First Application of the Local Ensemble Tangent Linear Model (LETLM) to a Realistic Model of the Global Atmosphere
Journal Article - Open Access
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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The local ensemble tangent linear model LETLM provides an alternative method for creating the tangent linear model TLM and adjoint of a nonlinear model that promises to be easier to maintain and more computationally scalable than earlier methods. In this paper, we compare the ability of the LETLM to predict the difference between two nonlinear trajectories of the Navys global weather prediction model at low resolution 2.5 degrees at the equator with that of the TLM currently used in the Navys four-dimensional variational 4DVar data assimilation scheme. When compared to the pair of nonlinear trajectories, the traditional TLM and the LETLM have improved skill relative to persistence everywhere in the atmosphere, except for temperature in the planetary boundary layer. In addition, the LETLM was, on average, more accurate than the traditional TLM error reductions of about 20 in the troposphere and 10 overall. Sensitivity studies showed that the LETLM was most sensitive to the number of ensemble members, with the performance gradually improving with increased ensemble size up to the maximum size attempted 400. Inclusion of physics in the LETLM ensemble leads to a significantly improved representation of the boundary layer winds error reductions of up to 50 , in addition to improved winds and temperature in the free troposphere and in the upper stratospherelower mesosphere. The computational cost of the LETLM was dominated by the cost of ensemble propagation. However, the LETLM can be precomputed before the 4DVar data assimilation algorithm is executed, leading to a significant computational advantage.