Ground-based remotely sensed observations

2.2.2 Ground-based remotely sensed observations

The high cost of measurements which need manual intervention and the need for better observations of convective processes with high temporal and spatial resolution have pushed forward new sensor technologies that have now reached various degrees of maturity. While some of the techniques presented below are already used in NWP, other challenges lie in the setup and operation of unattended all-weather observations and consistent quality assurance to make them

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suitable for NWP and eventually for climate monitoring. The geographical coverage of some exemplary networks is shown in Figure 3.

Figure 3. Maps of exemplary ground-based networks a) CWINDE network; b) European lidar and ceilometer network; and c) MWRnet from COST EG-CLIMET final report

Ground-based measurements of signals from the Global Navigation Satellite System (GNSS) that give the zenith total delay (ZTD) are starting to be widely distributed. ZTD (Yan et. al., 2009) is related to the total column water vapour in the atmosphere and can be measured under all weather conditions. The measurements are made in many countries but the dissemination of the data to NWP centres in real-time is still work in progress. Europe and the US have good networks which provide data in near-real-time time but there is scope for much more data to be made available from other countries.

For continuous profiling of the atmospheric state, i.e. winds, humidity, temperature, ozone, clouds and aerosol properties, various remote sensing instruments offer inexpensive, unmanned ground based observational possibilities. The following are examples of some networks:

• Ceilometer networks that are deployed by meteorological services for visibility and cloud base height also give aerosol backscatter profiles that can be used for operational boundary layer monitoring (Haeffelin et al. 2012).

• Doppler wind lidar networks exist, but the number of these instruments is currently much smaller than the number of ceilometers that are deployed. However, the Doppler lidar offers a more direct measurement of the turbulence together with the horizontal wind vector but

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are limited to the lower (aerosol rich) part of the atmosphere. In recent years their usefulness for wind energy applications has led to a rapid increase in the number of operating systems making them a candidate for future networks.

• Radar wind profilers can cover the full troposphere and have a much higher degree of maturity than Doppler lidars. Networks on both sides of the Atlantic, e.g. US and southern Canada, CWINDE (Co-ordinated wind profiler networking Europe) have already been established and data from several stations are already assimilated into NWP models and have shown substantial value (Benjamin et. al., 2004, Illingworth et. al. 2015). However the coverage of this network has declined in recent years particularly over the US.

• Microwave radiometers can operationally provide temperature profiles of the lower atmosphere though the vertical resolution rapidly decreases with altitude during nearly all weather conditions (Löhnert and Maier, 2012). In addition, cloud liquid water path and some information on the water vapour profile is available. More than 100 radiometers are

organized in the voluntary MWRnet (an International Network of Ground-based Microwave Radiometers) that develops joint quality assurance procedures and has supported first attempts to assimilate them into NWP. They are also used for climate studies and NWP validation. Experimental studies are also exploring the use of volume scanning microwave radiometry for severe convection forecasting.

• Infrared spectrometers are superior to microwave radiometers in terms of vertical resolution in the retrieved temperature and humidity profiles. These retrievals are limited to cloud free scenes; however, new research has demonstrated that accurate thermodynamic profiles can also be retrieved below cloud (Turner and Löhnert 2014). Ideas for a ground-based observing network, where ground-based infrared spectrometers serve as the core instruments at each station are currently under consideration.

• Water vapour lidar, i.e. Raman and differential absorption lidar (DIAL) allow the

determination of high vertical resolution water vapour and aerosol profiles under clear sky conditions or below cloud base. Though they are currently rather complex in terms of operation and cost, research efforts to develop low-cost versions are ongoing. Some Raman lidars are also configured to measure temperature profiles.

• Weather radars are now widely used in NWP models both for defining the wind field and mostly in research mode for defining areas of precipitation through assimilation of the radar reflectivity. Phased array radar techniques that make use of electronic scanning instead of mechanical scanning offer an exciting possibility for much faster and flexible scanning of precipitation and also of clear-air convection.

• Lightning networks that can observe cloud-ground lightning and Cloud-Cloud lightning (Rodger et. al., 2006) are increasingly used to investigate the evolution of convective cells, lightning activity and electrical vertical structure of heavy rainfall cells.

• GALION (GAW Aerosol Lidar Observation Network) is a network of networks as it is not feasible to implement a global aerosol lidar network by installing a homogeneous set of systems at a number of stations selected for optimal coverage. Instead GALION makes use of existing systems at established stations, of the experienced operators of these systems, and of existing network structures. The structure and development of GALION is described in the GAW Report No. 178.

• The international Network for the Detection of Atmospheric Composition Change (NDACC) is composed of more than 70 high-quality, remote-sensing research stations for observing and understanding the physical and chemical state of the stratosphere and upper

troposphere with an emphasis on the long-term evolution of the ozone layer (http://www.ndsc.ncep.noaa.gov/).

Each of the instrument networks above provides limited information on the atmospheric state.

Hardesty et al. (2012) summarize the suitability of the different instruments for operational thermodynamic profiling. Data assimilation into NWP models involving observation operators to convert model variables into the measured quantities provides an efficient means to exploit these non-standard measurements. Alternatively, the synergy of different instruments is exploited by combining their measurements within one retrieval algorithm providing products that can be

assimilated without the use of complex forward operators. Both methods are used in current NWP models depending on the type of measurement being assimilated.

Despite the developments within the last decade, some gaps in the current observation system exist, e.g. a true reference turbulence observation ‘away from the surface’. In this respect several efforts are made to characterize turbulence by deriving higher order moment distributions of

temperature, water vapour, and vertical wind in the convective boundary layer with specialized lidar systems (e.g. Turner et al. 2014, Lenschow et al. 2012) though much work needs to be done.

In order to demonstrate the utility of the different instruments for NWP, prototype networks have been setup for different applications and limited areas, e.g. the Front Range Observational Network Testbed (FRONT). For Europe the COST action TOPROF aims to specify an optimum European network of inexpensive, unmanned ground based profiling stations, which can provide continuous profiles of winds, humidity, temperature, clouds and aerosol properties. A ground-based

remote-sensing network consisting of 5 climatologically different stations for the observation of wind, aerosol particles, cloud and precipitation has recently been set up in Finland (Hirsikko et al.

2014). In addition tools for data assimilation, e.g. a version of the fast radiative transfer model, RTTOV, (Saunders et al. 1999) for up-looking geometry, are also being developed.