DECANO DE LA FACULTAD DE MEDICINA SAN FERNANDO

Water vapour is a key climate variable. In the troposphere, condensation of water vapour into precipitation provides latent heating which dominates the structure of tropospheric diabatic heating. Water vapour is also the most important gaseous source of infrared opacity in the atmosphere, accounting for about 60 per cent of the natural greenhouse effect for clear skies, and it provides the largest positive feedback in model projections of climate change.

In the troposphere, the radiative forcing due to direct anthropogenic sources of water vapour (mainly from irrigation) is negligible. Rather, it is the response of tropospheric water vapour to warming itself – the water vapour feedback – that matters for climate change. In GCMs, water vapour alone provides the largest positive radiative feedback, as it roughly doubles the warming in response to forcing such as from greenhouse gas increases.

In the stratosphere, there are potentially important radiative impacts due to anthropogenic sources of water vapour, such as from methane oxidation. In addition, water vapour is a source gas for OH, which is chemically active in the ozone budget. There is recent evidence that the Brewer Dobson circulation is changing in the Tropics. This may alter the balance of water vapour in the Upper Troposphere (UT) and Lower Stratosphere (LS), with potential feedback on climate change. There are also possible stratospheric water vapour feedback effects due to tropical tropopause temperature changes and/or changes in deep convection.

Total column water vapour representative of the water vapour in the lower troposphere can be estimated using microwave radiometers over the ocean and VIS/NIR radiometers over land. Because VIS/NIR observations are limited to clear skies and daylight, the capability to observe continuous total column water vapour with ground-based GPS receivers provides another alternative over land surfaces. The network of GPS receivers needs to be extended to provide coverage across all land areas, and the free exchange of these data needs to be implemented. Upper tropospheric humidity, often used in water vapour feedback studies, can be determined from strong absorption lines in the infrared and microwave spectral range. Infrared estimates are restricted to areas with no- or only low-level clouds, whereas microwave estimates are valid in all non-precipitating areas. This results in a global dry bias of the infrared estimates due to the clear-air-only sampling.

Observational capabilities in the stratosphere are not well co-ordinated. Microwave limb sounding and solar occultation can provide estimates of stratospheric water vapour profiles on a global scale, but these are currently provided by limited-term research missions. Comparisons between instruments flying at the same time (including such in situ data as is available) show differences on the order of 20 per cent or

greater, and there are problems in measuring water vapour at the lowest mixing ratios, both with remote sensing and in situ techniques.

Due to the importance of water vapour and the selective capabilities of different measurement techniques, a number of specialized water-vapour-related products are needed for this ECV:

Product A.5.1 Total column water vapour

Product A.5.2 Tropospheric and lower-stratospheric profiles of water vapour Product A.5.3 Upper tropospheric humidity

Benefits

• Determining of radiative forcing due to water vapour in both the troposphere and lower stratosphere and the nature of the water vapour feedback as greenhouse gases increase;

• Better understanding of the water cycle through water vapour transport estimates derived by using total-column water vapour together with precipitation and evaporation estimates;

• Better understanding of the processes controlling stratospheric water vapour and the links to stratospheric chemistry through Hox;

• Analysis of response of upper-tropospheric water vapour to deep-tropical convection;

• Better structural information on water-vapour distribution from 3-D fields derived from reanalysis. Target Requirements

Variable/ Parameter Horizontal

Resolution

Vertical Resolution

Temporal

Resolution Accuracy Stability

Total column-water vapour 25km N/A 4h 2% 0.3%

Tropospheric and lower- stratospheric profiles of water vapour 25km (troposphere) 100-200km (stratosphere) 2km 4h (troposphere) daily (stratosphere) 5% 0.3%

Rationale: The resolution and accuracy requirements are set by the need to fully describe water-vapour (specific humidity) profiles and general atmospheric climatology (monitoring) and for use of data in reanalysis. Total-column accuracy is driven by the need to reliably link humidity changes to changes in precipitation and evaporation. Four-hourly resolution in the troposphere is also needed to avoid aliasing of the diurnal signal and for synoptic-scale resolution. The hourly resolution for upper-tropospheric humidity is needed for the study of variability at synoptic to intra-seasonal scales for water vapour associated with continental tropical convection, including the diurnal cycle. Stability targets for A.5.1 and A.5.2 are based on constant relative humidity and 0.2K/decade temperature trend.

