Turbulent flow and ozone concentration at two (rooftop and sidewalk) sites in a high-rise building area of Seoul, Republic of Korea, were measured for the period of 24-27 June 2013 to examine their characteristics according to site location. During the observation period, the diurnal variations of air temperature, wind speed, and turbulent kinetic energy were distinct at both sites. The time series of ozone concentration exhibits a diurnal variation with daytime double peaks and one nighttime peak at both sites. The horizontal wind direction at the rooftop site has variations related to local winds, while the horizontal wind direction at the sidewalk site is mostly southerly (following a nearby street). The multiresolution spectra of horizontal and vertical velocities at the rooftop site confirm diurnal and turbulence-related variations. A quadrant analysis indicates that turbulence at the rooftop site is characterized by frequent ejections and less frequent but stronger sweeps, while turbulence at the sidewalk site is weaker and less characterizable than turbulence at the rooftop site.
Long-term changes in extreme hourly precipitation in Japan were examined using data at 983 stations for 1979 to 2013, with relation to the variations in air temperature (AT) and sea-surface temperature (SST). According to the statistics over the whole country, annual maximum and 95 percentile precipitations show increasing trends of 2-4%/decade, whereas AT and SST have risen at a rate of 0.2-0.3°C/decade. Analysis for each region and season indicates that the increase in extreme precipitation is a nation-wide feature and is not limited to a single season. Moreover, year-to-year variations in extreme precipitation tend to be in phase with those of AT and SST, with a correlation coefficient of 0.5-0.6 on the annual basis, although negative correlation is found for the summer season in western Japan and the Nansei Islands.
From daily precipitation data for June-July of 1971-2010, we examined the features and year-to-year variations of the Baiu precipitation around the Kyushu District, western Japan. The total precipitation in the Baiu season (June-July) at Nagasaki (located in northwest Kyushu) significantly decreased during the 2000s (2001-2010), mainly during June, at the 95% confidence level. The area with the largest decrease extended widely over northwestern Kyushu, while precipitation in southern Kyushu increased during the 2000s. Such regional precipitation contrasts appeared in late June, during the mature Baiu season. The decrease in total precipitation, as in Nagasaki, in late June during the 2000s was a reflection of the “heavy rainfall days” (with rainfall ≥ 50 mm day-1). It is suggested that less frequent appearance of Baiu frontal zone with wider meridional extension of the heavy rainfall area resulted in such features during the 2000s, although the Baiu fronts' appearance frequency on the surface weather maps was not very different from that before 2000.
The purpose of this paper is to quantify the sensitivity of the Global Precipitation Measurement (GPM) mission core observatory Dual-frequency Precipitation Radar (DPR) with focus on the Ka-band detectability of light rain and snow in comparison with the Ku-band capability. In this work, storm top height (STH) is utilized exclusively as the metric of radar sensitivity. The GPM DPR standard product level 2 version 3 is used in this analysis for the period from April to August 2014. The Ka high sensitivity (HS) mode and Ku have little systematic difference in STH over a broad range of the histogram, implying that the advantage of the Ka HS mode may not be as distinct as expected. The non-Rayleigh scattering effect may have partly offset the sensitivity advantage of the Ka HS over the Ku.
Cloud-to-ground (CG) and total (both CG and intracloud) lightning trends were investigated in a tornado-producing (F2) supercell thunderstorm that occurred over the Kanto Plain in Japan on 2 September 2013. The spatiotemporal signatures of mesocyclone (MC) and storm development were also analyzed using Doppler radar data. The results revealed that rapid increases of lightning flash rates (“lightning jumps”) occurred in CG and total flash rates 0-7 and 0-14 min before tornadogenesis, respectively, which are similar to those observed in the United States. However, the CG polarity change reported in previous studies was not observed, i.e., 96% of CG flashes were negative. The vertical vorticity of the MC was larger in the mid-level (4-6 km) than the low level (2 km) at the beginning stage of the lightning jumps, which might have led to updraft enhancement above the freezing level appropriate for the rapid increases in lightning flash rates.
