Data from the Japanese 25-yr reanalysis/Japan Meteorological Agency Climate Data Assimilation System (JRA-25/JCDAS) and the chemistry climate model (CCM) of the Meteorological Research Institute (MRI) were used in this study to investigate the annual synchronization tendency of the quasi-biennial oscillation (QBO) in the equatorial stratosphere. The annual synchronization refers to phase transitions of the QBO that tend to occur for specific seasons. The JRA-25/JCDAS reanalysis data exhibit similar features of the annual synchronization. The present diagnosis based on the transformed-Eulerian mean zonal momentum equation shows that zonal wind accelerations for the annual synchronization are largely unexplained by explicitly calculated terms, with a small contribution from resolved wave driving and a strong canceling effect from vertical advection. The CCM does not simulate the annual synchronization as observed, although it does simulate general features of the QBO, as well as annual variation in equatorial upwelling with some differences. The absence of the annual synchronization is related to an almost seasonally uniform distribution of parameterized gravity wave forcing. It is speculated from these results collectively that seasonal variation of convection and gravity waves plays a key role in inducing the annual synchronization tendency of the QBO under actual conditions.
The Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) 2A23 algorithm attempts to classify rain echo as stratiform or convective. Updates to the 2A23 algorithm for Version 7 (V7) have resulted in an increase (decrease) in the fraction of rain echo classified as convective (stratiform) compared with the previous version of the algorithm. This is caused in part by more shallow non-isolated echoes, which exhibit strong concentrations over the Pacific intertropical convergence zone (ITCZ), being classified as convective in V7. The tropics-wide (20°N-20°S) stratiform rain fraction has decreased correspondingly from 40% to 38% between Version 5 (V5) and V7. It is suggested that two additional stratiform categories could be considered convective because of the prevalence of warm-rain processes in each, particularly, higher altitude rain over mountainous terrain and shallow non-isolated rain over tropical oceans; this would decrease the tropics-wide stratiform rain fraction to 35%.
A parameter set expressing the potential response used to simulate evapotranspiration was inferred from pooled data of leaf-level stomatal conductance in the Far East. We used Jarvis-type parameterization, which consists of restriction terms of light, air dryness, temperature, and soil water. Two parameter sets, pooled common (PC) and within-site (WS) sets, were determined using stomatal conductance data of eight species at nine sites. Seasonal changes of energy and water fluxes observed at four tower sites are well reproduced by a land surface model, despite using a different set of parameters. The results suggest that energy and water fluxes can be simulated using only one common parameter set with no parameter tuning, at least for forests in a wide region of the Far East. We discuss why similar fluxes were simulated using PC and WS parameters despite the clear differences between these two parameter sets. Sensitivity experiments showed that the effects of maximum stomatal conductance and soil water content mutually cancel each other in Yakutsk and that an optimum temperature effect was added.
The dual-polarization (dual-pol) Doppler radar provides more information on precipitation particles compared with the traditional Doppler radar. In this study, the dual-pol radar observations of differential reflectivity (ZDR), specific differential phase (KDP), and radial velocity (VR) collected by the C-band Advanced Radar for Meteorological and Operational Research are assimilated for a convective storm on June 23,2008. Two different warm-rain radar forward operators for ZDR and KDP data assimilation are built on the basis of empirical relationships between the dual-pol variables and water content in precipitation. The advanced research version of the Weather Research and Forecasting (WRF) model and three-dimensional variational (3DVAR) data assimilation system are used in this study. The main goal of this study is to demonstrate the sensitivity of initialization and prediction of a real case event to the dual-pol radar data assimilation with different radar forward operators. The results show that both forward operators successfully assimilate the ZDR, KDP, and VR data and help with the initialization and short-term forecast for the thunderstorm event. In addition, significant sensitivity caused by the use of different radar operators is found in the thermodynamic, kinematic, and microphysical fields of the storm, especially over the storm region. However, the storm location and movement are not largely influenced; hence, short-term precipitation forecast is not largely affected by the use of different radar operators.
Global climate metrics are often used for model evaluation and selection of general circulation models. However, most future projection studies have a certain target region or phenomenon, while global climate metrics do not necessarily reflect model performance in reproducing local climate and/or phenomena. In this study, we introduce an iterative selection method of variables and regions to derive a regional climate metric that is also related to model performance in reproducing local phenomena, utilizing a collection of various phenomena metrics. Using the 20th century experiment outputs of the Climate Model Intercomparison Project phase 3 (CMIP3) models, we demonstrate the derivation of a prototype summer Eastern Asian metric. Three locally important phenomena, the seasonal progress of the jet stream, the seasonal progress of the Baiu, and the inter-annual variation of the Pacific-Japan pattern are chosen for the prototype metric. The variables iteratively selected for the new metric were 200-hPa zonal wind, 850-hPa meridional wind, and precipitation. The selected region included Siberia and the western North Pacific. The prototype metric correlates with the three phenomena metrics significantly better than the global climate metric. The resulting metric is strongly influenced by the chosen set of phenomena to be considered, and can be modified by altering the choice accordingly to the purpose of study.
