Satellite data has been efficiently used to monitor changes in the Earth's environment, from ever-changing weather conditions to long-term climate variability such as global warming. This month's article presents a few players contributing to the space-based Earth observation and describes Japan's future plans in this field.
Increasing Demand for Space-based Earth Observation
The U.S. launched the worlds first meteorological satellite more than forty years ago. Named TIROS for Television Infrared Observation Satellite, the satellite demonstrated the advantage of mapping the Earths cloud cover from satellite altitudes. Given the fact that observations from the Earths surface had not provided meteorologists with a sufficient interpretation of cloud patterns, satellite imagery was a breakthrough in cloud research.
Since the launch of TIROS, space-based Earth environment observation programs have improved both in the quality and quantity of the relevant data. Recent international cooperative programs have been aimed at monitoring the Earth comprehensively in combination with satellites.
Taking microwave observation data as an example, we will present the current and future remote sensing programs here. Some programs call for us to monitor the Earth continuously over twenty years, while others are being conducted to monitor daily changes.
Long-term Temperature Changes Observed from Space
We have made maximum use of in-situ data obtained by meteorological agencies throughout the world as a tool to research long-term global climate changes. Recently remote sensing satellite data has emerged as a player in this field.
A typical example is the TIROS-N satellites being operated by the NOAA. TIROS-N satellites carry instruments that provide useful data for researching global changes in the environment. Hereinafter we will present the Microwave Sounding Unit (MSU), a sensor aimed at measuring temperatures in the stratosphere and troposphere.
The MSU measures the thermal emission of molecular oxygen at specific spectral frequencies. Using the sensitivity of radiant intensity to temperature change, MSU data provides us information on temperature. In addition, we can derive temperature profiles at different altitudes by shifting spectral frequencies within a specified measurement range.
Fig. 1 shows MSU data indicating monthly-averaged temperature anomalies (departures from seasonal normals) in the lower troposphere and the lower stratosphere. Launched successively in the TIROS program, the MSUs have collected data for more than twenty years and have provided a global record of various influential events on temperature fluctuations.
The large increases in 1982 and 1991, shown in the left graph, were caused by the volcanic eruption of El Chichon and Mt. Pinatubo, respectively. Except for these large temporary warming perturbations, the lower stratospheric data reflects a significant cooling trend. This may be due to depletion of ozone that is considered to be a contributor to global warming.
The lower troposphere shown on the right is located closer to the Earth, which may cause temperature in this region to be more strongly influenced by oceanic activity, particularly the El Niño and La Niña phenomena. For example, temperature fluctuations between the middle of 1997 and the end of 1998 occurred slightly after the generation and decay of El Niño, showing the trend that atmospheric change following the oceanic change.
As it has seen, long-term satellite data provides us with overall trends of the Earths temperature. Now can we cite the lower troposphere data as evidence for global warming, an internationally growing concern? Yes and no, there remains much to be discussed how to interpret the data. However, considering the twenty-year contribution of satellites to the monitoring of climate changes on a global basis, particularly for the oceans that are difficult to monitor using surface thermometers, space-based Earth observation is being viewed as a new approach. Future plans call for us to make every effort to validate the accuracy of satellite data with the combined use of other observation methods such as balloons.
From the Long Term Observation to Frequent Observation
Remote sensing satellites are required to conduct observations not only over the long-term, but also at frequent intervals. As seen in the cloud images obtained by the Himawari meteorological satellite, many weather phenomena changes every minute. In addition, some natural events present diurnal changes because of their close relationship with solar radiation. Observing such ever-changing phenomena frequently is not only useful in weather prediction, but also offers valuable information for long-term monitoring activity.
Fig. 2 shows yearly-averaged rates of rainfall on oceans in the morning and in the afternoon observed by the SSM/I onboard the Defense Meteorological Satellite Program (DMSP) satellite. The SSM/I is a passive microwave radiometric system capable of measuring rainfall rates, and water content in clouds, soil, and the like.
