This study numerically examined how the locally strong “Karakkaze” wind in the Kanto Plain of Japan is affected by terrain shape, particularly by a convex feature in the mountain range. Our method involved running idealized numerical simulations using the Weather Research and Forecast model with a horizontal grid spacing of 3 km. The results revealed that a strong-wind region formed in the lee area of the convex feature, hereafter the semi-basin, and leeward of the semi-basin. In contrast, weak-wind areas formed adjacent to the strong-wind region. These results were consistent with the basic features of the observed surface wind pattern of the Karakkaze during the winter monsoon. However, such a flow pattern did not appear in the numerical simulation with a mountain range that lacked a convex feature.
Sensitivity experiments were also conducted to evaluate the detailed effects of a mountain range with convexity. Sensitivity experiments with different convex shapes revealed that strong winds appeared within and leeward of the semi-basin when the aspect ratio of convexity (ratio of the wave amplitude to the wavelength of the convexity) exceeded about 0.5. Sensitivity experiments on terrain shape suggested that saddles in the mountain range were not essential to the formation of the Karakkaze, but they could affect its strength. Sensitivity experiments on the mountain Froude number, Frm, showed that locally strong winds within and leeward of the semi-basin appeared only when the Frm was in the range 0.42-1.04. Sensitivity experiments with surface heat fluxes (SHFs) showed that the basic structure of the strong-wind region in the leeward plain of the convex feature did not depend strongly on SHFs. However, the addition of SHFs reduced the surface wind speed, but increased the size of the strong-wind region.
This study evaluates possible changes in tropical cyclone (TC) precipitation over Japan under a future warmer climate using an ensemble projection generated by a non-hydrostatic regional climate model with a resolution of 5 km (NHRCM05) under the RCP8.5 scenario. NHRCM05 reproduces TC precipitation and TC intensity more accurately than does a general circulation model with a resolution of 20 km. The number of TCs approaching Japan is projected to decrease under the future climate, while the TC precipitation rate increases. As these two effects cancel each other out, total TC precipitation, and the frequency of the moderate TC precipitation that is usual under the present climate, shows no significant change. On the other hand, the frequency of extreme TC precipitation increases significantly because the intensification of the TC precipitation rate outweighs the reduction in TC frequency. The increase in the TC precipitation rate is caused primarily by the increase in water vapor around the TCs, which in turn results from the increase in environmental water vapor. The intensification and structural changes to TCs also contribute to the enhanced TC precipitation.
In this present study, we analyzed the synoptic and mesoscale dynamics and underlying mechanism of an extreme rainfall and flood event that occurred in Sri Lanka between 14-17 May 2016, using the Weather Research and Forecasting Model simulations with a horizontal grid size of 3 km and observational data. This extreme rainfall event was associated with a low-pressure system (LPS) that originated over the Bay of Bengal in the Indian Ocean and passed over Sri Lanka. The observed maximum accumulation of rainfall during the event exceeded 300 mm at several weather stations on 15-16 May and it resulted in severe flooding and landslides, particularly in the western part of the island. The model closely simulated the timing of the initiation of the LPS and its development along the east coast of Sri Lanka. The model could capture the overall rainfall tendency and pattern of this event. Synoptic and mesoscale analyses indicated that this extreme rainfall event occurred as the cumulative effect of a sustained low-level convergence zone, generated by an enhanced westerly monsoon flow and the circulation of the LPS, alongside a continuous supply of high-magnitude moisture, strong vertical motion, and orographic effects of the Central Mountains of Sri Lanka. Model sensitivity experiments indicated that the rainfall over the western slope area of the mountains was enhanced by mountain lifting, whereas western coastal rainfall was reduced because the mountains blocked the northeasterly flow of the LPS.
In this work, long-term (10 years) raindrop size distribution (RSD) measurements from the Joss–Waldvogel Disdrometer (JWD) installed at the National Central University (NCU) (24°58′6″N, 121°11′27″E), Taiwan, and the vertical profile of radar reflectivity were used to analyze the variations in the gamma parameters of six seasons (winter, spring, mei-yu, summer, typhoon, and autumn) and types of precipitation. The normalized gamma distribution of RSD revealed that the highest mean Dm (mass-weighted average diameter) values occurred in the summer, whereas the highest mean log10 Nw (normalized intercept parameter) values were found in the winter. Furthermore, most of the rain falling at a rate of less than 20 mm h−1 occurs in Northern Taiwan. In this study, we used radar reflectivity to differentiate between convective and stratiform systems. It was revealed that the mean Dm values are higher in convective systems, whereas the mean log10 Nw values are higher in stratiform systems. The structure of RSD in stratiform systems remains constant in all seasons; however, convection is similar to maritime type. The microphysical characteristics that are responsible for the different RSD features in different seasons and types of precipitation are illustrated with the help of contoured frequency by altitude diagrams of radar reflectivity.
