Medspiration – System Requirements Document - Data User Element

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
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