%, etc. It is clear that with microwave radiometers, we obtain more information than with infrared radiometers. And here on the right, we have a time series where this percentage is shown from the second half of 2002 and then 2003. So for the microwave radiometer, except for specific places where there were some failures in the instrument, most of the time the coverage is above 90%. While with the infrared radiometer, we see that the percentage is much lower and oscillating strongly. Depending on the weather conditions. So let's now see, what is measuring our microve radiometer that we can use to extract information on seasonal phase temperature. First, let examine the penetration. Here, we see that for different temperatures these are the different curves. We have for a different frequencies, a different penetration. Around 5 gigahertz, the penetration is close to 5 millimeters. So as we said before, if we are observing the ocean with an infrared radiometer, we will have the temperature very close to the surface what is called the skin temperature. While if we are using a microwave radiometer, we will have the sub skin temperature a few millimeters. Then what is the optimum frequency to measure the surface temperature at microwaves? Here, we have figure that shows what is the sensitivity of the different frequencies depending on the temperature. It means that around 4 to 6 gigahertz is where we have the maximum sensitivity for the different temperatures. This means that we want to use this frequency to measure the surface temperature of microwaves. Well what is in fact measuring a microwave radiometer? We have here a satellite with a radiometer and we want to capture the emission that is coming from the ocean surface. But at the same time, the atmosphere can be emitting at the same frequency. So some radiation goes directly from the atmosphere to the satellite. Then radiation from the atmosphere can go to the surface being reflected to the surface and going outside to the satellite. And at the same time, we can have external bodies like sun, moon, galaxies and so that can be emitting at the same frequency. This radiation arrives to the ocean surface and goes to the satellite. So there are several physical processes that are participating in the radiation that is recorded by the radiometer. So what do we do, is that we have to measure independently these sources of radiation. Or modeling these physical processes in such a way that through a mathematical model, we can compute what are these different fx. And then obtain what's the sea surface temperature that in fact has been recorded by our radiometer. This is an image of one of the microwave radiometers that has been used for ocean observation. This is the advanced microwave scanning radiometer, what we call AMSR. This is a Japanese sensor that was developed and put in different satellites. For example, in one of the NASA satellites that was observing the ocean surface. This is an image of one week of data. So accumulating one week of observations, we can build this sea surface temperature map. This kind of radiometers usually operate at different frequencies. What is the advantage of this? From one side, we can obtain different information that we can use to correct for the atmospheric effects. And on the other side, we can use this information to derive different variables. And here, you see from just one day of operation of this AMSR sensor different variables that can be obtained. Not only sea surface temperature but also the wind speed, rain rate, water vapor, concentration and cloud liquid water. Another possibility is that we can use different satellites with different sensors combining the data and building a single map. This is one of the example, provided by an US company that is offering the service free through Internet. And this is the result of obtaining data from different microwave radiometers and building a single sea surface temperature map for one day. So then why we don't use always this instead of using infrared? Because infrared has the problem of clouds. Here is the reason. The different frequencies provide different spatial resolution. At the frequency used by infrared radiometers, we can obtain one resolution at the frequency used by microwave radiometers will have a different resolution. This is what a microwave radiometer would observe in terms of thermal status on the ocean surface. In this case, we can say that the spatial resolution is of the order of 30 kilometers. We're obtaining formation on what we call mesoscale status of the ocean. But if the same area is observed by an infrared radiometer, what we have is this, much higher resolution. Even some parts are covered by clouds, but the areas where we observe the temperature of the ocean, have a much higher spatial resolution. You can see here, you can go up to one kilometer. It is clear that using this for a infrared radiometres, even we have the problem of cloud cover. We can go to what we call the submesoscale structure of the ocean surface. We can capture details of thermal structures at much higher resolution than with microwave radiometers.
