Regional differences in occurrence frequency of dust events (dust storms, sand storms and blowing dust) observed in spring (March, Apri1, and May) in 1999-2003 in the northern half of China (north of 35°N), and Mongolia are studied from the viewpoint of the relationship with strong wind and precipitation occurrence. In order to categorize the weather conditions, World Surface Data were used. Dust events are frequently observed in the regions of (I) West Tarim Basin, (II) East Tarim Basin and East Xinjiang, (III) the Hexi Corridor, (IV) southern Mongolia to Inner Mongolia, (V) a part of southern Dongbei Plain (Northeast China Plain), and (VI) the Loess Plateau. However, dust event occurrence frequency under strong wind condition, is not the same among regions and these 6 areas are divided into 2 groups as follows. In areas (I), (II), (III), and (VI), dust event frequency under strong wind conditions is high, even though the number of strong wind occurrences is small. In contrast, in areas (IV) and (V), the number of dust event occurrences is large, but the dust event frequency under strong wind conditions is low. Dust events were observed not only in arid, or semi arid region. At almost all of observatories in the study area, dust events were reported at least once in 1999-2003. However, there are areas where both the number of dust events, and dust event frequency under strong wind conditions are low, even close to arid/semi-arid land, such as Dzungaria Basin. It is attributed to high frequency of precipitation events under strong wind and land surface characteristics. We made composite maps of sea level pressure, surface air temperature, and surface wind for 10 severest “Widespread Dust Event Cluster” (WDE Clusters) by using NCEP/NCAR reanalysis data for the above 6 areas respectively. It is found that severe dust events are observed in these 6 areas on the condition that anti-cyclones develop over Kazakhstan to the Altai Mountains and western Mongolia, associated with strong cold wind over each area. In areas (I), (II), (III), and (VI), dust events occurred only under the influence of these anti-cyclones. On the other hand, in areas (IV) and (V), a clean contrast was found between these anti-cyclones and cyclones located around 47.5°N, 120°E, and the role of these cyclones seems to be important. Only in area (V), surface air temperature tends to increase at the time of the 10 strongest WDE Clusters. This is caused by southerly wind associated with the cyclone. It is suggested that the possible causes of low dust event frequencies, under strong wind condition in areas (IV) and (V), are precipitations and land surface characteristics (e.g., snow cover) caused by the cyclone development.
The relationship between large-scale seasonal (June-October) circulation over the western North Pacific, and the frequency of tropical cyclone (TC) approach or landfall on Japan, is examined using the reanalysis datasets (ERA-40(1958-2001) and JRA-25(1979-2004)). The result shows that there are two dominant modes over the western North Pacific; the El Niño Southern Oscillation (ENSO) mode, and a mode that is correlated with the variation of the subtropical high. In years when the subtropical high extends westward from its climatological position and the maximum of the high pressure anomaly is located along the Tropic of Cancer, the frequency of TC approach/landfall on Japan tends to be reduced. In contrary, when the subtropical high retreats eastward, and the maximum of low pressure anomaly is located along the Tropic of Cancer, there is a tendency to have more number of tropical cyclones approach or landfall on Japan.
A number of observational and modeling studies have shown a tendency for typhoon strength vortices to develop upward motion and produce precipitation, particularly in the eyewall, on the downshear to downshear-left side of the tropical cyclones (TCs). However, the directional relationships obtained from the observational studies have been mostly confined to the TC cases in the Atlantic basin. Furthermore, little evidence has been presented so far for the relationship in magnitude, between shear and rainfall asymmetry. In the former part of the present study, the observational analysis on TC rainfall asymmetries is extended to the western North Pacific TCs in 2004, using the two types of rain-rate data, the Radar-AMeDAS precipitation data, and satellite-based rainfall estimates, such as TMI and AMSR-E rain rates. It is well demonstrated from the analysis that rainfall in the inner-core region of a TC tends to occur on the downshear to downshear-left side, irrespective of data type used and latitudes where TCs are located. However, as far as the relationship between shear and storm motion is concerned, a sharp contrast is found between low and middle latitudes. In middle latitudes TCs have a tendency to move to the left of the shear, consistent with previous studies, while in low latitudes they tend to move to the right of the shear. The contrasting shear-relative storm heading between the two latitudes is attributed to the difference in vertical structure of the ambient wind. In the latter part of the study, to explore the quantitative relationship between shear and rainfall asymmetry, a formula for the shear-induced vertical motion is derived from the thermal wind balance equation for TC-like vortices. The formula states that the shear-induced vertical motion should be a function not only ofshear magnitude, but also of vortex strength. To validate the formula a set of idealized numerical experiments are conducted, with realistic wind profiles, in which the initial environmental winds are specified from the 6-hourly JMA global analyses for two major typhoon cases in 2004. It is found from the numerical study that the magnitude of wavenumber-one vertical motion, predicted by the formula, is much more strongly correlated with that of model-produced rainfall asymmetry, than the shear alone, suggesting that the vortex strength is one of the main factors determining the magnitude of shear-induced rainfall asymmetry. The results from the idealized simulations also suggest that vortex tilt would have only a minor contribution to the rainfall asymmetry in the inner-core region, at least for well-developed TCs.