The Local Ensemble Transform Kalman Filter (LETKF) is implemented with the Non-hydrostatic Icosahedral Atmospheric Model (NICAM) to assimilate the real-world observation data. First, the NICAM-LETKF system was developed using grid conversions between the NICAM's icosahedral grid and LETKF's uniform longitude-latitude grid to take advantage of the existing codes of Miyoshi. The grid conversions require additional computations and may cause additional interpolation error. Therefore, the LETKF code is modified, so that the LETKF reads and writes the NICAM's icosahedral grid data directly. We call this new version ICO-LETKF. In this study, the two systems are tested and compared using real conventional observations. The results show that the ICO-LETKF successfully accelerates the computations and improves the analyses.
Water vapor variations associated with a meso-γ scale convection were investigated using GNSS (Global Navigation Satellite System) derived PWV (precipitable water vapor) and high resolution numerical model data with a 250 m horizontal grid interval. A rapid increase of GNSS-derived PWV that occurred prior to the initiation of surface rainfall was well simulated by the numerical model. In the model, PWV values began to increase 16 min before the rainfall occurred at the surface. A local maximum of PWV was formed because of the generation of shallow free convection and surface water vapor flux convergence due to a lifting of an air parcel at approximately 1 km elevation by a preceding surface wind convergence. Due to the existence of a stable inversion layer between 2.2 and 3.5 km elevation, the shallow free convection took 11 min to rise above the inversion layer to form a deep convection. These results suggest that observation of local distributions of GNSS-derived PWV is useful for monitoring the generation of deep moist convection.
Precipitable water vapor (PWV) derived from a ground-based global navigation satellite system (GNSS) can be regarded as a representative value of the PWV above each GNSS station. It is inherently difficult to capture local-scale water vapor distribution using GNSS-derived PWV. Shoji et al. (2014) proposed a new method that utilizes GNSS slant path delays (SPDs) to estimate the PWV distribution around each GNSS station. To evaluate this new method, we simulated GNSS SPDs using a high-resolution numerical weather prediction model result, emulated GNSS analysis, retrieved PWVs, and compared their accuracy with the conventional method for a severe tornado that occurred in Japan on 6 May, 2012. Comparison results demonstrate the validity of the new method for this case. The conventional procedure introduces a 0.3 to 0.7 mm root mean square error (RMSE) at the GNSS site. Errors made by simple extrapolation increased with distance and reached 1.5 mm at about 1 to 3 km. The distance dependency of PWV errors in the new procedure varied with SPD elevation angle. Using SPD with an elevation angle exceeding 15°, we were able to estimate PWV with 1.5mm or better RMSE within 6km from a GNSS station.
This study applies the chemistry version of the Weather Research and Forecasting model (WRF-Chem) to examine impacts of three additional HONO sources, i.e., the reaction of photo-excited NO2 (NO2*) with water vapor (NO2* chemistry), the NO2 heterogeneous reaction on aerosol surfaces and HONO emissions, on concentrations and deposition of individual NOy species over the Beijing-Tianjin-Hebei region (BTH) in summer and winter periods of 2007. The results show that the three additional HONO sources produce a 20%∼40% (> 100%) increase in monthly-mean OH concentrations in many urban areas in August (February), leading to a 10%∼40% (10%∼100%) variation in monthly-mean concentrations of NOx, NO3− and PAN, a 5%∼10% (10%∼40%) increase in the total dry deposition of NOy, and an enhancement of 1.4 Gg N (1.5 Gg N) in the total of dry and wet deposition of NOy over this region in August (February). These results suggest that the additional HONO sources aggravate regional-scale acid deposition, emphasizing the importance of the additional HONO sources in the NOy budget.
