Satellite active sensing

Active remote sensing uses artificially-generated radiation emitted by a satellite or ground-based instrument, modified by the Earth's surface or atmosphere and received by the same instrument, or another receiver. Typical wavelengths used include microwaves (wavelengths of a few millimetres) to the optical and ultraviolet regions. The method contrasts with passive remote sensing techniques where an instrument measures the natural radiation signature from the atmosphere or surface.

Meteorological information from these 'active' measurements can be used in numerical weather prediction models. Quantities that can be derived include:

• near-surface wind speed and direction over the oceans
• vertical profiles of horizontal wind speed and aerosols
• profiles of pressure, temperature and humidity
• vertically integrated water vapour
• cloud depth
• rainfall rates

The Met Office uses:

• Wind scatterometers
• Radio occultation
• GPS ground-based sensing of integrated water vapour
• Doppler Wind Lidar (in the future)

Wind scatterometers

A scatterometer is a radar system that measures the level of transmitted microwave energy which has been backscattered from the surface at two or more look angles. Over the ocean, backscattering is due to in-phase reflections of the microwave radiation transmitted by the scatterometer.

This backscattering is due to the presence of trains of wind-generated waves of 5-20 cm wavelength. Because these small waves tend to lie at right angles to the wind direction, there is a larger backscatter in the up- or downwind direction than the crosswind direction.

Wave amplitude, and hence backscatter, also increases with increasing wind speed. Measuring the backscatter at two or more angles allows both wind speed and direction at the surface to be derived, which can then be assimilated into our models. Soil moisture amounts can also be derived from backscatter signatures over land.

Radio occultation

Radio occultation (RO) is based on measuring the path of radio waves passing through the atmosphere from one satellite to another. This path is bent by atmospheric density gradients. The variation of ray bending with height above the surface can be inverted to give the refractive index as a function of height. In regions where the atmosphere is dry, this information can be combined with the ideal gas law and the hydrostatic equation to give a temperature profile. At the Met Office, we assimilate RO refractive index directly, from which we can also extract information on humidity in the lower atmosphere.

Ground-based GPS

Signals transmitted by GPS satellites to ground-based receivers are slowed down due to the refractive index of the atmosphere. With a dry atmosphere, these path delays can be modelled with millimetric accuracy and so any residual delay is due to water vapour. These residual delays can be used to produce estimates of vertically integrated water vapour. Assimilation experiments have shown improvements in some short-range forecasts and data from the European network are used operationally in our regional and UK models.

Doppler Wind Lidar

The Doppler Wind Lidar (DWL) technique uses a lidar to detect the Doppler shift of the light backscattered from atmospheric molecules, cloud droplets or aerosols. The Doppler shift is directly related to the wind speed in the line-of-sight of the lidar beam. Since vertical wind speeds are relatively small, the instrument effectively measures the horizontal wind speed at the direction of the lidar beam — the 'horizontal line-of-sight' wind (HLOS). If two or more lidar beams are used at different azimuth angles, both wind speed and direction can be obtained.

The ESA Atmospheric Dynamics Mission (ADM) or 'Aeolus' is planned for launch in 2009. This mission is dedicated to the measurement of wind profiles using the DWL technique, but being a demonstrator mission, is limited to deriving the HLOS only. Such a measurement can only be easily interpreted by assimilation into NWP models. We are currently researching how HLOS winds can be assimilated, so this data source can be used soon after launch of ADM-Aeolus.

GRAS Satellite Application Facility

As a partner in the EUMETSAT GRAS Meteorology SAF we:

• developed data assimilation methods using the radio occultation instruments on Metop and other satellites
• deliver the Radio Occultation Processing Package (ROPP) to assist other numerical weather prediction centres to exploit radio occultation data in their models
• monitor the data flow and validate the quality of near-real time RO data delivered to users