Requirements for satellite instruments and satellite datasets

FCDR of passive microwave and infrared imager radiances, VIS/NIR spectrometer radiances, for example through:

• Continuity of microwave imager radiances on at least two polar-orbiting satellites;

• Continuity of geostationary water-vapour imager radiances;

• Continuity of VIS/NIR spectrometer radiances over land;

FCDRs of IR and Microwave Spectral Range (MW) sounders, for example through:

• Continuity of polar-orbiting IR sounders (e.g. CrIS, IASI);

• Continuity of microwave humidity sounders on at least two polar-orbiting satellites;

• FCDR of limb sounder measurements (see also section 3.1.8); Supplemented by:

• Continuous ground-based measurements of GPS zenith total delay. Calibration, validation and data archiving needs

• Reference profiles from balloon-borne in situ instruments, particularly important for the validation of

water vapour profiles;

• Commercial aircraft observations such as those from the Measurement of Ozone and Water Vapour by In-Service Airbus Aircraft (MOZAIC) programme, providing an important validation source for upper tropospheric humidity products;

• Upward-looking MW radiometers for total column-water vapour and Raman and DIAL lidars, used for water-vapour profiles (those instruments are ideally placed at GRUAN sites);

• Ground-based GPS receiver network;

• The proposed CLARREO mission, to provide an SI traceable standard for calibration in the IR;

• Development of SI-traceable standards for absolute calibration of microwave instruments. Adequacy/inadequacy of current holdings

• Total column-water vapour over oceans from SSM/I is well established, and different datasets have systematic global mean differences of less than 0.5 kg/m2;

• HIRS, Meteosat and AMSU-B/MHS UTH products are used for model validation, climate variability analysis and trend identification;

• Stratospheric water-vapour measurements are available only from research instruments. The principal historical record (SAGE, HALOE) ended in 2005 and has not been adequately linked to the current record (from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), SMR, ACE-FTS, GOMOS and SCIAMACHY); there are currently no plans for future measurements of sufficiently high vertical resolution.

Immediate action, partnerships and international coordination

• Maintenance of planned deployment of instruments, to be consistent with existing

capabilities/datasets and to improve pre-launch characterization of IR instruments;

Construction of improved FCDRs – from HIRS data from 1979 to present – by using IASI and AIRS;

• Extension of SSM/I FCDR from 1987, with earlier data from SMMR (1979-1984), and with SSMIS data beyond the SSM/I era;

• Extension of MW sounder FCDR datasets with data from Special Sensor Microwave

Temperature/Humidity Sounder (SSM/T2) (from 1994);

• Development of a strategy for systematic measurement of lower stratosphere water vapour, building on experience already obtained from limb-sounding and solar occultation;

• Encouragement of new initiatives (e.g. SCOPE-CM) to sustain data set generation and quality assessment;

• Coordination, by GEWEX Radiation Panel and CGMS ITWG climate WG with the GEWEX Radiation Panel, to initiate and perform a formal assessment of existing water-vapour datasets;

• Enhancement of spectroscopy databases to improve radiative transfer for hyperspectral IR sounders;

• Research toward derivation of water-vapour profiles from radio occultation, to clarify the potential of GPS-RO data to benefit the analysis of humidity;

• Free exchange, for research purposes, of data from ground-based GPS receivers, lidars, and MW radiometers used to observe total-column water vapour (via total zenith delay).

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Link to GCOS Implementation Plan

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• [IP-10 Action A22] Finalize standard and implement exchange of data globally from the networks of ground-based GPS receivers;

• [IP-10 Action A26] Establish long-term limb-scanning satellite measurements of profiles of water vapour, ozone and other important species from the UT/LS up to 50km

Other applications

• As regards hydrology, surface humidity is important in the calculation of potential evapotranspiration;

• As regards oceans, surface humidity (with surface wind and surface temperature) is a key variable in determining the latent heat flux over oceans.