The vertically-integrated atmospheric energy and moisture budgets have been computed for all available months for the Japanese reanalysis (1979 to 2004), and results are described in detail for the month of January 1989 and compared with those of other reanalyses. Time series are also presented. The moistening, diabatic heating and total energy forcing of the atmosphere are computed as a residual from the analyses using the moisture, dry energy (dry static energy plus kinetic energy) and total atmospheric (moist static plus kinetic) energy budget equations. These fields are also computed from the model output based on the assimilating model parameterizations. Moreover, some component fields can also be computed from observations to evaluate the results. In particular, when the vertically-integrated forcings computed from the model parameterizations are compared with available observations and the budget-derived values, significant JRA model biases are revealed in radiation and precipitation. The energy and moisture budget-derived quantities are more realistic than the model output and better depict the real atmosphere. However, low frequency decadal variability is spurious and is mainly associated with changes in the observing system. Results also depend on the quality of the analyses which are not constructed to conserve mass, moisture or energy, owing to analysis increments. Although there has been considerable progress in depicting the diabatic components of the atmosphere, some problems remain, and suggestions are made on where research can make further improvements.
Pentad rainfall data from 46 stations in West Sumatera Province, Indonesia in 1992 reveal temporal and spatial variations of rainfall in coastal and inland regions. Distinct contrasts in rainfall amounts and variations are evident between the coastal and inland mountainous regions. Locality index is defined by pentad rainfall time series in coastal and inland regions to classify characteristic rainfall distribution patterns into four types: coastal-, inland-, active-, and inactive-type. In 1992 the coastal- or active-type rainfall patterns tend to appear throughout the year, whereas the inland-type rainfall pattern tends to appear during the Southern Hemisphere summer. The inactive-type tends to appear during the Southern Hemisphere winter. Correlation and composite analyses suggest that lower tropospheric zonal wind variations over central western Sumatera tend to intensify the contrast between coastal and inland rainfall distribution. The correlation coefficient between locality index and zonal wind of 850 hPa NCEP/NCAR reanalysis is 0.77 (99% confidence level) for characteristic rainfall distribution patterns during 1992-1993. This implies that rainfall in the windward side of steep mountain is relatively greater than that of leeward side with increasing wind speed. The occurrence of coastal-type pattern is relatively well related with lower tropospheric westerly wind. Furthermore, although the number is limited, diurnal variations of convective activity are clear when active-type rainfall appears.
The long-term behavior of Rossby wave breaking (RWB) events is investigated by a diagnosis separating in different asymmetric types of RWB, i.e., cyclonically sheared waves breaking predominantly pole-ward (P1) or equatorward (LC2) and anticyclonically sheared waves breaking predominantly poleward (P2) or equatorward (LC1). Generally, RWB can be identified by meridional overturning of potential vorticity (PV) on specific isentropes, but a separation in poleward or equatorward asymmetry is too difficult based on PV maps alone. For this paper, we use that northward or southward direction of the meridional wave flux component for quasi-stationary Rossby waves indicates cyclonically or anticyclonically sheared RWB. We demonstrate that the magnitude of the meridional wave fluxes gives a reliable measure of the asymmetric types of RWB when combining with PV diagnostics as well as with the geometry of large-scale diffluent/ confluent flow. Based on 45 winter periods of ECMWF Reanalysis (ERA-40), we found two pronounced regions for P1- and LC2-type events, over the northern North Pacific and the northern North Atlantic, and two extended belts of P2- and LC1-type RWB events, over the North Pacific/North America region and the North Atlantic/European-West Asian region. The results reveal that the long-term mean occurrence of poleward RWB is generally as large as that of equatorward RWB with local differences. Since poleward or equatorward RWB events influence different regions efficiently, e.g., by associated cut-off cyclones or anticyclones, the proposed diagnosis gives an important tool for interpreting long-term general circulation patterns and large-scale weather regimes.
Based on a freezing rain and ice pellet event observed on 3 January 2003 in the Kanto Plain, Japan, the atmospheric conditions necessary for the occurrence of such phenomena in this area were investigated, and a simple method of discriminating precipitation types, including freezing rain and ice pellets, was examined. Freezing rain and ice pellets fell from 1800 to 2400 JST on 3 January in the western portion of the Kanto Plain. During the precipitation, northwesterly cold-air advection near the surface dominated in this region, with southerly warm-air advection associated with the approach of a synoptic low present above the cold air layer. This atmospheric structure produced the conditions required for freezing rain and ice pellets to form, namely a melting layer with an air temperature above 0°C to melt snow particles into raindrops, and a refreezing layer with an air temperature below 0°C to supercool the raindrops into freezing rain or freeze them into ice pellets. To establish a method of discriminating between freezing rain and ice pellets, the conditions required for the complete melting of snow particles and the complete freezing of raindrops during the event were examined theoretically. We found that the mean air temperatures, the mean relative humidities, and the depths of the melting and refreezing layers can be used to distinguish the environmental conditions required for freezing rain and ice pellets to form. In addition, determination of whether the wet-bulb temperature at the surface is above or below 0°C was used to discriminate rain and freezing rain. The distribution of precipitation types at 2100 JST on 3 January 2003 in the Kanto Plain was estimated using the method described above, and the results agreed closely with the distribution actually observed. The diagnostic method presented in this study enables the discrimination of precipitation types, including freezing rain and ice pellets.
