Systematic errors in northern Eurasian short-term weather forecasts induced by atmospheric boundary layer thickness

Abstract

Systematic errors in forecast near-surface air temperature (SAT) still constitute a considerable problem for numerical weather prediction (NWP) at high latitudes. Numerous studies in the past have attempted to reduce this problem through recalibration of physical parameterization schemes and better approximation of the surface energy budget. The errors, however, remain despite notable improvements in the overall weather forecast performance. This study looks at the problem from a different perspective. It analyzes asymmetries in the SAT forecast errors. The study reveals a statistical pattern of warm SAT biases under cold weather conditions and cold SAT biases under warm weather conditions. The largest errors were found in shallow atmospheric boundary layers (ABLs). The study attributes the problem to the modeled excessive ABL thickness in northern Eurasia (the NEFI region). The ABL thickness is considered as a scaling factor controlling the efficacy of the applied surface heating. Too thick an ABL damps the magnitude and agility of the SAT response. The study utilized the operational model SL-AV of the Russian Hydrometeorological Centre. Two turbulence schemes were evaluated in the northern European and western Siberian regions of Russia against observations from 73 meteorological stations. The pTKE (old) scheme is based on the local balance of the turbulence characteristics. The TOUCANS (new) scheme incorporated the total turbulence energy equations in an energy-flux balance approach. Neither scheme uses the ABL thickness as a prognostic parameter. The study reveals that the SAT errors are consistent with the damped response of temperature and reduced agility of temperature fluctuations in too thick ABLs. The TOUCANS scheme did not improve those features, probably because it links the turbulent fluxes and the ABL thickness. The SAT errors in shallow ABLs persist in the new scheme. This study emphasizes the need for a closer look at the ABL thickness in the NWP models.