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Improving Atmospheric Temperature Monitoring Capabilities In this letter report the discussion of remotely sensed upper air temperatures focuses on microwave-based datasets, but many of the recommendations regarding the MSU/AMSU (Microwave Sounding Unit/Advanced Microwave Sounding Unit) satellite program will benefit other sensors (e.g., infrared sensors) which provide observations for CDRs. Although this report focuses on atmospheric temperature monitoring, it also addresses issues pertaining to factors such as CDRs for humidity and winds that, in combination with temperature, are important for improving understanding of climate dynamics. Data on upper air temperature variations can provide critical information in helping to explain the responses of the global climate to natural and human-related forcing mechanisms. The various remotely-sensed and in situ observations of upper air and surface temperature provide complementary information. Information from multiple sensing systems is valuable for ensuring the integrity of each individual system through cross-comparison/cross-calibration. These data are vital in determining the nature of the coupling between the troposphere and stratosphere, as well as between the troposphere and surface. Some of the datasets that have proven most useful for climate analysis purposes, such as the global temperatures from the MSU and radiosondes, have been produced by individuals or small teams of researchers working on projects with limited and intermittent funding. On June 8, 2000, a sub-panel of the full RTO panel (see Appendix A) met and heard presentations from NOAA and NASA representatives, as well as POES contractors from Aerojet. These presentations and the ensuing discussions helped inform the deliberations of the sub-panel, which produced the first draft of this letter report. This draft was then submitted to the full panel for its deliberation and comments. The report was then peer-reviewed following NRC guidelines and modified according to the reviewers' comments (see Appendix E). The report reflects a consensus of the panel and has been approved by the NRC. The panel focuses its recommendations on climate monitoring concerns. A few of the recommendations may require additional expenditures to implement. Although the panel believes that these recommendations are appropriate, it has not undertaken a cost analysis comparing its climate-related recommendations with other resource allocation possibilities. The report consists of succinct findings and recommendations in three categories: Satellite Observing System, In-situ Observing System, and Climate Data Record Issues. The findings and recommendations in each of these categories are not explicitly ranked in terms of priority. However, the ordering places a very slightly higher priority on the items at the top of each section than those at the bottom. The panel feels that all of the recommendations made in this report are important and should be given due consideration by NOAA. To keep the report brief, the bulk of the background information is indicated by appropriate citations. 1. SATELLITE OBSERVING SYSTEM 1.1 Rigorous Station-Keeping Finding: Climate monitoring using polar orbiting satellites is significantly impacted as a spacecraft drifts from a fixed local crossing time and fixed altitude (Waliser and Zhou, 1997; Christy et al., 1998, 2000; Wentz and Schabel, 1998). Some polar orbiting spacecraft drifted over four hours from their initial crossing times while still providing operational data (e.g. NOAA-11). The four remaining POES spacecraft (NOAA-L, M, N and N') are not equipped with on-board propulsion systems. NESDIS has formulated an orbit insertion solution to reduce local crossing time drift to approximately +/−15 minutes in the a.m. node and +/−30 minutes in the p.m. node for a nominal six-year mission.
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Improving Atmospheric Temperature Monitoring Capabilities Recommendation: The orbit insertion solution formulated by NESDIS will substantially mitigate systematic biases resulting from orbital drift and should be adhered to within the above-indicated local crossing time tolerances, beginning with NOAA-L. 1.2 Overlapping Observations Finding: A minimum of one year of overlapping observations of operational spacecraft is required to fully sample the annual cycle effects on both the platform and the quantity being sensed to determine inter-satellite biases in the construction of CDRs (Karl et al., 1995; Christy, 1995; NRC, 1999). Recommendation: NESDIS should schedule launches of new satellites to allow for overlap of at least one year with soon-to-be decommissioned satellites and should accommodate the associated increase in data transmission from multiple platforms. 1.3 Launch Strategy Finding: The “launch-on-failure” strategy 1 entails significant risk of temporal gaps in the record and of instrument failures of a magnitude that degrade the precision of climate observations, without seriously impacting operational products. NESDIS now includes in the definition of “failure” the significant drift from initial local crossing time in addition to the significant dysfunction of critical instruments. (At scheduled launches, the next satellite is on-call in case of launch failure.) The present launch dates for NOAA -L, -M, -N and -N' are September 2000, May 2001, December 2003, and January 2008, respectively. Combined with METOP 2 launches, this generation of POES apparently will operate until about 2012. Proposed launch dates are often overly optimistic and actual launches are delayed for various reasons if orbiting spacecraft provide data minimally suitable for operations. NOAA-12 for example was in operation for seven years (due in part to NOAA-13's failure after one week on orbit) even though various instruments experienced functional problems of a magnitude considered minor for operations, but significant for climate (Christy et al., 2000). Recommendation: NESDIS should attempt to perform a launch-on-schedule strategy in which a platform is replaced within its projected operational life (currently five years or less) to ensure continuity in observations for CDRs. Such a schedule would also address concerns indicated in Findings 1.1, 1.2 and 1.5. 1 This is a strategy in which a replacement satellite is not launched until failure of the active satellite is imminent or has already occurred. 2 For the last two decades the United States has supported two NOAA satellites in polar orbit, one with a nominal equator crossing time in the morning, the other with a nominal equator crossing time in the afternoon. Cooperation between Europe and the United States has led to the development of the Initial Joint Polar System (IJPS), which will comprise the continuation of the current NOAA afternoon satellite series with NOAA-N and -N', together with the new Meteorological Operational (MetOp) morning satellite series (MetOp-1, -2, and -3) run by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT). These two satellite series will be produced independently by the United States and Europe, but will carry a core set of nearly identical instruments to ensure operational data continuity and coherence of the key meteorological observations. The MetOp satellites will carry instruments provided by EUMETSAT, NOAA (which will contribute, e.g., AMSU-A), and the French space agency, CNES. MetOp-1 is scheduled for launch in 2003.