The Global Change Observation Mission-Water “Shizuku” (GCOM-W) satellite, with a newly developed microwave radiometer: Advanced Microwave Scanning Radiometer-2 (AMSR2) developed by Japan Aerospace Exploration Agency, was launched successfully in May 2012. The standard geophysical products of AMSR2/GCOM-W were released a year after launch date. Here, we use data from three buoys moored in the Kuroshio Extension region to test the accuracy of AMSR2 sea surface temperature (SST) and near surface wind speed (SSW). The Kuroshio Extension region is subject to large multi-scale variability and intense air-sea interaction and thus provides a challenging test for the satellite sensor. From the year-long comparison, we confirm that the root mean square difference (RMSD) of AMSR2 SST observations was 0.75°C and meets the criterion for release accuracy (0.8°C). On the other hand, the RMSD of SSW was 1.6 m s−1, slightly worse than the criterion (1.5 m s−1), suggesting that the algorithm for SSW needs to be further improved. The analysis also showed that seasonal variations and characteristics of the relationship between SST and SSW are similar to those observed by previous satellite sensor (AMSR-E). Overall, the results give confidence that AMSR2 products can be used for many air-sea interaction, climate, and water cycle studies.
This study develops and tests a quality control (QC) algorithm for reflectivity from the single polarization phased array weather radar (PAWR) in Osaka, with particular focus on clutter detection, in preparation for radar data assimilation into a high resolution numerical model. The QC algorithm employs a Bayesian classification that combines the information from different parameters based on reflectivity and radial velocity. To take advantage of PAWR's unique high temporal and vertical resolutions, a new parameter based on the temporal variability of reflectivity is included. In addition, clutter probability estimations from previous volume scans are also included. The newly developed QC algorithm performs properly in two events characterized by heavy convective precipitation and stratiform precipitation.
In this study, the usefulness of the Adjoint Sensitivity-based Data Assimilation (ASDA) method in assimilating radar data was investigated by comparing it with the Three/Four Dimensional Variational (3/4D-Var) method. A total of 10 heavy rainfall cases over the Korean Peninsula were selected and classified as one of four Heavy Precipitation Systems (HPSs) according to their phenomenological properties. The Quantitative Precipitation Forecasting (QPF) skill is evaluated by computing the threat and bias scores, and the Root Mean Square Errors (RMSEs) of the simulated radial velocity are also calculated. The forecast skill of the ASDA method is comparable to that of the 4D-Var method in most of cases. This is because the ASDA method has some of the advantages of the 4D-Var method such as flow-dependent background error covariance and well-balanced analysis. In addition, the dependence of the QPF skill of the ASDA method on the characteristics of heavy rainfall cases is analyzed by calculating time-lagged autocorrelations of the observed radar data.
The influence of large-scale atmospheric blocking over the northwestern Pacific on heavy snowfall events in the Kanto area and on the Pacific Ocean side of northern Japan (PNJ) within the time scale of ∼10 days was examined through blocking case studies. The past 14 blocking cases over the Pacific, including the blocking during early- to mid-February 2014 that influenced a record-breaking snowfall over the Kanto area, were analyzed using a long-term reanalysis dataset and local meteorological observation station data. Results reveal that blocking over the Pacific causes large precipitation over the Kanto area and the PNJ by shifting cyclone (storm) tracks towards the east coast of Japan from their usual eastward course across the mid-Pacific via the south coast of Japan. Excessive passing of cyclones caused large precipitation in the Kanto area and snowfall in the PNJ. In the blocking cases with heavy snowfall events in the Kanto area, a strong cold-air inflow over Japan also existed in the lower troposphere originating from east Siberia, which initiated synoptic ground cold-air environments in the Kanto area.
The 2011/12 La Niña, referred to as a double-dip La Niña event with following a previous La Niña event, was not well predicted by most climate models when starting from early-mid 2011. Based on a developed El Niño-Southern Oscillation (ENSO) ensemble prediction system, this paper investigates the key predictors for the 2011/12 La Niña, to determine which conditions favor a double-dip La Niña event up to one year in advance. The key predictors were isolated in a 100-member ensemble hindcast experiment. Results show that continuous easterly surface winds and persistent subsurface cold conditions preceded the second-year cooling in mid-2011. And a significant difference can be viewed between the best and the worst ensemble forecasts arose from the stochastic model-error perturbations. The detailed comparisons between the best and the worst ensemble forecasts further illustrate that the stochastic model-error perturbations play a significant role in improving the prediction skills of the best ensemble members during the 2011/12 12-month forecast process, through capturing the transition of sea surface temperature (SST) over the tropical Pacific (i.e., from a warm condition to a cold condition) in boreal spring.
