Description of the TPW Retrieval Method

One method to retrieve atmospheric TPW is to deduce it from the vertical profile of temperature and humidity.

Satellite sensors measure radiation emitted from the Earth's atmosphere, surface and clouds. Active sensors (e.g. RADAR) emit a signal and measure the time the signal needs to return to the sensor. The distance between sensor and reflecting object can be deduced from the time lapse of the signal.

Here, only passive instruments are considered for the retrieval of vertical temperature and humidity profiles. They receive radiation from the Earth's atmosphere without emitting a signal.

The main problems to solve are:

  • How can we know from which atmospheric level the radiation comes from?
  • How can we infer the vertical distribution of temperature and moisture from the received signal?

Let's start with a special case:

Some channels are so called "window channels". This means the radiation reaching the satellite is emitted from a surface (e.g. soil, snow, cloud water droplets, ...) and reaches the satellite instrument without interacting with the atmosphere. In this simple case, we just need to know that the measurement is not affected by clouds to get the surface skin temperature by applying the Planck formula which establishes a relation between the temperature and the radiation emitted by an object. As radiation emitting surfaces are usually not "black bodies", an emissivity correction term has to be applied.

Figure 2: In atmospheric windows, radiation from the Earth's surface directly reaches the satellite instrument without interacting with the atmosphere. In case of clouds, radiation from higher atmospheric levels (cloud tops) reaches the satellite. Credits: Marianne König (EUMETSAT)

Fortunately for our purposes, most channels are absorption channels, which means that atmospheric trace gases like ozone (O3), carbon dioxide (CO2) or water vapor (H2O) prevent a "clear view" from the ground to the satellite. Radiation emitted from the ground is "weakened" by trace gas molecules because they absorb the radiation and scatter it in all directions (Figure 3).


Figure 3: H2O molecules absorb the radiation energy emitted from the Earth's surface. Credits: Marianne König (EUMETSAT)

Some trace gases like water vapour are so obtrusive that all radiation arriving at the satellite instrument stems from the atmosphere alone, since all direct radiation from the ground has been absorbed on the way to the satellite (Figure 4).


Figure 4: Contribution profile of a water vapour absorption channel (red) compared to a window channel (blue). Credits: Marianne König (EUMETSAT)

For a better understanding of the retrieval method, imagine a landscape covered with thick fog. Depending on the thickness of the fog, you will be able to discern objects 100, 300 or even 600 meters away from you.


Figure 5: Dense fog on a motorway prevents a view of more than 50 meters. All radiation from greater distances is absorbed and scattered by the fog droplets.

By knowing the amount of radiation emitted from the ground (window channels) and by measuring the radiation in the absorption channels, the retrieval algorithm will estimate the amount and distribution of trace gases. Of particular importance is the vertical distribution of water vapour, which is one of the most effective absorbers and greenhouse gases in the atmosphere.

The retrieval algorithm thus tries to find an atmospheric profile of temperature and humidity that best reproduces the observations from the passive satellite instrument. In general, this is a multi-solution problem, i.e. many different vertical distributions of temperature and humidity would lead to the same measurement. Hence a so-called "background profile" is needed to provide a first guess as to the real state of the atmosphere. This background profile comes from a numerical model.

For many satellite measurements, this first guess will represent the real atmosphere adequately. In cases where synthetic radiances based on NWP profiles and satellite-measured radiances diverge, an iteration process is used to adapt the NWP profiles of temperature and humidity to the satellite instrument measurements. The radiation transmission model is recalculated with a modified NWP input until its output fits the observed radiation. The iteration process produces a modified profile which includes the primary NWP information corrected by satellite observations.

The temperature and humidity profiles thus derived are the basis for TPW, layer precipitable water (LPW) and stability parameters.


Note

  • The higher the spectral resolution of the satellite instrument, the better the resolution of the vertical sounding of the atmosphere. Imager on GEO satellites have fewer spectral channels than the sounding instruments on LEO satellites.
  • The spectral range in microwave bands is better suited for water vapour sounding than the VIS/IR spectral bands of the imagers on GEOsatellites. (See next chapter)
  • GEO TPW products are generated for clear sky measurements only. A cloud mask is a mandatory input to the retrieval algorithms.