Active remote sensing uses artificially generated radiation,
emitted by a satellite or ground- based instrument and received
by the same instrument or another receiver. This radiation interacts
with the earth's surface or atmosphere, and from the measurements
meteorological and other information can be derived for eventual
use in numerical weather prediction models. Typical wavelengths
used include microwaves (wavelengths of a few millimetres) to
the optical and ultraviolet regions, depending on the application.
The method contrasts with passive remote sensing techniques where
an instrument measures the natural radiation signature from the
atmosphere or surface.
A scatterometer is a radar system which measures the level of
transmitted energy which has been backscattered from the surface
at two or more azimuth angles. Over the ocean, backscattering
is due to in- phase (Bragg) reflections of the microwave radiation
transmitted by the scatterometer. This backscattering is due to
the presence of trains of wind-generated waves of the order of
5- 20 cm wavelength. Because these small waves tend to lie perpendicular
to the wind direction, there is a larger backscatter in the up
or downwind look direction than the crosswind direction. Wave
amplitude, and hence backscatter, also increases with increasing
wind speed. From an empirical model relating wind vector and backscatter
for a given geometry, both wind speed and direction close to the
surface can be derived from two or more backscatter measurements
of the same area taken from different look directions.
The second generation of scatterometer was included as part of
the Advanced Microwave Instrument (AMI) on board the European
Space Agency's European Remote Sensing Satellite, ERS-1, launched
in 1991. An identical AMI instrument was launched on ERS-2 in
1995; this satellite took over from ERS-1 as the 'operational'
satellite in June 1996, and continued to produce data - although
a technical failure limits coverage to the North Atlantic. The
Satellite Active Sensing Group is currently involved in investigating
new scatterometer instruments for use in NWP. Wind retrievals
from the QuikScat/SeaWinds
instrument, launched in 1999, have been assimilated into the Met
Office's operational NWP model since December 2002, and similar
data from the ASCAT
instrument on MetOp
will become operationally available by early 2007.
|GPS temperature &
humidity sounding by radio occultation
The GPS radio occultation (RO) technique is based on measuring
the path of radio waves propagating between two satellites through
the atmosphere. This path is bent by atmospheric refractive index
gradients. The variation of ray bending with height above the
surface can be inverted to yield 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. The accuracy of the temperature
profiles is ~1.5K with a vertical resolution of ~1 km. The measurements
are globally distributed and have an-all weather capability. Recent
work within the Satellite Active Sensing Group has focused on
how GPSRO data should be assimilated into an NWP system. A 16-day
NWP global model assimilation trial performed at the Met Office,
using refractivity profiles from the CHAMP
instrument and all other available data sources, demonstrated
significant improvements in the 250 hPa temperature field RMS
fit to radiosondes in the southern hemisphere, with an improvement
in fit of ~0.1K at 24 to 96 hour forecast ranges. We have
developed the capability to operationally assimilate GPSRO data
from missions such as CHAMP, COSMIC and GRACE, and became the
first operational NWP centre in the world to begin operational
assimilation of GPSRO on 26 September 2006, using data from the
CHAMP and GRACE- A instruments.
The Met Office is a partner in the EUMETSAT
GRAS Meteorology SAF:
- to develop data assimilation methods using the GRAS radio
occultation instrument, scheduled for launch on Metop;
- to deliver the Radio Occultation Processing Package (ROPP)
to assist other NWP centres to exploit RO data in their models;
- to analyse the data flow and validate the quality of data
delivered to users by near- real time continuous monitoring.
sensing of integrated water vapour
Path delays to GPS satellite signals during propagation through
the troposphere to a ground receiver can be modelled with millimetric
accuracy. These delays can be used to produce estimates of vertically
integrated water vapour(IWV); ground-based GPS data is therefore
of potential use for both NWP and nowcasting applications. We
worked closely with several other groups from the geodetic and
meteorological communities in Europe through the recently completed
Action 716 and TOUGH projects
and ongoing E-GVAP project to
develop the networks, processing and assimilation techniques necessary
to exploit this potential source of data. Assimilation experiments
have demonstrated slight positive forecast impacts overall, and
significant improvements to short range forecasts in some cases.
The Doppler Wind Lidar technique, as its name suggests, uses
a lidar to detect the Doppler shift of the light backscattered
from atmospheric molecules 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
is assumed to measure the horizontal wind speed at the azimuth
angle of the beam - the 'horizontal line-of-sight' wind (HLOS).
It would need an azimuth-scanning instrument, or two synchronised
instruments in different orbits, to measure the true vector wind.
The ESA Atmospheric Dynamics Mission (ADM) or 'Aeolus'
is planned for launch in 2008. 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 sensibly interpreted by assimilation
into NWP models. The Satellite Applications section is starting
to investigate how HLOS winds can be assimilated so this data
source can be used soon after launch of ADM-Aeolus.