No abstract

Four recent reanalyses—the 55-yr Japanese Reanalysis Project (JRA-55), Interim ECWMF Re-Analysis (ERA-I), NCEP Climate Forecast System Reanalysis (CFSR), and NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA)—are assessed to clarify their quality in representing the diurnal cycle over East Asia. They are found to present similar patterns/structure and summer progress of the mean wind diurnal cycle, whereas they exhibit some differences in diurnal amplitude, particularly for the low-level meridional wind. An evaluation with intense soundings suggests that the amplitude difference mainly results from the diurnal variation of mean bias that differs among reanalyses. The root-mean-square (RMS) error is found to have a diurnal variation more evident in CFSR and MERRA than that in JRA-55 and ERA-I, which strongly affects the representation of the varying diurnal amplitude at the peak hours of RMS error. Compared with satellite-derived rainfall, the four reanalyses are shown to reproduce well the rainfall diurnal cycle over East Asia in terms of large-scale terrain contrast, summer progress, and interannual variability. JRA-55 even presents a long-term increase of morning rainfall percentage over the east China plain over the past four decades, consistent with rain gauge observations. The four reanalyses exhibit some considerable discrepancies at regional scale; JRA-55 gives the best capture of the rainfall diurnal cycle over the Tibetan Plateau and the eastward propagation to the eastern lees. These results suggest that new reanalyses are potentially applicable for studying the large-scale diurnal variability over East Asia, whereas their different preferences, especially at regional scale, should be of concern in data application.

An analysis method is proposed for polar cold airmass streams from generation to disappearance. It designates a threshold potential temperature θT at around the turning point of the extratropical direct (ETD) meridional circulation from downward to equatorward in the mass-weighted isentropic zonal mean (MIM) and clarifies the geographical distributions of the cold air mass, the negative heat content (NHC), their horizontal fluxes, and their diabatic change rates on the basis of conservation relations of the air mass and thermodynamic energy. In the Northern Hemispheric winter, the polar cold air mass below θT = 280 K has two main streams: the East Asian stream and the North American stream. The former grows over the northern part of the Eurasian continent, flows eastward, turns down southeastward toward East Asia via Siberia, and disappears over the western North Pacific Ocean. The latter grows over the Arctic Ocean, flows toward the eastern coast of North America via Hudson Bay, and disappears over the western North Atlantic Ocean. In their exit regions, wave–mean flow interactions are considered to transfer the angular momentum from the cold airstreams to the upward Eliassen–Palm flux and convert the available potential energy to wave energy.

A new formulation is proposed to diagnose wave-mean flow interactions and Lagrangian-mean meridional circulations within a finite amplitude nongeostrophic system. In this scheme, the vertical coordinate is defined by the zonally averaged pressure along isentropic surfaces and all variables representing zonal mean fields are averaged with normalized weights proportional to the mass of air between two isentropic surfaces. Although the zonally averaged momentum, continuity and thermodynamic equations are formulated in a way similar to the transformed Eulerian mean (p-TEM) equations by Andrews and McIntyre, the thermodynamic equation of the new scheme does not have eddy terms. This makes it possible to correct for `Stokes drift' in meridional circulations for finite amplitude disturbances.Using a perpetual January run of the NCAR community climate model, a preliminary diagnostic study demonstrates a practical advantage of the new scheme. Comparisons of the new scheme with the p-TEM show considerable differences in meridional cross sections of the mass streamfunctions and the Eliassen-Palm flux divergence. In particular, the mass streamfunction obtained with the new scheme forms a hemispheric single cell circulation in the lower stratosphere (the so-called Brewer-Dobson circulation).As a further application of the new coordinate system, a two dimensional axisymmetric tracer transport equation is formulated, where diffusion due to adiabatic motions is expressed by a symmetric tensor involving only one independent variable.

[1] Using the satellite data, spatial patterns of precipitation diurnal cycles and their seasonality were examined with emphasis on southeastern China (SEC). Results show that spatial distributions of diurnal cycles over SEC have a robust large-scale seasonality in which the regional differences are evidently embedded. Rainfall diurnal variability is weak in spring but it becomes more pronounced from presummer. Both the mean rain rates and amplitudes of diurnal cycles experience remarkable amplification during presummer. The widespread and strong morning rainfall dominates the SEC area, especially inland valleys and plains, and offshore areas. The morning peak rainfall over western SEC is largely contributed by the increasing rain frequency and diurnally varying intense rain rates. Even over eastern SEC, morning rainfall still has a comparable magnitude to afternoon rainfall. In contrast, spatial distributions of diurnal cycles in midsummer are dependent primarily on topography. The morning (afternoon) rainfall is mainly located over valleys, basins, and oceans (plateaus and mountains). The afternoon peak rainfall becomes a notable feature over southern China. The signature of widespread morning rainfall decays during midsummer and remains apparent only in central eastern China, which is likely related to the north shift of summer rainband.