Calibration of pyrgeometer was performed using blackbody radiation emitted from a conical cavity. For its improvement, optical processes in the pyrgeometer were reconsidered, and a detailed steady state equation of heat transfer was derived. The equation additionally includes three factors; 1) heat transfer between sensor and body, 2) heat transfer from sensor to the air or gas in the body and dome, and 3) form factor for the convergence of dome radiation. Expansion coefficients or calibration parameters in the equation were determined precisely by least squares technique with the experimental data of temperatures and thermopile voltages. In addition, the coefficients were evaluated for the first time theoretically with numerical values of optical properties of its components such as dome, sensor and body. It is successfully demonstrated that both coefficients well agree with each other at a special value of thermopile constant, while there was no agreement in the past investigations.
We examine how cloud cover is determined in shallow-cloud areas by using large-eddy simulation with an extremely wide domain, which covers the transition phase from cumulus-under-stratocumulus to shallow-cumulus regimes. The relationship between two distances is critical to cloud cover. One characteristic distance is the horizontal distance between cumulus clouds, and the other is the broadening distance of anvil-like stratiform cloud at the top of the boundary layer. High cloud cover occurs with a long distance of broadening and short distances between cumuli. In contrast, low cloud cover appears with a short distance of broadening and a long distance between cumuli. The contrast of the two distances is rooted in aerosol amount and the strength of the surface heat flux. The relationship between these two distances can be applied to estimating the cloud cover below sharp inversions.
The onset of the Indian summer monsoon (ISM) in 2012 was investigated using the nonhydrostatic icosahedral atmospheric model (NICAM), a global cloud-system resolving model. We focused on the effect of tropical disturbances on ISM onset and considered the potential extension of onset predictability. A series of NICAM experiments was performed under various initial conditions for the period 10 May to 10 June, 2012. NICAM showed promising performance by realistically simulating ISM onset based on the initial conditions two weeks before the onset. ISM onset in both observations and simulations was accompanied by northward-migrating tropical disturbances over the Bay of Bengal and the Arabian Sea. These disturbances were generated by the eastward propagating disturbance along the equatorial Indian Ocean. As indicated by a comparison of NICAM experimental results with those obtained by the Japan Meteorological Agency Operational Ensemble Prediction System, we suggest that the better reproducing the tropical disturbance enhances the potential to extend the predictability of the transition phase in the Asian summer monsoon.
A new numerical method to calculate full-nonlinear steady lee-wave solutions with high accuracy is proposed. This method is based on the charge simulation method, which is known as an efficient way to solve some kinds of partial differential equations numerically. Numerical solutions obtained by the new method are compared with analytical solutions or numerical solutions obtained by the boundary element method in order to show the proposed method yields highly accurate numerical solutions with a low computational cost.
Future changes in surface air temperature and precipitation over Japan by the end of the 21st century are projected by a well-developed non-hydrostatic regional climate model with a grid spacing of 5 km under the RCP8.5 scenario. Uncertainties in the projected temperature and precipitation are also evaluated with the results obtained from ensemble simulations using this high-resolution model. The projected future climate indicates robust increases in the annual-mean surface air temperature for all regions in Japan. In contrast, many regions do not exhibit statistically significant changes in annual precipitation. In some regions and months, however, monthly precipitation in a couple of members of the ensemble simulations has a statistically significant decrease or increase. Monthly precipitation over the eastern Japan Sea side (EJ) region in December has relatively robust decreases. These decreases are attributed to decreases or weakening of convection over the Japan Sea polar air-mass convergence zone, which is accounted for by the weakening of large-scale low-level northwesterly winds associated with the winter monsoon. The relationship between precipitation and convergence in the EJ region is consistent with the results above: Convective clouds are shallower in the future climate compared with those in the present climate.