According to Fig. 2, most of the oceanic areas tend to have a higher rainfall rate in the morning than in the afternoon. The notable difference can be seen particularly in areas adjacent to so-called maritime, continent such as the Philippines and Indonesia, and oceans between and along the borders of North and South America.
In contrast, the rainfall rate over land exhibits a tendency to rain more from afternoon to evening. The ground surface is more susceptible to solar radiation than the oceans, which makes the ambient air warm and consequently unstable. This factor is thought to account for daily changes opposite to the tendency observed on oceans. Although the direct cause has not been identified, the cumulative effect of diurnal changes over land may contribute to the specific diurnal change mechanism in the oceanic areas around maritime continent.
Further research will be conducted to determine differences between simulation models and actual measurements.
In order to understand the diurnal change mechanism and ever-changing phenomena, it is necessary to increase the daily observational frequency. As do most Earth observation satellites, the DMSP satellite follows a near-polar sun-synchronous orbit in which the satellite always passes over a location at the same local solar time (LST). The DMSP satellite crosses the same points on the Earth in the morning and again in the evening. Frequent coverage can be implemented by combining satellites having different observing LSTs.
Careful attention must be given to time-dependent biases that often accompany data collected over long periods of time. These biases are due to the use of different satellite generations and an observation period ranging over decades. When the repeat cycle of a satellite is limited to one or two a day and the observation target entails diurnal changes, the collected data contains time-dependent biases as the true climate signal. Because of this, it is necessary to increase the number of revisit times.
The Tropical Rainfall Measuring Mission (TRMM) satellite adopted anon-sun-synchrnous orbit, which makes the satellite to observe the same region at slightly different local time every day. A combined use of such non-sun synchronous orbiting satellites as the TRMM satellite and multiple sun synchronous orbiting satellites would be effective for implementing uniform observations.
Expectations for ADEOS-II and EOS-Aqua
The National Space Development Agency of Japan (NASDA) plans to conduct the Global Change Observation Mission (GCOM) over a fifteen-year period, beginning with the Advanced Earth Observing Satellite (ADEOS)-II, scheduled for launch in late 2002. The ADEOS-II will accommodate such optical sensors as the Global Imager (GLI), and microwave sensors such as the Advanced Microwave Scanning Radiometer (AMSR) and SeaWinds. The satellite is expected to measure various geophysical parameters by taking advantage of the different capabilities of multiple sensors. The modified ADEOS-II sensors will be continuously operated onboard observation satellites of the second generation and beyond in order to create a database containing a long-term observation product.
As part of their efforts toward international cooperation, NASDA is also providing the AMSR-E for use on NASAs EOS-Aqua, scheduled for launch on May, 2002. As its name implies, EOS-Aqua collects various data required to understand the global water cycle. The AMSR-E, having a modified design of the ADEOS-II AMSR, collects data related to water vapor in the atmosphere, cloud water, precipitation, soil moisture, sea ice, snow cover and sea surface temperature. AMSR-E data is expected to help in achieving the goals of the EOS-Aqua mission. In addition to the AMSR-E, the AMSU a modified version of the MSU and the Humidity Sounder for Brazil (HSB) a microwave radiometer for measuring the vertical profile of water vapor will be accommodated in the Aqua satellite.
Since the Aqua and the ADEOS-II will follow the afternoon orbits of 13:30 and 1:30 LST and the morning orbits of 10:30 and 22:30 LST, respectively, the number of revisit times will be increased. Further, more detailed information on diurnal changes will be provided with the combined use of the Aquas AMSR-E and Moderate Resolution Imaging Spectroradiometer (MODIS) and the ADEOS-IIs AMSR and GLI.
As described earlier, a satellite configuration in which sun-synchronous satellites and non-sun synchronous satellites co-exist is being examined for use in the Global Precipitation Measurement (GPM) mission.