Air-sea fluxes were observed to elucidate the lower boundary conditions in the development of an atmospheric boundary layer over sea from 14 to 27 August 2002. Fluxes were observed at the tip of a narrow cape on Miyako Island, in Japan’s Southwestern Islands. Flux measurements and turbulence properties were clearly divided by fetch condition or wind direction. Analysis of the footprint area and homogeneity indicated that air-sea fluxes over an upwind sea surface were observed in cases of approximately 30% for all observation periods. The bulk transfer (BT), eddy covariance (EC), and variance (VA) methods were used to evaluate air-sea fluxes. The general features of the estimated fluxes were similar. Averaged sensible heat flux of approximately 6 W m-2 was obtained by all three methods, and the latent heat fluxes were 92, 60, and 71 W m-2 by the BT, EC, and VA methods, respectively. Daily mean net radiation input RNET was 223 W m-2. The sea surface was a large energy sink during the observation period.
In this study, twelve tropical storms between 2002 and 2005 over the western North Pacific Ocean, namely, typhoons, were simulated through medium-range forecast experiments with the 20 km-mesh Japan Meteorological Agency (JMA) and Meteorological Research Institute (MRI) Atmospheric General Circulation Model (JM-AGCM). These simulations were compared with the 60 km-mesh JMA Global Spectral Model (GSM) to evaluate differences in resolution. They are verified with the best-track data as an observation. The verification was conducted in terms of estimating error of position, intensifying tendency, radius of 50 knot, 30 knot, and composite wind profile. This paper addresses the importance of such high resolution to predict typhoon’s intensity and inner-core structure. As a result, the JM-AGCM shows slightly smaller position error than the GSM. Moreover, much improvement was seen in the intensity prediction. The JM-AGCM outstandingly can both decrease the central sea level pressure and increase maximum wind velocity, while the GSM can not simulate them because of low resolution. The JM-AGCM also shows better intensifying and decaying tendency than the GSM. The verifications of 50 knot and 30 knot radii, and the composite wind profile indicate that the typhoon structure by the JM-AGCM is quite realistic. Moreover, the JM-AGCM expresses the drastic transformation of inner-core wind profile within 100km from the typhoon center more realistically than the GSM. These results indicate that a high resolution global model, such as 20 km-mesh, is vital when discussing the intensity and wind profile of tropical storms.
Hourly data from a 400-MHz Wind Profiler Radar (WPR) during the warm season months of three years are used to develop a climatology of the low level jet (LLJ) over Okinawa, a Japan subtropical island. Long-term observations of the wind with high temporal and vertical resolution allowed an investigation of the LLJ statistics. Characteristics of the LLJ are investigated in two subperiods: during the Baiu season over Okinawa Island and the post-Baiu season when the Baiu front has moved further north of Okinawa. The investigation result shows that stronger LLJs occur more frequently during the Baiu season, when heavy precipitation is mostly observed compared to the post-Baiu season. The appearance frequency of the maximum speed height of LLJs has a double peak structure in the vertical; lower and higher peaks appear around heights of 600 m and 1.3 km, respectively. In the rain cases with daily rainfall equal and more than 1 mm, another peak also appears around a height of 3 km. The frequency of LLJ occurrence exhibits a diurnal variation with the maximum in the nighttime and early morning, especially in the non-rain cases. The examination with upper-air sounding observations suggests that the nocturnal preference of LLJ occurrence is brought from the reduction of vertical mixing in the atmospheric boundary layer after sunset. Additional investigations of LLJs, using data from the WPR network of the Japan Meteorological Agency for three other stations surrounding Okinawa Island, provide more evidence for the role of the atmospheric boundary layer in generating the diurnal variation of LLJ occurrence.
