Simulations of Spaceborne Lidar Measurements of Boundary Layer Temperature and Water Vapor

Currently there are numerous efforts to quantify the boundary layer top using lidar technologies where such knowledge can indirectly lead to improvements in the estimates of the profiles of temperature and water vapor (hereafter called thermodynamic profiles) within the planetary boundary layer. For example, ceilometer networks are being developed in Europe, the US and elsewhere to respond to the need for improved knowledge of boundary layer evolution. But accurate quantification of the thermodynamic profiles within the boundary layer is of such primary importance to weather forecasting, radiative transfer and understanding land surface-atmosphere interactions and convection initiation that direct measurement of these profiles within the boundary layer is an important research goal (Wulfmeyer et al., 2015). Ground based profilers using both active and passive sounding approaches are under development with the goal of establishing networks of thermodynamic profilers that can improve numerical modeling on the regional scale. But only spaceborne measurements offer the potential for significant improvement in our knowledge of the boundary layer thermodynamic profile on a global scale (Di Girolamo et al., 2006, 2018). And only active sounding systems such as lidar possess the vertical resolution to resolve water vapor and temperature gradients within the planetary boundary layer (Wulfmeyer et al., 2015)

In response to the European Space Agency Call for Earth Explorer-10 Mission Ideas released in September 2017, a combined Mie-Raman vibrational, rotational lidar for direct measurements of the boundary layer thermodynamic profiles was proposed by an international group led by Dr. Paolo Di Girolamo of the University of Basilicata. The proposed system called ATLAS (Atmospheric Thermodynamics Lidar in Space) uses a 4 m telescope with 25 microradian field of view and 250 W laser emitting pulses at 100 Hz at 355 nm and follows a 450 km high dawn/dusk orbit. We have performed simulations of the measurements of such a system using a lidar simulator with well-established heritage (Whiteman, 2001, 2010, 2018). Input to the lidar simulator was provided by the NASA/GSFC GEOS Earth system model (Colarco et al., 2010) which was used to simulate a 24-hr orbit of the Calipso satellite on July 15, 2009. This is the same orbit that was used in a recent study of spaceborne lidar aerosol measurement capability (Whiteman et al., 2018). Operating in reanalysis mode, GEOS provides realistic profiles of atmospheric temperature, pressure, aerosols and water vapor at a horizontal spacing of 80 km for the 24-hr orbit resulting in 8640 profiles. The scene radiance was provided by the VLIDORT radiative transfer model run in parallel with GEOS.

An example of the simulated lidar temperature retrievals from the Calipso orbit simulation can be seen in Figure 1. The input temperature field from GEOS is shown on the left and on the right is the simulated ATLAS temperature. Calipso is part of the A-train and does not follow the desired dawn/dusk orbit proposed for ATLAS. Therefore, further work is in progress to modify the radiance fields used in the current GEOS output to more accurately reflect a dawn/dusk orbit. We also are pursuing higher spatial resolution (7km horizontal) GEOS profiles with the goal of performing data assimilation studies using simulated lidar measurements at this higher spatial resolution. Updated results and a status report of the investigation will be provided at the conference.

References

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