The number-size distribution and shape factors of water-insoluble particles (WIP) in four dirty snow layers in the spring of 2001 at Mt. Tateyama, Japan, were analyzed using scanning electron microscopy, and using optical microscopy with a digital camera. Results show that the median aspect ratio (ratio of the longest dimension b to the orthogonal width a; b/a) of the WIP varied within 1.22-1.31. Only a few particles with an aspect ratio of more than 2.5 were observed. The median circularity factor (4πS/l2; S is projection area, and l is periphery length) varied from 0.83-0.97. Combined with backward air trajectories, visibility reducing surface weather reports, additional results of rain containing Saharan dust and dry deposition samples, the number and shape factor distributions versus particle size were characterized as (1) narrower distributions of number-size and shape factors for the Saharan dust, and (2) bi-modal number-size distribution for dry deposited dust. Results suggest that the proportions of nearly spherical (higher circularity nearly 1) and less elongated (lower aspect ratio close to 1) particles were higher in the sample that had been transported a longer distance from the dust source areas.
A mass-flux type cumulus parameterization scheme is evaluated in the Korea Meteorological Administration (KMA)’s Global Data Assimilation and Prediction System (GDAPS). Two sets of parallel assimilation runs, with a horizontal resolution corresponding to the spectral truncation of T213, approximately 50 km, are designed, one with the operational Kuo convection scheme, and another with a simplified Arakawa-Schubert (SAS) convection scheme, for the month of July 2001, which was a typical summer monsoon period in Korea. An additional sensitivity experiment, that excludes the moist downdrafts in the SAS scheme, is conducted. A 10-day forecast cycle is run, within a data assimilation window, beginning at every 1200 UTC in July 2001. A heavy rainfall event, that occurred in the Korean peninsula on 14-15 July 2001, is investigated in detail. For the heavy rainfall case, the SAS scheme tends to reduce the precipitation amount, as compared to the Kuo scheme. It is found that the exclusion of moist downdrafts in the SAS scheme, increases drying and heating rates, resulting in enhanced rainfall, as shown in the Kuo scheme. The monthly verification demonstrates that the systematic warm bias in the lower troposphere in the operational model is nearly absent when the SAS scheme is employed. In terms of statistical measure of the skill for the predicted sea-level pressure, and that for the upper level patterns, the experiment with the SAS (Kuo) scheme out-performs in the northern (southern) hemisphere. In the case of precipitation forecasts, the SAS scheme effectively corrects the spurious excessive precipitation over the tropical land area, due to the inclusion of moist downdrafts. However, the reduction in precipitation in the SAS scheme, deteriorates the forecast skill for high precipitation categories.
The present report studies synoptic-scale variations of the polar air-mass transformed over the Japan Sea, during the “East Japan Sea Observation” in the winters of 1976 and 1977. Rawinsonde observation data at a research vessel are used, in addition to data at operational upper observation stations and the objective analysis data. The multi-layer structure of the transformed air-mass, including the unstable lowest layer and the mixed layer capped by the stable layer, is formed in the transformed airmass over the Japan Sea. The mixed layer, which includes the sub-cloud layer and the cloud layer, is characterized by nearly venically-uniform equivalent potential temperature. The height of the mixed layer top is 1000-1500 m, on the average, over the Japan Sea. The height increases toward the south, and reaches 2000-2500 m over the coastal area of Japan. The height of the mixed layer top, and the apparent total heat source in the transformed air-mass, vary widely in association with the synoptic-scale changes of polar-air outbreak. The height of the mixed layer top tends to increase in association with the increase of the apparent total heat source in the mixed layer, and the decrease of the vertical stability in the lower-middle troposphere. The snowfall depth over the coastal area of Japan tends to increase in association with the increase of the apparent total heat source over the Japan Sea. The important synoptic conditions for the occurrence of the heavy snowfalls are the intense polar-air outbreak, andlor the passage of the cold vortex or the deep cold trough over the Japan Sea.
An efficient non-linear normal-mode initialization (NNMI) scheme for a super high-resolution atmospheric general circulation model (AGCM) has been developed. The conventional NNMI method, based on Machenhauer (1977), is difficult to apply to such an AGCM, with a grid spacing of 20 km, since it demands an extremely large computational memory. The new scheme combines two techniques; one is the venical-mode initialization method proposed by Bourke and McGregor (1983), and the other is the incremental NNMI method proposed by Ballish et al. (1992). Comparisons with the conventional Machenhauer scheme, using a 60 km-resolution AGCM, show that the new scheme favorably suppresses undesirable gravity-wave oscillations, while maintaining meaningful tidal waves in the initial states. A medium-range forecast experiment with data assimilation shows that the new scheme is also competitive with the conventional scheme in terms of forecast skill. It is demonstrated that the new scheme is efficient and effective for a 20 km-resolution AGCM.