The effect of historical land-use change on winter precipitation in Hokkaido, Japan is evaluated by sensitivity experiments using a regional climate model and past and current land-use maps. The regional climate model successfully simulates winter precipitation characteristics, such as heavy precipitation in central mountain areas and around the Japan Sea coast, and a gradual decrease in precipitation from west to east. A comparison of model simulations using 1850 and 1985 land-use maps shows that precipitation has decreased approximately 1.7 mm mon-1, especially over deforested areas. This decrease is mainly a result of reduced evapotranspiration (approximately 2.4 mm mon-1) from lower net radiation associated with increased snow cover and higher surface albedo. Meanwhile, precipitation has increased in mountainous areas and on leeward slopes as a result of intensified upward air motion, which is attributed to a decrease in surface roughness caused by deforestation and the strengthening of horizontal wind speeds.
From November 2012 to October 2013, we measured the eddy covariance flux of carbon dioxide (CO2) in the middle of Tokyo, Japan. The study area is characterized as having low-vegetated mid-rise residence with some arterial roads. Contribution of human activities to the measured CO2 flux was evaluated by hourly inventory analysis. The study area serves as a net source of CO2 whose annual total was +4300 gC m−2 yr−1. The resulting diurnal CO2 flux has one peak in the morning and one in late evening. This diurnal amplitude is larger in winter and also larger on weekday. An hourly inventory analysis for winter shows that the morning peak arises from both traffic and household gas use, whereas the evening peak arises from just the household gas. The significant difference in the morning flux between weekday and holiday should be attributed to the traffic emission.
The Beaufort High (BH) is a well-known climatological feature of the Arctic with close ties to sea ice variability. Here, an objective cyclone-tracking algorithm was utilized to link the summertime interannual variability of the BH to cyclone migration pattern and its intensity. The results indicated that rather than the in situ change of anticyclone in the region, the variability of BH is influenced more by passive effects of cyclone activity with its center near the North Pole. The effect of cyclones is twofold. One is the effect of more intense cyclones entering the peripheral region of the BH where three times as many cyclones below 980 hPa reach the Arctic for years in which BH is weak. The other is intensification of cyclones forming within the Arctic contributing to an average difference of 4.9 hPa between years with strong and weak BH. We argue that along with the in-situ change in the BH, such peripheral cyclone activity spreads into the BH region strongly affecting the observed interannual variability.
Ground-based microwave radiometer (MWR) has been used for high-frequency retrievals of thermodynamic environments. However, raindrops on the radome of MWR and in the air cause errors in retrievals during precipitation events. Although a recent study has noted that off-zenith observations with neural networks (NN) reduce the retrieval errors, the effect of off-zenith observations with one-dimensional variational (1DVAR) technique, which is known to be more accurate than other methods, has not been studied. We developed a new 1DVAR technique that considers the effect of cloud liquid water. We statistically investigated the accuracy of vertical profiles of atmospheric temperature and water vapor retrieved by NN and 1DVAR techniques by using zenith and off-zenith observation at 15° elevation angle under no-rain and rainy conditions and compared them with results of radiosonde observations. The results showed that the 1DVAR technique outperforms NN and numerical model simulation in the estimation of thermodynamic profiles under no-rain conditions. The results also indicated that the error in retrieved profiles in the low-level troposphere can be reduced by the 1DVAR technique by using off-zenith observations even under rainy conditions with rainfall rate less than 1.0 mm h−1, especially when the environment cannot be accurately reproduced by a numerical model.
We analyzed trends in precipitation characteristics at 12 weather stations in Bangladesh using data from 1950 to 2008 and a non-parametric Mann-Kendall's tau test. It was found that the annual precipitation amount and the number of wet days tended to increase at almost all the stations. The number of days with weak precipitation (< 3 mm) showed increasing tendency; the number of days with daily precipitation exceeding 10 mm increased at three stations; and the number of days with daily precipitation exceeding 20 mm increased at two stations. Heavy rainfall index showed no clear trend, and was dominated by inter-annual and/or decadal variations. During the pre-monsoon season, seasonal precipitation amount and the number of wet days increased, and the number of days with weak precipitation (< 3 mm) also increased. Changes in the monsoon and post-monsoon seasons were insignificant.