Comparisons of relative humidity (RH) measurements between the Meisei RS-06G radiosonde and a chilled-mirror hygrometer revealed that the RS-06G radiosonde shows a stepwise change of ~3% RH at 0°C (drying when air temperature is decreasing). This is due to a discontinuous correction factor in the processing software that compensates for the temperature dependence of the RH sensor. Results from chamber experiments regarding the temperature and RH dependence of RS-06G RH sensors under steady-state conditions showed a wet bias exceeding 7% RH below ~+10°C. As this result contradicted previous in-flight intercomparisons that used the original manufacturer's correction, we investigated a possible additional dry bias caused by a thermal lag in the RH sensor. We speculated that the thermal lag of the RH sensor typically causes a dry bias during a tropospheric ascent, which largely compensates for the wet bias related to the temperature and RH dependence of the RH sensor. We observed that the experimental results of the temperature and RH dependence considering the thermal lag were in agreement with the extrapolation of the original manufacturer's correction. Consequently, we proposed to extrapolate the original manufacturer's correction, which is currently applied at temperatures between −40°C and 0°C, up to +14.5°C to resolve the artificial stepwise change at 0°C. Because the RS-06G radiosonde is a successor to the Meisei RS-01G and RS2-91 radiosondes, which have adopted the same RH sensor material installed since July 1999 and have used the same processing software, the current results should be applied to the data obtained by those radiosondes. The bias of RS-06G RH measurements using this new correction is estimated to be within 7% RH, which is within the manufacturer's specifications, being drier at +40°C and wetter between −40°C and +10°C.
The impact of optimizing the air-sea exchange coefficients and the initial state is examined for a tropical cyclone (TC) with the aim of improving an operational mesoscale four-dimensional variational data assimilation system. In our optimization approach for TC Chaba that approached the Ryukyu Islands in October 2010, the drag coefficient values are adjusted so that they saturate at extreme conditions, while the first guess increases with increasing wind speed. In addition, the latent heat exchange coefficient values are adjusted so that they significantly increase under extreme conditions relative to the first guess. Consequently, the term of the cost function, which represents the discrepancy between the model results and the observational data, decreases by 4.1-22.4% relative to the existing system after some spin-up cycles. The intensity and location of the TC are brought close to those of the corresponding best track produced at the Japan Meteorological Agency. Our optimization approach has the potential to improve the forecast skill.
Monthly mean temperatures for April and August have been strongly and negatively correlated from 1998 to 2011 in northern Japan. When monthly mean temperatures in April were either significantly below or significantly above normal, the temperatures in the following August had the opposite anomalies. We attribute this seasonal behavior of temperatures to a displacement of the core of upper-level westerly winds. When monthly mean temperature was higher than normal in August, the subtropical jet stream had been strengthened in April and a continental polar air mass affected northern Japan in April. In August of that year, if a jet located north of Japan moved further north, Japan was covered by a maritime tropical air mass, and a maritime polar air mass rarely affected summer weather in northern Japan. In the opposite case, when temperatures were cool in August and warm in April, we inferred that the jet had been weak and the continental polar air mass did not move south and affect northern Japan in April; thus, in August the jet shifted southward and the maritime polar air mass could affect summer weather. An empirical orthogonal function analysis of the 200-hPa height field revealed that two principal component modes were associated with the anomalous temperatures in these two months. On the basis of these results, we identify these modes as the cause of upper level westerly wind variations on the northern hemispheric scale. Based on a singular value decomposition analysis of the 200-hPa height field and the sea surface temperature, the year 1998 marked one of the several pronounced climatic shifts of the last century.
In this study, a data analysis was conducted to describe the common characteristics of “large-scale cloud separations,” in which zonally elongated cloud bands extending a few thousand kilometers over the western tropical Pacific are simultaneously separated into two or three zonally elongated bands. The separated cloud bands maintain their shapes for more than a day. In the present study, a case study was performed on four separation cases observed during the Intensive Obervation Period (IOP) of the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA-COARE). The northern and southern cloud bands consisted of clouds with fine line structures. Their orientations were in good agreement with the horizontal winds near the cloud top, indicating that the northern and southern cloud bands consisted of upper-tropospheric cirriform clouds. After the convective activity in the original cloud band weakened considerably, strong meridional winds remained in the upper troposphere and advected the separated cloud bands northward and southward. All the cloud separations occurred about half a day after the convective activity had peaked, and the onset time of the separations were fixed from evening to midnight. These facts indicate that the typical diurnal convective activity over the western tropical Pacific may play a role in the cloud separations. Westward-propagating cloud clusters were observed to the west of the original cloud band during the separations, which may be related to convectively coupled equatorial waves.
A long-term (1948-2009) frontal data set was created with an objective method by using NCEP-NCAR (National Centers for Environmental Prediction-National Center for Atmospheric Research) reanalysis data. This method utilizes a gradient and a thermal front parameter (TFP) of a potential temperature and an equivalent potential temperature at 850 hPa (dθ, TFP(θ), and TFP(θe)). The TFP defined as the directional derivative of a gradient of a thermodynamic variable along its gradient is one of the measures of frontal intensity and is often used for objectively analyzing frontal positions on surface weather maps. On the basis of the frontal data set, the average seasonal behavior of the frontal zone around Japan, its influence on the weather in mid-summer, and the seasonal march of the frontal zone during El Niño/La Niña events were examined. The main results are summarized in the following points: 1) The frontal data set generated under the conditions of dθ > 0.04 K (100 km)−1, TFP(θ) > 0.05 K (100 km)−2, and TFP(θe) > 0.69 K (100 km)−2 showed the strongest correlation to that compiled by counting the number of fronts on the surface weather maps around Japan. 2) Although the long-term frontal data set created in this study retained some differences in frequency, the seasonal march of the frontal zone was consistent with that created from fronts on the weather maps. 3) The relationship of interannual mid-summer variations (July 20-August 16) between the clear-sky ratio of Japan and the frontal zone and various mean characteristics of the Japanese climate during El Niño/La Niña events, most of which have been discussed in previous reports, were verified from the perspective of the variability in the frontal zone, which has not been clarified so far. These results of this work show that the frontal data set created herein has the advantage of being simple and objective and is useful for research on the detailed relationship in interannual variations between the regional climate around Japan and large-scale atmospheric conditions.