An intensive observation of the stratosphere has been made using 10 radiosondes every 3 h for the time period of 11-12 May 2006 at Shigaraki, Japan (34.85°N, 136.11°E). Horizontal wind and temperature data were successfully obtained with high accuracy in the height region up to about 36 km. The sampling time intervals are 2 s corresponding to a nominal vertical resolution of about 10 m. Two packets of wavelike fluctuations whose phases propagate downward are detected around a height of 34 km (hereafter referred to as Wave-A) and of 24 km (Wave-B) in the obtained vertical profiles of horizontal winds. Wave parameters are estimated using a hodograph analysis under an assumption that these fluctuations are due to inertia-gravity waves (IGWs). The ground-based wave periods are 11 and 21 h, the horizontal wavelengths are 850 and 900 km, and the vertical wavelengths are 6.0 and 2.6 km, for Wave-A and Wave-B, respectively. It is also shown that both IGWs propagate energy upward and northnorthwestward relative to the background wind. The validity of the assumption is confirmed by the accordance of two independent estimates of the ground-based frequency. The horizontal structure seen in the horizontal divergence field calculated from European Centre for Medium-rangeWeather Forecasts (ECMWF) operational analysis data is consistent with the estimated wave parameters. Sources of the two IGWs are examined by a ray tracing method. Both IGW rays are traced back to the level and latitude of the mid-latitude westerly jet. Detailed examination for temporal variation of the wave structure indicates that the IGWs meandered eastward slightly south of the mid-latitude jet, turned north-northwestward, ascended rapidly where the background wind direction was changed to southward, and reached the middle stratosphere over the observation site. An interesting point is that both locally-defined Rossby number and cross-stream Lagrangian Rossby number are large in the regions where the IGW packets were situated during propagation around the jet from several days. Therefore, it is likely that the IGWs were generated in the vicinity of the unbalanced westerly jet through the spontaneous adjustment processes.
A non-hydrostatic, mesoscale model, LOCALS, is used to investigate the effect of land surface changes on precipitation in an urban area. The horizontal mesh used in the model is approximately 5 km and the domain includes Tokyo and central Japan. Land surfaces identified as pavement or urban are transformed into forested land and model simulations are performed for both the current urban land surface and the hypothetical forested land surface. Detailed analysis is performed for two sample cases. On 5 August 2003, increased heating by the urban surfaces increases the height of the urban boundary layer resulting in much more rainfall. On 25 July 2001, air that is advected over Tokyo but forms a precipitating cloud away from the urban core carries less moisture and therefore produces less rainfall. These case studies suggest that changes to land surface types in urban areas may increase or decrease rainfall in the urban area and further research is necessary before it is possible to generalize these results.
A series of GPS radiosonde and oceanic observations was conducted over the Okhotsk Sea for the first time in July 1998. Under the prevalent anticyclone over the Okhotsk Sea during the observations, distinct differences in the atmospheric and oceanic boundary-layer structure were observed between the fog and non-fog periods. During the fog period, the observed strong surface winds and upward sensible heat fluxes promoted vertical mixing in the atmosphere and ocean. The height of the well-mixed marine boundary layer varied from 200 to 1000m in the atmosphere, and 10 to 15 m in the ocean, respectively. The fog formation occurred when surface air temperature (SAT) was cooler than the underlying sea surface temperature (SST). By trajectory analyses for the observed well-mixed atmospheric boundary layer during the fog period, the boundary-layer air mass that has passed over warm (cool) SST tended to form stratus-like (stratocumulus-like) fog. In the non-fog period, by contrast, atmospheric and oceanic boundary layers were stably stratified due to weak surface winds and downward sensible heat fluxes. Regardless of the air mass trajectory, SAT is warmer than underlying SST during the no-fog period.
The formation mechanism of an intensified cold air mass around East Siberia in December 2005, which brought a heavy amount of snow over northern Japan, is investigated using a regional climate model. The strong cold air mass is simulated by the CTL run with full diabatic processes. However, the experiment with the adiabatic process (ADIABATIC run), which is conducted to investigate the effects of the atmospheric response and the advection of large-scale atmospheric circulation from the outside of the calculation domain, does not simulate the intensified cold air mass below 235 K at 500 hPa. Another sensitivity experiment with diabatic heating processes, except for surface heat fluxes (NO-SFH run), reproduces the intensified cold air mass due to the diabatic heating process in the middle to upper troposphere. The intensification and maintenance of the cold low in the typical events cannot be reproduced by the experiment without radiation process (NO-RAD-R run), while the NO-SFH run can simulate those important features of the cold low. These results indicate that the radiation process in the middle to upper troposphere is indispensable to intensify the cold air mass below 235 K at 500 hPa. From the satellite images, it is evident that the formation of clouds is accompanied by a cold low in the middle troposphere. It is speculated that the clouds are the source of the cooling due to the infrared cooling process, which may intensify the cold low. Therefore, the infrared cooling of the clouds is supposed to be the primary process in the intensification of the cold air mass in December 2005.