To consider the growth of cloud droplets by condensation in turbulence, the Fokker–Planck equation is derived for the droplet size distribution (droplet spectrum). This is an extension of the statistical theory proposed by Chandrakar and coauthors in 2016 for explaining the broadening of the droplet spectrum obtained from the “Π-chamber”, a laboratory cloud chamber. In this Fokker–Planck equation, the diffusion term represents the broadening effect of the supersaturation fluctuation on the droplet spectrum. The aerosol (curvature and solute) effects are introduced into the Fokker–Planck equation as the zero flux boundary condition at R2 = 0, where R is the droplet radius, which is mathematically equivalent to the case of Brownian motion in the presence of a wall. The analytical expression for the droplet spectrum in the steady state is obtained and shown to be proportional to Rexp(−cR2), where c is a constant. We conduct direct numerical simulations of cloud droplets in turbulence and show that the results agree closely with the theoretical predictions and, when the computational domain is large enough to be comparable to the Π-chamber, agree with the results from the Π-chamber as well. We also show that the diffusion coefficient in the Fokker–Planck equation should be expressed in terms of the Lagrangian autocorrelation time of the supersaturation fluctuation in turbulent flow.
Turbulent heat flux is the main passageway for air–sea interactions. However, owing to a lack of long-erm observations of turbulent heat flux, it is difficult to investigate the mechanisms of coupled ocean–atmosphere variabilities, such as the Pacific Decadal Oscillation. In this study, we reconstructed the long-term turbulent heat flux in the North Pacific from 1921 to 2014 on the basis of observations in the International Comprehensive Ocean-Atmosphere Data Set–International Maritime Meteorological Archive. Sea surface temperature, air temperature, wind, and humidity were used to reconstruct the turbulent heat flux by using the Coupled Ocean–Atmosphere Response Experiment 3.5 algorithm. The modied Fisher–Tippett distribution was employed to calculate the turbulent heat flux at each grid square, and missing values were further derived on the basis of data interpolating empirical orthogonal functions. The reconstructed turbulent heat flux was shown to be in accordance with the commonly used short-term heat flux datasets. This reconstruction is further examined by comparing it with long-term data from the European Center for Medium-Range Weather Forecasts twentieth-century reanalysis (ERA-20C) and the Twentieth Century Reanalysis (20CR) dataset from the National Oceanic and Atmospheric Administration. This reconstruction displays good agreement with ERA-20C both in spatial and temporal scales but shows some differences from 20CR. These examinations show that the reconstructed turbulent heat flux can reproduce well the main features of air–sea interaction in the North Pacific, which can be used in the studies of air–sea interaction in the North Pacific on multidecadal timescales.
The northward shift of the Western North Pacific Subtropical High (WNPSH) in July 2018 broke the historical record since 1958 and resulted in extreme heat waves and casualties across Northeast Asia (NEA). In the present work, we associated this extreme WNPSH anomaly with the anomalies of barotropic anticyclone above NEA originating from the strongest positive tripole pattern of sea surface temperature anomaly (SSTA) in the North Atlantic in July. Both data analysis and numerical experiments indicated that the positive tripole SSTA pattern could produce an upper-tropospheric wave source over Europe, which stimulated an eastward propagating wave train along the subpolar westerly jet over the Eurasian Continent. When its anticyclonic node reached NEA, the WNPSH started to shift northward. After the cyclonic node in the circulation anomaly encountered the Tibetan Plateau (TP), atmospheric diabatic heating was enhanced over the eastern TP, initiating another subtropical wave train, which furthered the northward shift of the WNPSH. Therefore, the wave source over Europe was critical for the northward shift of the WNPSH in July, connecting the tripole SSTA pattern in the North Atlantic with the WNPSH anomaly and maintaining the downstream effects of thermal forcing over the eastern TP on the East Asian summer monsoon.