| Summary: | Marine air temperature observations are collected by volunteer observing ships carrying automatic meteorological sensor packages. Close proximity of the package mount to a dark colored ship's deck is known to introduce a warm bias on sunny afternoons in summer under low wind conditions. This observation set is used as an influential constraint in the MERRA-2 reanalysis, as well as several other reanalyses, in the poorly sampled surface marine boundary layer. Here we explore the consequences of bias in the marine air temperature observation set during the 36 year period 1980-2015. Marine air temperature observations have mainly been collected along major shipping routes, with the densest coverage in the North Atlantic and North Pacific. In the first part of this presentation we review the historical observation coverage and compare air temperature and contemporaneous SST measurements to identify systematic differences. The comparison suggests the presence of diurnal bias in the air temperature observations peaking in subtropics and midlatitudes latitudes in mid-afternoon local time. In the second part we explore the impact of the marine air temperature observations on MERRA-2 through analysis of the terms in the temperature forecast tendency equation, the equation which describes the contributions to changes in temperature during each analysis cycle. Examination of the temperature forecast tendency equation allows us to separate systematic errors due to such processes and radiation, mixing, and advection and allows us to identify the major source of systematic forecast tendencies. The results show that the marine air temperature measurements introduce an error that can reach 0.4K/dy which also peaks in local afternoon at subtropical latitudes. This bias varies with season and has gradually reduced from the beginning of the MERRA-2 reanalysis in 1980. In the final part we conduct numerical experiments with a version of the MERRA-2 analysis system, comparing experiments when the marine air temperature observations are included or withheld from the analysis system.
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