Methods and Science used in Medspiration
SST Definitions
The thermal structure of the top few metres below the sea surface is quite complex, as shown in figure 1; Because of this, different methods for measuring SST may record different values. This has important consequences for the accuracy and the precise calibration of satellite SST datasets. In order to help to clarify the issues, the science team of GHRSST-PP has distinguished between a number of different representations of SST, set out as follows:
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| figure 1 Schematic diagram showing (a) idealised night-time vertical temperature deviations from SSTfnd and (b) idealised day-time vertical temperature deviations from SSTfnd in the upper ocean. Although the distinctions between the different definitions of SST may seem arcane, they are very significant when uncertainties of temperature measurement less than 0.3°C are being sought. |
- 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 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 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 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 models. It is never observed directly by satellites, but it comes closest to being detected by a microwave radiometer which penetrates the skin, at dawn when the previous day's diurnal stratification can be assumed to have decayed and SSTsubskin, SSTdepth and SSTfnd are equal.
Diurnal Variability of SST
As outlined in the SST definitions section, SST can vary dramatically depending on the instrument used, the depth it is measured at, and the time of day (diurnal variability).During calm conditions, shortwave radiation from direct insolation can penetrate the sea surface and heat the upper layer of water by as much as 5K. Once the source of heating is removed (ie the sun goes down) the upper layer loses heat, convective overturning starts, and the stratification built up during the day is rapidly eroded. Daytime measurements may not accurately represent the upper mixed layer temperature, depending on the strength of the diurnal stratification.
A single satellite may always observe a part of the ocean at the same time of day, and so not be able to extract the diurnally variable part of the SST measurement. In situ measurements, depending on their depth, may not detect some of the diurnal stratification. It is therefore important to sample SST often enough to accurately resolve diurnal variability. Resolving the diurnal cycle is one of the aims of the Medspiration project.
Measuring SST from Space
There are two types of sensor used to measure SST from space, infra-red radimeters and microwave radiometers.
Infrared radiometers
Infra-red sensors (operating in the wavebands 10 to 12.5 and 3.5 to 3.9 microns) cannot penetrate cloud, but in cloud-free conditions they can resolve in fine spatial detail down to length scales of about 1 km.Microwave radiometers
Microwave sensors (operating in the frequency band 6 to 11 GHz) are less affected by the atmopshere apart from heavy rain. They can therefore penetrate cloud, but their spatial resolution is presently no better than about 50 km.Polar orbiting satellites
The orbits of these satellites carries them within a few degrees of the Earth's poles, and they complete around 15 orbits a day (approximately one every 100 minutes). The satellite measures a swath of the earth's surface below it as it travels. Due to the relatively low orbit of these satellites, data is usually of high spatial resoulution.However, the temporal resolution of these satellites is dependent on their orbit and sensor characteristics. Satellites such as Envisat have a 35 day repeat cycle - the time taken to re-visit the same spot above the earth's surface. The time taken to make a repeat measurement at a point on the earth's surface may be significantly reduced if the instrument has a wide swath width, or the point is at high latitude, where the orbit tracks are closer together.
Geostationary satellites
The Orbit of Geostationary satellites is much higher - around 24,000 km. The satellite is positioned directly above the equator, and its speed is presicely matched the the speed of rotation of the Earth. The result is that the satellite stays in the same location relative to the earth's surface. The satellite can continuously monitor a large area, and a few well placed satellites can cover a large part of the earth's surface. The major disadvantages are that higher latitudes are not well observed, and the higher orbit leads to lower spatial resolution.In Situ Data
Collecting data using instruments that are at the sea surface is a vital part of measuring SST. In-situ SST measurements are made by a wide variety of buoys, floats, research ships, and ships-of-opportunity. These instruments can sample at high frequency, and with high accuracy, but do not have the wide spatial coverage of satellite sensors. The primary function of making measurements in this way is the calibration and validation of satellite data. However, it is also important to make measurements when satellites cannot see the sea surface (for example, when it is cloudy) to eliminate any biases that may arise.Measurements from these sources that are coincident with Medspiration L2P data will be input into the Medspiration MDB