We produced a preliminary high-resolution probable hourly precipitation (mm h−1) and probable Soil Water Index (SWI) for a 50-yr recurrence interval over the Japanese archipelago from 5-km grid-cell Radar/Raingauge-Analyzed Precipitation (R/A) with a 26-yr time series (1988-2013). To date, the selection and applicability of a probability density function such as the Gumbel distribution (GUM) or the generalized extreme value distribution (GEV) has been investigated mainly from a statistical perspective, whereas this study examines them from the viewpoint of disaster prevention. Results show detailed spatial patterns of the probable hourly precipitation and SWI in Japan with ranges of 17.0-158.0 (±1.1-51.1) mm h−1 and 82.1-638.6 (±3.7-93.0), respectively, by spatial resolution of 5-km grid-cell. The probable hourly precipitation by GUM is less than that of GEV with a shape parameter of less than 0.0. Our results demonstrate that R/A-based probable hourly precipitation and SWI by GUM, which has robust estimation, is appropriate for landslide hazard assessment. There results revealed the potential and limitation of estimating the probable precipitation from the R/A dataset. Further study should verify the R/A dataset and the probable precipitations with accumulation of R/A.
Nitrate and its precursor (gaseous HNO3) in China are generally overestimated by current chemical transport models in comparison to in-situ observations. In this study, we used an observation-based box model and in-situ measurements at a rural site in southern China to investigate possible missing sinks of nitrate. We found that a heterogeneous reaction of HNO3 to NOx on soot can better balance the NOx/HNO3 chemistry in models and improve model performance with regard to nitrate, particularly where an additional HONO source (the heterogeneous reaction of NO2 on soot) is incorporated into current models. Through a series of sensitivity simulations, the uptake coefficients of heterogeneous reactions were suggested to be 3.0 × 10−3 and 1 × 10−4 for HNO3 and HONO over southern China in fall, respectively. A 3-D simulation with the suggested uptake coefficients further confirmed that heterogeneous reactions significantly decreased the nitrate concentrations (PNO3) in southern China, by up to 10 μg m−3 (50%-80% of simulated PNO3) in polluted regions (e.g., the Yangtze River). In contrast to nitrate and HNO3, NOx concentrations in China were enhanced, which partly explained the underestimation of NO2 in current models compared to satellite observations.
Typical parameter calibration techniques in land surface models often limit to solve the spatiotemporal discrepancy of modeling performances due to high heterogeneity of land surface, especially for regional applications. We evaluated a coupling system of micro genetic algorithm (micro-GA) and the Noah land surface model with multi-physics options (Noah-MP) for its usability for regional applications. Four different regions having different climatic characteristics over East Asia were selected, and for each region Noah-MP provides two to four scheme options in eight scheme categories each of which represents different land surface processes, and the model was optimized through searching the best scheme combination by micro-GA. The optimization focused on the surface water balance, comparing model simulations of evapotranspiration and runoff with the European Centre for Medium-Range Weather Forecasts ERA-Interim land products. The optimizing process was controlled by micro-GA using natural selection and evolution techniques. This study demonstrated that the coupling system assures not only the effectiveness of the scheme-based optimization but also the skill of the used model diagnosis in quantifying model performance during the micro-GA evolution process. Since each region has its own advantageous scheme combination, multiple scheme-combinations are a possible solution for the spatiotemporal discrepancy of modeling performance.
We evaluated a method by which to fit gamma distributions with parameters of snowflake size distributions by measuring three physical quantities using an optical sensing disdrometer. The three physical quantities are the diameters of the snowflakes that have 50 and 99 percentiles of volumes (D50 and D99, respectively) and the sum of the sixth powers of the diameters in a unit volume (Z). Snowflake size distribution was well fitted to a gamma distribution between D50 and D99, inclusive. This method prevented the snowflake volume from being considerably underestimated. Although the mean absolute error based on snowflake volume for this method was large compared with that obtained using a moment method, good estimates of snowflake volume were obtained for some samples using this method, for which the snowflake size distribution was less influenced by snowflakes 1 mm or less in diameter. The correlation coefficient was 0.989, as determined by regression analysis based on the observed and estimated snowflake volumes using this method. The estimation of the snowflake volume using this method depends on the quality control of the optimum shape parameter and requires a continuous probability distribution of snowflakes for diameters above 1 mm.
