Medspiration â System Requirements Document - Data User Element

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Medspiration System Requirements Document MED-SOC-RS-001_1 Issue F 2. SYSTEM OVERVIEW 2.1 System Context The purpose of the Medspiration sea surface temperature (SST) processing system is to fulfil the role of the European regional data assembly centre (RDAC) for the Global Ocean Data Assimilation Experiment (GODAE) High Resolution Sea Surface Temperature Pilot Project (GHRSST-PP). The primary aim of GHRSST-PP is to develop and operate an operational demonstration system that will deliver global coverage SST data products for the diverse needs of GODAE and the wider scientific community, at high resolution (better than 10 km and resolving the diurnal cycle) and within a short interval (~24 hours) from real-time acquisition. In parallel with RDACs serving other parts of the world, Medspiration will produce in near real time a new generation of SST data products for Europe, based on the combination of existing SST products from both infrared and microwave radiometers (nominally level 2 although some are level 3) already being generated and served by several different agencies. The ingested SST data are referred to generically as L2 SST inputs The function of the Medspiration processing system is threefold: • To generate specific combined SST products (described in 2.1.2) in near-real time and to serve them to the GDAC and to European operational ocean models. • To populate an off-line archive in which to curate them. • To provide a data product dissemination service for all types of users. 2.1.1 The importance of SST definitions used in Medspiration In its search for accuracy and rigour in calibration when combining SST detected by different classes of instrument, the GHRSST-PP processing model [RD.1] has had to take into account the thermal structure of the near-surface layers of the ocean. Since this has significant consequences for a number of the data processing stages, it is appropriate to provide a brief explanation at this stage. Figure 2-1 illustrates schematically how temperature varies in the upper few metres of the water column. Five different expressions of SST are shown. • SSTint is the hypothetical concept of the temperature of the interfacial layer of water and air molecules at the sea surface. It is not measurable and not used in GHRSST- PP. • SSTskin is defined within GHRSST-PP as the radiometric temperature of the surface © 2004 SOC Page 14 of 193
Medspiration MED-SOC-RS-001_1 System Requirements Document Issue F measured by an infrared radiometer operating in the 10 – 12 µm waveband. Physically it represents the temperature of the water at a depth of approximately 10 – 20 µm. • SSTsubskin represents the temperature at the base of the thermal skin layer. The thermal skin layer is a region less than 1 mm deep in which the convective exchange of heat by turbulent mixing is inhibited by the proximity of the sea surface, so that the net outward flow of heat through the surface creates a steep reduction of temperature towards the surface. Below the skin layer, turbulent processes ensure that the temperature is nearly uniform over a depth of at least a few centimetres. By definition within GHRSST-PP this corresponds to the SSTsubskin temperature. In practice SSTsubskin is assumed to be approximately equal to the radiometric temperature measured by a microwave radiometer operating in the 6-11 GHz frequency band, although the relationship is not exact or fully known. • SSTdepth is the generic term used to represent the temperature measured by a contact thermometer within the upper few metres of the water column and generally referred to as the “bulk” SST. If the water column below the skin layer is uniform (typically the case at night and when wind mixing is strong) then SSTdepth is the same as SSTsubskin, irrespective of the actual depth of the measurement (see Figure 2-1a). However, when daytime solar shortwave radiation penetrates to heat the water below the skin layer, and if wind mixing is weak, stable stratification develops in the upper few metres of the water column, in which the temperature increases towards the sea surface (apart from the cool skin layer which lies right at the surface - see Figure 2-1b). This phenomenon is called a diurnal thermocline. When it occurs a measurement of SSTdepth, made from a buoy or ship-mounted thermometer which does not precisely specify the sampling depth, is of limited value. A diurnal thermocline almost always collapses to a uniform temperature some time after sunset when surface cooling removes the excess heat. Under calm conditions, however, it may take several hours for the diurnal thermocline to decay entirely. • SSTfnd is defined within GHRSST-PP as the temperature at the base of the diurnal thermocline. It is so named because it represents the foundation temperature on which the diurnal thermocline develops during the day. SSTfnd changes only gradually along with the upper layer of the ocean, and by definition it is independent of skin SST fluctuations due to wind- and radiation-dependent diurnal stratification or skin layer response. It is therefore updated at intervals of 24 hrs. SSTfnd corresponds to the temperature of the upper mixed layer which is the part of the ocean represented by the top-most layer of grid cells in most numerical ocean © 2004 SOC Page 15 of 193
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