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Improving Atmospheric Temperature Monitoring Capabilities 1.4 Sensor Calibration Finding: Nearly all satellite microwave radiometers (MSU, SMMR, SSM/I, TMI) have exhibited calibration drifts that seem to be related to both the temperature of the radiometer components (receivers, mixers, antenna) and the non-linear response of the system gain function to incoming radiation (e.g., Christy et al., 1998, 2000; Wentz et al., in press). For the measurement of long-term climate change, the removal of these sensor calibration drifts can be a formidable problem (Hurrell and Trenberth, 1998). Recommendation: NOAA should put a high priority on measuring all aspects of the radiometer 's system gain function and baseline offset during pre-launch testing. The usual set of thermal-vacuum tests should be expanded and done more rigorously, and the test results should be made readily available to the scientific community for evaluation. Since the calibration drift seems to be related to temperature, a sufficient number of precision thermistors should be mounted on the various radiometer components (antenna, feedhorn, front-end receiver, detector, etc.) for on-orbit monitoring and drift detection. More robust on-board calibration systems (e.g., additional reference loads) should be considered for future missions. 1.5 Ensuring the Climate-Monitoring Effectiveness of Polar Orbiting Satellites Finding A: NASA's EOS-class satellites (Terra, Aqua, Aura, Jason, ACRIMSAT, QuikScat) are (or will be) providing data of significant importance for climate analysis. Many of these will cease observations well before 2010. NPOESS, with (mostly) similar quality instrumentation, is scheduled for launch in 2009. The NPOESS Preparatory Project (NPP, about 2006) is designed to bridge the gap between the EOS and NPOESS eras with high-quality overlapping observations (particularly from infrared sensors), the continuity of which may be in jeopardy if the gap is longer than three years. The last POES (NOAA-N' p.m.) and METOP-2 (a.m.) are presently scheduled for launch in 2008 with nominal lifetimes of five years of operational service. Budgetary pressures may force a delay of several years in the launch of NPOESS based on the view that the operational requirements are met by the POES series, and thus NPOESS would not be needed until the last POES reaches a failure status. Recommendation A: NESDIS and NASA should plan launch schedules to ensure that NPP will have a minimum of one-year overlap with both EOS and NPOESS regardless of the status of POES. Agencies should recognize that the continuity of the measurements is critical to climate science (NRC, 1999). Finding B: A single-platform NPOESS will not be capable of continuing CDR-quality observations for variables such as sea-level height, solar irradiance, and active microwave scatterometer data (NRC, 2000b). Recommendation B: NESDIS and NASA should consider the cost effectiveness of including NPOESS free fliers (i.e., a spacecraft separate from the main operational platform in an orbit customized for the unique measurement required) as part of an effective strategy for preserving the continuity of high-quality climate observations of the above-mentioned quantities (NRC, 2000b).
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Improving Atmospheric Temperature Monitoring Capabilities 1.6 Continued Production of Remotely-Sensed Stratospheric CDRs Finding: A thorough examination of the time series of stratospheric temperatures from the infrared-sensing Stratospheric Sounding Unit (SSU), including assessment of error characteristics, has not been done. The existing time series has not been extended beyond 1998. These factors reduce confidence in temperature trend estimates based on SSU and impede the merging of the soon-to-be-terminated SSU data with soundings from AMSU. The United States does not support research to produce CDRs of stratospheric temperatures derived from a combination of the older SSU data and the newer AMSU data. Recommendation: NESDIS should take responsibility for the construction and validation of CDR-quality bulk-layer temperature time series from the SSU and AMSU for the analysis of stratospheric climate variations and trends. 1.7 Communicating Instrument Performance Finding: Instrument problems that do not exceed design specifications can be significant for the construction of CDRs. Information regarding changes in, or new knowledge of, instrument health and performance is difficult for climate investigators to obtain and apply. Investigators have also found instrument problems that were difficult to communicate back to NESDIS. Recommendation: NESDIS should create a web site that includes information on spacecraft and instrument condition and changes that are of interest for the construction of CDRs. In addition to the official NESDIS TIROS Operational Anomaly Reports (TOAR), this site should be interactive to allow climate investigators to communicate their findings and opinions concerning the behavior of specific instruments and/or channels. The site should be well organized, with cross-referencing by category, and should include a good search capability to enable interested parties to find what they want. An attempt should be made to hierarchically construct the site so that issues judged by NESDIS to be of greatest importance to the climate record are most prominently featured. The information contained on this web site would become part of the permanent metadata record for each instrument. 1.8 Unused Assets in Space Finding: Several decommissioned spacecraft, such as NOAA-10 AND NOAA-12, carry instruments capable of monitoring climate at atypical local times. Reconciling Observations of Global Temperature Change (NRC, 2000a) discussed the difficulties associated with adjusting the MSU tropospheric temperature record to account for satellite drift-induced changes through the diurnal cycle. Recommendation: NESDIS should investigate the potential to observe the earth with still-functioning instruments of decommissioned spacecraft in order to better characterize the diurnal cycle of the atmosphere's vertical structure. This information could also be useful in helping to better characterize the instrument itself and to account for satellite drift-induced changes in the tropospheric temperature record.
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