Interactive comment on “Estimating the State of a Geophysical System with Sparse Observations: Time Delay Methods to Achieve Accurate Initial States for Prediction” by Z. An et al

important, particularly for ocean and climate reanalyses that attempt to reconstruct the past state of the Earth system (e.g. Compo et al., 2011; http://www.esrl.noaa.gov/psd/data/gridded/data.20thC_ReanV2.html). The experiment scenarios described here by the authors are perhaps most applicable to the estimation of the global ocean state after the introduction of satellite altimeters, e.g. TOPEX/Poseidon in late 1992 (https://sealevel.jpl.nasa.gov/missions/topex/), with their final set of experiments having a potential application to leverage data from the Global Drifter Program (http://www.aoml.noaa.gov/phod/dac/index.php). Thus from a practical point of view, the time-delay method has potential merit for operational scale data assimilation (DA) and reanalysis.