Status report on the key climate variables technical supplement to the
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- Satellite soundings
- Main climate application
GUAN
Specific recommendations for GUAN are:
And for the radiosonde network in general:
Satellite soundings
Variable: Upper air humidity (surface to 30 hPa) Main climate applicationUpper air humidity and related quantities such as precipitable water in layers must be measured accurately to validate models of hydrologic processes, to calibrate satellite and other remote sensing water vapour retrieval methods, to determine the radiative forcing due to water vapour and the nature of the water vapour feedback as greenhouse gases increase, and to increase knowledge of atmospheric chemistry processes in the ozone layer. Contributing baseline GCOS observations The Global Upper Air Network (GUAN) of 150 stations, described under the upper air temperature parameter, provides baseline upper air humidity measurements. Uncertainties in the measurement of humidity are much larger than in the measurement of temperature. Differences in the average reported humidity between instrument types can exceed 10% near the surface, with newer instrument types usually showing drier readings than older instrument types. The largest disparities are in very moist and very dry conditions. In the upper troposphere and stratosphere, most older instrument types are almost completely unresponsive to moisture changes. Many instrumental error sources are not well enough understood to develop robust corrections. Metadata describing instrument types and dates of changes are incomplete, even for GUAN stations. Other contributing observations The global upper air network includes several hundred land-based radiosonde stations in addition to those in GUAN, ships, dropsondes, and aircraft observations. Almost all soundings and many aircraft observations report both temperature and humidity measurements, so the discussion in the upper air temperature parameter is not repeated here. Large areas have few or no radiosonde or other in situ observations. Many experimental efforts to remotely sense atmospheric moisture using satellites have been published, but only a few retrievals are applied operationally. Some operational retrievals include the TIROS Operational Vertical Sounder (TOVS) instruments on NOAA satellites (but such retrievals are usually not made in cloudy areas), the Special Sensor Microwave/Imager (SSM/I) instrument on Defense Meteorological Satellite Program (DMSP) satellites (over oceans only), and Global Positioning System (GPS) measurements (water vapour changes the path length, so, given the temperature, the amount of water vapour can be extracted from the path length between 2 known locations). Validation by comparison with nearby radiosondes has been limited due to satellite and radiosonde errors, including differences between satellites, calibration drifts in each satellite, and differences between radiosonde types. Calibrated satellite retrievals can provide thousands of moisture observations per day with global coverage, but the vertical resolution of remotely-sensed observations is limited. Significant data management issues Radiosonde and other in-situ observations and satellite data are routinely archived and made publicly available through various data centres, as described in the upper air temperature parameter. Some data management issues relating to archived radiosonde data are as follows:
Analysis products Objective model analyses, including reanalyses (which process a long period using consistent methods), incorporate both upper air temperature and moisture observations. Without systematic application of adjustments for each instrument type to a hypothetical 'reference' instrument, assimilated radiosonde data introduce artificial discontinuities into the model, and interactions among data elements in the model may cause additional unwanted trends. For example, the global mass of dry air should be constant except for a very slow increase from anthropogenic greenhouse gases (primarily the carbon in carbon dioxide), and it may be difficult to obtain a reliable modelled moisture trend (derived from total pressure minus dry air pressure) if the quantity of dry air is unreliable. Current Capability Instrument discontinuities make radiosonde-based climate trends questionable. A modern instrument type can be more than 10% drier than an older instrument, and global instrument-caused drying from 1973 to 1996 averaged 4% (using preliminary adjustments). Global average tropospheric warming around 0.4 °C (if similar to the surface warming from the middle 1970s to 2000) with unchanged relative humidity would raise total atmospheric water vapour content by nearly 3%. Therefore, global water vapour trends are not reliably detected using unadjusted radiosonde data. Complete inferred metadata and validated instrument adjustments, currently being developed, should eventually allow precipitable water variations on interannual scales to be identified reliably. Upper-tropospheric humidity measurements have historically been considered so unreliable that many radiosonde observations omitted dew points in cold temperatures, usually under -40 or -50 °C. More recent sensors have moderate responsiveness at all temperatures, so the United States started reporting all dew points starting 1 October 1993. However, many stations using Vaisala radiosondes have stopped reporting dew points in cold temperatures in the last few years. Even at stations where dew points have always been reported, moisture trends from current operational radiosondes in the upper troposphere and lower stratosphere will probably never be considered reliable. Continuous global satellite data exist back to late 1978 from which atmospheric water vapour data can be constructed, but operational satellite moisture retrievals are not routinely published in a form suitable for climate trend monitoring. NESDIS computes precipitable water in 3 layers using each non-cloudy NOAA satellite sounding, but does not compute area or time averages. SSM/I monthly averages of precipitable water over global oceans are published in the Climate Diagnostics Bulletin, but no global average time series is included. Several long-term projects, including the international Pathfinder program, are developing climate datasets by reprocessing long satellite records with consistent algorithms. The NASA Water Vapour Project (NVAP) combines radiosonde, TOVS, and SSM/I data but so far covers only 1988 to 1997, and global averages contain the biases of the individual sources. Ground-based water vapour retrievals have also been made using lidar, and are considered to have good accuracy if calibration is carefully maintained. Ground-based remote sensing can provide high time resolution at each observing site. Ground-based GPS receivers show considerable promise for obtaining total column water vapour observations over land. Issues and priorities Directory: pages -> prog prog -> Regional Association IV (North America, Central America and the Caribbean) Hurricane Operational Plan prog -> Review of the ra IV hurricane operational plan prog -> Cyclone programme prog -> World meteorological organization prog -> World meteorological organization technical document prog -> Regional Association IV (North America, Central America and the Caribbean) Hurricane Operational Plan prog -> World meteorological organization ra IV hurricane committee thirty-fourth session prog -> World meteorological organization ra IV hurricane committee thirty-third session prog -> Review of the past hurricane season prog -> Ra IV hurricane committee thirty-fourth session ponte vedra beach, fl, usa Download 0.61 Mb. Share with your friends:
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