An Observing System Simulation Experiment for the Laser Atmospheric Wind Sounder (LAWS)

Abstract

A series of observing system simulation experiments (OSSES) was conducted to assess the potential impact of the Laser Atmospheric Wind Sounder (LAWS) instrument on a 5-day forecast using the Florida State University (FSU) primitive equation multilevel spectral global circulation model. This proposed Earth Observing System satelliteborne instrument is a CO2 Doppler lidar wind sounding system. The instrument's requirement for usable measurements is that clouds or high concentrations of tropospheric aerosols must exist within the sample volume. Two different orbits, a 55° inclined and a 98° sun synchronous, were examined by adding simulated LAWS wind profiles into a global four-dimensional data assimilation system and comparing the analyses and forecasts to a control experiment. Also, two different sets of simulations were examined for the 55° inclined orbit. The first set followed the assumption of other previous Doppler lidar wind sounding OSSES; a global concentration of aerosols exists such that observations will be usable at each pulse location. The second set of simulations incorporated the effects of subvisual cirrus, aerosols, molecular attenuation, and sampling-scale turbulence. All simulations of LAWS wind observations are degraded when the lidar pulse encounters the earth's topography and when the downward-integrated cloud amount reaches a critical threshold. The four-dimensional data assimilation system consists of a multivariate optimum interpolation analysis and a nonlinear normal-mode initialization using the aforementioned FSU global circulation model. In this set of assimilations only upper-air data was used with the exclusion of temperature sounder data, which may reduce the overall skill of the forecasts in the largely data-void Southern Hemisphere, which has been seen in temperature sounder OSSES. The inclusion of LAWS wind observations exhibits an overall improvement of the forecast skill for this study. The greatest increase in skill is in the Southern Hemisphere, as can be seen in both the motion and mass fields. The 98° sun-synchronous orbit resolved the polar meteorological features much better than the 55° inclined orbit. Otherwise, the two different orbits were very similar, with the 55° inclined showing a better analysis of the tropics. A comparison of the two different sets of simulations for the 55° inclined orbit is also revealing. The incorporation of physical effects into the LAWS simulations shows a reduced forecast skill as compared with the other set of simulated LAWS observations. However, a global increase in forecast skill can still be seen over that of the control experiment. This is an attempt at creating more ?realistic? simulations, and it is difficult to draw robust conclusions without completing a large number of forecasts, examining many different meteorological phenomena. In the future, the atmospheric physical effects will be much better incorporated into the LAWS simulated observations. The analysis of Global Backscatter Experiment data will aid in a better definition of global concentrations of aerosols and will attempt to relate specific concentrations of aerosols to chemical composition and associated large-scale meteorological features.