We investigated the seasonal variation of snow cover at different altitudes using station data, satellite data, and a high-resolution numerical model around the Japan's Northern Alps during three winters (2011/12, 2012/13, and 2013/14). The satellite data showed that the snow cover fraction was largest in 2012/13 before late December, which indicates that much of snowfall occurs at higher elevations in the early winter. In midwinter, the snow cover fraction was over 90% in 2011/12, while it was approximately 70% and 60% in 2012/13 and 2013/14, respectively. The station data also showed the greatest snow depth at lower elevations in 2011/12 among the three winters. The numerical model well simulates the year-to-year and monthly variations of the snow cover fraction, although the threshold of snow depth are larger than that of the satellite data. The numerical simulations indicate that the total amount of snowfall is controlled by spring snowfall as much as by winter snowfall at higher elevations. The year-to-year variations of spring snowfall are relatively larger than those of winter snowfall, resulting in different year-to-year variations of snow cover at lower and higher elevations.
Comparisons are made between two computational methods for associated Legendre functions across truncation wave numbers between 39 and 10239. One of the two methods uses the four-point recurrence in double precision (the Fourier method) and the other uses the three-point recurrence in the extended arithmetic (the X-number method). Both methods avoid the shortcomings of the traditional method using the three-point recurrence in double precision and generate values accurate enough to enable stable Legendre transforms at large truncation wave numbers (> 1700). The errors for the Fourier method are found to be much smaller than those for the X-number method and have little latitudinal dependencies. The errors for the Fourier method, however, are found to grow rapidly with large degrees n > 2048. Two alternatives are proposed to calculate the scaling factor of the Fourier coefficients of the associated Legendre functions accurately with errors in O(√n).
To investigate the variability of the structure and evolution of meso-α-scale precipitation systems generated in the Baiu frontal zone, numerical experiments using a cloud-resolving non-hydrostatic model were performed with idealized Baiu-front-like environments. The environment was constructed based on hydrostatic and geostrophic balances, and temperature and relative humidity were designed by using Gaussian functions to realize the frontal structure and moist conveyor belt in the lower atmosphere. In order to generate meso-α-scale precipitation systems, temperature perturbation associated with a shallow depression was introduced. Long-lived band-shaped meso-α-scale precipitation systems with the internal multiscale structures as are often observed in the Baiu frontal zone were simulated under the given simplified environments. The variability of features of the meso-α-scale precipitation systems with respect to relative humidity in the middle troposphere in the Baiu frontal zone was examined as an example. The moister environment produced the more rainfall. Additionally, rainfall was intensified under a specific humidity condition.
A dust event occurred in the Gobi Desert on 22-23 May 2013 due to strong wind reaching a speed of 16 m s−1. The strong wind was caused by a developing low pressure system and a cold front. The vertical structures of the dust layer and cold frontal system were observed by a ceilometer installed at Dalanzadgad, Mongolia, which is located in the central part of the Gobi Desert. The dust layer had extended over the ground surface for 12 hours, and its top height reached 1.8 km above ground level (AGL). During the dust event, the cold front passed, and the cold air mass replaced the warm air mass. The top height of the cold air mass was less than 1.8 km AGL. In addition, the observational results suggest that some dust was transported from the atmospheric boundary layer to the free troposphere by the warm air ascending along the surface of the cold front.
We investigated the validity of column-averaged dry air mole fractions of methane (XCH4: V02.21) retrieved from shortwave infrared (SWIR) spectra obtained by Greenhouse gases Observing SATellite (GOSAT) over Siberia, which is known as a major source area of methane (CH4). We compared the GOSAT XCH4 dataset with aircraft measurements of XCH4 that have been collected in Novosibirsk and Surgut, West Siberia since the 1990s. The average difference between the GOSAT XCH4 and aircraft-based XCH4 was −1.0 ± 22.0 ppb in Novosibirsk and −0.4 ± 14.8 ppb in Surgut when we selected appropriate data pairs. These results indicate that GOSAT XCH4 data obtained over West Siberia is consistent with aircraft measurements and assure the reliability of the GOSAT XCH4 product for scientific analysis.
