Workshop Introduction

The quality of measurements from the ASCENDS lidar required to address its primary science questions need to be better quantified, along with that of any ancillary measurements, such as temperature and water vapor.  This workshop will provide an important step in the process of defining the ASCENDS mission’s measurement requirements.  Several types of Observing System Simulation Experiments (OSSE’s) have been outlined to allow evaluating a realistic range of overall CO₂ measurement performances needed for this mission.  This workshop will update the definition and status of the CO₂ source/sink OSSE’s which will be conducted. It will also start the evaluation of the potential CO₂ measurement precision and accuracy of a generic laser absorption spectrometer (LAS) lidar instrument.

Several different global carbon cycle and/or atmospheric models will be considered for conducting the OSSE’s.  These must have sufficient spatial and temporal resolution to adequately incorporate simulated ASCENDS along-orbit CO₂ column measurements and to assess the simulated measurement’s impact on uncertainties in regional-scale CO₂ source/sinks.  The workshop will discuss the assumptions used in each model type. An approach will be used to ensure the results from the different models can be readily compared with respect to the impact of the simulated ASCENDS measurements.  The some relevant input parameters are surface fluxes, flux uncertainties, cloud and atmospheric scattering statistics, orbit, and CO₂ weighting function profiles with a range of realistic measurement precision and accuracy estimates over a horizontal distance.

A preliminary description of the expected random errors in CO₂ column measurements from a generic space based LAS instrument has been defined and will be discussed.  These errors depend generally on surface reflectivity and atmospheric attenuation, as well as the lidar’s power aperture product. Calculations of representative vertical sensitivity, or weighting functions, will also be discussed.   The initial OSSE’s will address the level of random measurement errors needed for different CO₂ flux questions. Subsequent work will address random errors in O₂ column measurements.

Approaches to subsequently model and assess the impact of potential systematic CO₂ measurement error sources will be discussed. Some sources might include errors in modeling CO₂ and O₂ laser line absorption (i.e., spectroscopy), errors in knowledge of atmospheric temperature or water vapor, and errors from cloud and aerosol backscatter Other potential measurement biases could be caused by non-linearities in instrument response, or a response changes which might depend on instrument temperature or sun angle.

The result from this workshop will be an informal report documenting the discussions. Also a recommended plan to complete the initial OSSE’s and develop an initial set of Level-1 measurement requirements for the ASCENDS lidar by the time of the fall/winter 2011 ASCENDS workshop.