ATBD - GHG-CCI
ESA Climate Change Initiative (CCI) Algorithm Theoretical Basis Document Version 2 (ATBDv2) – UoL-FP for the Essential Climate Variable (ECV) Greenhouse Gases (GHG) Page 18 Version 2 – Draft 1 18 March 2013 the site altitude is lower than the lowest level of a grid point the pressure is calculated with respect to the lowest level, where the temperature and molar mass are extrapolated downwards based on the lapse rate and gradient of the 5 lowest levels above, respectively. The surface pressure for the site can then be resolved by interpolating the pressures with latitude, longitude and time. To validate the surface pressure two sites were chosen; Southern Great Plains (SGP)/USA, and Black Forest/Germany, which are flat and mountainous in topography respectively. The Atmospheric Radiation Measurement (ARM) program has a central facility in SGP which takes in situ ground based measurements of surface pressure using the Temperature, Humidity, Wind and Pressure Sensors (THWAPS) instrument. Figure 3-3 shows that the surface pressure determined is within 1 hPa of THWAPS observations. Surface Pressure observations were also made by the ARM mobile facility that visited Black Forest, Germany, in 2007 with the Surface Meteorological Instrument (MET). Figure 3-2 illustrates that over a mountainous topography the surface pressure can still be resolved to within 1 hPa. Hence, the surface pressure a priori can be constrained to less than 0.1%, although we use a 1- sigma error of 4 hPa on the a priori surface pressure estimate to allow for more difficult topographies. Figure 3-2. Validation of a priori surface pressure with in-situ ground based measurements from ARM instruments in Southern Great Plains/USA and Black Forest/Germany. 126.96.36.199 Temperature and Water Vapour The ECMWF specific humidity data can be used to generate water vapour volume mixing ratio profiles using the equation: where R d and R w are the gas constants in dry and wet air respectively. ECMWF also provides temperature profiles which, along with the H2O VMR , are interpolated with latitude,
ESA Climate Change Initiative (CCI) Algorithm Theoretical Basis Document Version 2 (ATBDv2) – UoL-FP for the Essential Climate Variable (ECV) Greenhouse Gases (GHG) Page 19 Version 2 – Draft 1 18 March 2013 longitude and time to the specific observation. A constant O 2 VMR profile of 0.2095 is also used as a priori. We have assumed the temperature and H 2 O profiles are correct but allow the retrieval to scale them. 188.8.131.52 Aerosol Profiles Aerosols are currently poorly characterized and little data is available. Therefore the same a priori for aerosol is used for all retrievals and a large a priori covariance (50 times the standard deviation of an aerosol profile) is used to allow a large flexibility and force the retrieval to rely more on the measurement in order to determine the parameter. Three atmospheric extinction profiles are used. Two of these profiles are the same shape, Gaussian profile with a 2 km height and 2 km width, and have a total column aerosol optical depth of 0.05 each. The only difference between these profiles are the aerosol properties used for each; based on Kahn et al, 2001, one uses classification 4b and the other uses classification 5b. Therefore providing one profile designed for carbonaceous and dusty environments and the other profile for carbonaceous and sooty environments. The third profile cases allows for the possibility of a cirrus cloud and is based on the height and thickness described in Eguchi et al, 2007. A Gaussian profile of 0.05 optical depth was created for each of the latitude regions described using the height and thickness given and then a look up table based on every 1 degree of latitude was created globally by interpolating between these regions with respect to latitude. Each retrieval uses the cirrus profile defined in the look up table that is closest to that retrievals specific latitude. The cirrus properties are constructed from ice cloud models based on the Baum models and an effective radius of 60 μm is used always. Figure 3-3. Gaussian profiles based on the height and thickness described for various regions by Eguchi et al, 2007.