The purpose of this work is to create indices to represent the atmospheric conditions throughout Japan based on frontal zone data at 5-day intervals for the period 1948-2013. The index was created by conducting principal component analyses of the distribution of frontal frequencies near frontal zones. The distribution of the factor loadings were interpreted as variations of the frontal zone in the following manner: PC1, northward/southward shift of the frontal zone; PC2, increase/decrease of frontal frequencies near the frontal zone; PC3, the extending direction of the frontal zone, whether northwest-southeast or southwest-northeast; and PC4, west-east variations of the frontal frequencies near the frontal zone. The result of correlation analyses between the indices defined as the factor scores and those representing global climatic and oceanic phenomena or teleconnection patterns revealed, in detail, periods with strong connections and comprehensive relationships reported by previous works. Furthermore, trends were found in the recent climate, such as southward shift of the frontal zones in mid- and late spring and increased frontal frequencies around frontal zones in mid- and late summer.
The roles of topography on the propagation of the Madden-Julian Oscillation (MJO) are discussed using an aqua-planet of the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) with a 220-km horizontal mesh. Four topographical configurations with different land-sea masks and elevations are tested using a zonally non-uniform fixed-SST distribution. Explicit cloud microphysics is used to obtain MJO-like signals. Broad land cover generally weakens convection because of reduced surface latent heat flux (LHF). Forced lifting because of topography enhances local convection on the upwind side of high topography. It is suggested that the zonal contrasts of LHF are one reason for the delayed eastward propagation of the MJO-like disturbances. When only the eastern portion of the convective envelope is over land where the LHF is small, the LHF becomes rear-heavy, resulting in delayed eastward propagation. As the entire convective envelope proceeds over land, its contrast decreases or even reverses, resulting in faster eastward propagation.
This study investigated two aspects of the relationship between cloud-to-ground (CG) lightning activity and lightning indices (LIs) based on updraft strength recorded in the Kanto region of Japan: (1) the relationship between CG lightning frequency and LIs based on updraft strength, and (2) the relationship between the occurrence of CG lightning and LIs based on updraft strength. Data from two X-band polarimetric radars recording at a high temporal resolution (every 1-2 min) were used to investigate updraft evolution associated with CG lightning activity. The LIs of updraft volumes were found to be well correlated with CG lightning frequency for threshold values of updraft speeds of > 5 and > 10 m s−1 (correlation coefficient (r) > 0.7); this result supports the findings of a previous study in the North America. The LIs derived from updraft strength studied here, updraft volumes at vertical velocities of > 0, > 5, and > 10 m s−1 and maximum updraft speed, were also well correlated with the occurrence of CG lightning (the highest hit rate (HR) was > 0.7 when probability of detection (POD) was > 0.9). The results demonstrate the potential for updraft parameters to be used as more accurate LIs than those based on reflectivity parameters.
Around 2000, El Niño-Southern Oscillation (ENSO) began to experience weaker-intensity, but more frequent events (i.e., the increased central-Pacific (CP) ENSO), when a climate transition characterized by a La Niña-like mean state coincidently occurred. Associated with this climate shift, the ENSO simulation and prediction skills of most models in the 2000s were significantly lower than those in the 1980-90s, which had common biases that were amplified through this climate transition. In this work, a model bias is examined with the heat flux (HF)-sea surface temperature (SST) coupling processes over the tropical Pacific, which experienced coupling strength modification around 2000 over both the regions west and east of 160°W. A simple ENSO coupled model trained with the observations before 2000 is used to demonstrate that this model bias can be alleviated by updating the HF-SST coupling parameters. After updating these parameters, the performance of the ENSO model is significantly improved for reproducing observations over the tropical Pacific after the year 2000, suggesting the potential for a substantial improvement in ENSO simulation and prediction.
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