This study investigates diurnal variations in surface wind in Japan during June-August of 1992-2006 using data from the Automated Meteorological Data Acquisition System (AMeDAS) and the Sapporo City Multisensor (MULTI). Harmonic analysis and hodograph analysis are employed to investigate the rotation direction and rotation rate of the wind vector at about 1000 stations. An analysis of six major plains in Japan reveals distinct clockwise and anticlockwise hodograph regions within each plain. The rotation direction is attributed to two lagged pressure-gradient forces of contrasting orientations: one between the land and sea, and another between the plain and adjacent mountains. An analysis of the linearized equations of motion reveals that rotation of the wind vector is mainly controlled by the balance between the pressure gradient force and the frictional force, with a small but non-negligible contribution by the Coriolis force, particularly near the coast. The observed rotation rates of the hodographs show a brief stagnation during the mid-afternoon and a longer stagnation during the nighttime. This irregular rotation rate is well explained by taking into account the semidiurnal component of the wind. The linearized equations of motion indicate that this semidiurnal component results from the semidiurnal component of the pressure gradient force, which is generated by non-sinusoidal solar heating over the course of a day.
Researchers have examined relationships between wind speeds and thermal conditions above a slope for pure drainage flow with nearly no disturbance by shear flow, though disturbances by shear flow are commonly present. This study examined the relationship between wind speeds and thermal conditions above a slope when wind speed is affected by shear as well as drainage flow. Data used in this study were obtained at a sloping forest site in Thailand, which had been also used in our previous studies (Komatsu et al. 2003, 2005). The canopy height (h) of the forest was approximately 30 m above ground level and wind speeds were measured at 34 m, 43 m, and 50 m (i.e., 1.13h, 1.43h, and 1.67h, respectively). After classifying measured wind speed data based on vertical air temperature differences, the mean and minimum wind speeds of each temperature-difference class were determined. Here, mean and minimum wind speeds refer to means and minimums among wind speeds for a specific temperature difference class. Regarding the 34-m wind speeds, mean wind speeds generally increased with increasing temperature difference and minimum wind speeds were non-zero, indicating the effect of drainage flow was generally dominant over the effect of shear flow at this height. Regarding the 50-m wind speeds, mean wind speeds decreased with increasing temperature difference and minimum wind speeds were nearly zero regardless of the temperature difference, indicating the effect of shear flow was dominant over the effect of drainage flow at this height. The 43-m wind speed data showed results that were intermediate to those of the 34- and 50-m wind speed data. Results of such analysis bring information on the significance of the drainage flow effect on wind speed data measured at different heights above a slope. Thus, such an analysis would be useful for examining vertical structures of wind flow above a slope when both drainage and shear flow effects are present.
Using an ensemble of wintertime hindcasts with a high-resolution (T106L60) Atmospheric General Circulation Model (AGCM) forced by observed sea surface temperatures (SSTs) and extending into the stratosphere, we investigate the formation and lifecycle of the Aleutian-Icelandic low Seesaw (AIS) during the 1978 to 1993 period. The AIS has been newly proposed to be an important mode of variability, linking the major wintertime surface lows, the Icelandic Low and the Aleutian Low, in late winter, and thereby linking climate variability over the North Pacific and the North Atlantic. We demonstrate for the first time with a stratosphere-troposphere model, that a coherent, ensemble-mean AIS extension into the stratosphere exists, where its presence modulates ultra-long planetary wave propagation and the polar night jet intensity. The model AIS peaks in February, when the Aleutian and Icelandic Low anti-correlation maximizes at -0.59. The AIS provides a new way to describe the El Niño-Southern Oscillation (ENSO) phenomenon influence into the stratosphere. For example, El-Nino conditions correspond to a deeper than normal Aleutian Low, extending its influence into the Icelandic sector as an AIS negative phase (weakened Icelandic Low), hence enhanced planetary wave vertical propagation and a weakened stratospheric polar vortex. This maturation of the AIS in late winter explains the intra-seasonal variability of the stratospheric response to ENSO, which peaks in late winter. Internal model variability is large and enhanced potential predictability is found primarily in the western hemisphere, with a western Atlantic maxima being more pronounced in the stratosphere than in the upper troposphere.
The rainy season is an important climate feature over Eastern China where anomaly in either its timing or length can lead to adverse economic and social consequences. Here, we illustrate that the records of daily precipitation description at Beijing and Shanghai contained in Memos-to-Emperor during the Qing Dynasty provide a unique source to extend the rainy season information to 1736. The information together with the instrument measurements since 1875 in both cities reveals significant inter-annual and decadal variations of the beginning and ending dates, and length of the rainy season. The analysis further reveals that, on the decadal time scale, the length of the rainy season increased in Shanghai since 1961 with more frequent extreme rainfall events, but decreased in Beijing since 1975 with persistent dry conditions. This pattern of changes was not seen in any other periods of the data, in particular during 1736-1820 when both cities showed an increase in the length of the rainy season.