This study investigated the representative height of low-level water vapor field that can be used to examine the occurrence possibility of heavy rainfall in East Asia. First, cloud base heights (CBHs) of moist convection were statistically examined by performing simulations with a 1-km-resolution numerical model during April–August 2008, with a focus on Kyushu and Shikoku Islands, western Japan. CBHs of moist convection with strong updrafts were simulated mainly around 500 and 300 m heights above sea level over land and over the ocean, respectively. This result indicates that low-level humid air below a height of 500 m is very important for the initiation of strong moist convection. Moreover, the equivalent potential temperature θe at the CBHs was examined to clarify θe values of lifted air parcels initiating cumulonimbus development. This result showed that, below the CBHs, θe was usually around 355 K.
Given these results for the CBHs, θe at 500 m height from 10-km-resolution objective analysis data was statistically compared with θe at various heights and pressure levels over the ocean south of 35°N in East Asia during June–September 2008. These comparisons showed that analyses at the 850-hPa level could not represent the low-level water vapor field, while the θe field at 850 hPa in the Baiu season was strongly influenced by convective activity over the Baiu frontal zone. The θe field at 925 hPa also could not adequately represent the low-level water vapor field, but the difference in θe between heights of 250 and 500 m was very small. Because high θe layers must have some thickness, data at 500 m height can be considered representative of the low-level water vapor field in analyses examining the initiation of moist convection leading to heavy rainfall.
The steering-flow analysis based on potential vorticity (PV) diagnosis is used to examine the reasons why the National Centers for Environmental Prediction Global Forecast System (NCEP-GFS) model showed large track forecast errors with over-recurving movement in Typhoon Fengshen (2008). In particular, two forecasts initialized at 0000 UTC 19 and 20 June 2008 are demonstrated in this study. The deep-layer-mean (DLM) steering flow between 925 and 300 hPa with tropical cyclone components filtered out is directed to the west or northwest in the analysis field, which can account for the continuous westward and northwestward movement in the best track. However, the DLM steering flow is shown more toward the north in the forecast fields. Four distinct PV features associated with the corresponding subtropical high, monsoon trough, continental high, and midlatitude trough are identified to diagnose their balanced steering flows around the storm. The result based on PV analysis indicates that the reduced westward steering flow in the forecast field is mainly attributed to the subtropical high which is over-predicted to extend southwestward, as well as the continental high with underestimated coverage, as characterized by the geopotential height at 500 hPa. The steering flow associated with the monsoon trough plays an essential role while Typhoon Fengshen (2008) experiences northward recurvature in both analysis and forecast fields. Therefore, the associated reduced westward steering flow in the NCEP-GFS model leads to the over-recurvature of Fengshen.
Although both the tropical cyclone (TC) peak seasons in 2016 and 1998 are in the decaying stage of a super El Niño, TC activities over the western North Pacific (WNP) exhibit vast differences. The TCs in 2016 were greater in number and intensity and had distinct monthly variations in TC activity in contrast to those in 1998. The detailed comparison shows that the warm sea surface temperature anomaly over the WNP in 2016 had higher magnitude and a more eastward extension than that in 1998. In August, coincident with the enhanced Madden–Julian oscillation westerly phase, more TCs clustered within the eastward-extending convective belt caused by the southwesterly surge. The mean longitude of TC genesis in 2016 shifted more eastward, which is favorable for the longer lifetime and greater intensity of the TCs. In terms of the extratropical influences, the cyclonic circulation anomaly associated with the Silk Road Pattern from the middle latitude penetrated southward and split the WNP subtropical high (WNPSH) into two components in August of 2016, thus causing deep-tropospheric southerly steering flows in between and TC northward-prone tracks. During the boreal autumn in 2016, the WNPSH strengthened and stretched westward, producing the robust easterly steering flows that led to successive TCs affecting the coastal areas of East Asia.
The dependence of the intensification rate (IR) of a tropical cyclone (TC) on its initial structure, including the radius of maximum wind (RMW) and the radial decay rate of tangential wind outside the RMW, is examined based on ensemble of simulations using a nonhydrostatic axisymmetric cloud-resolving model. It is shown that the initial spinup period is shorter, and the subsequent IR is larger for a storm with an initially smaller RMW or with an initially more rapid radial decay of tangential wind outside the RMW. The results show that the longevity of the initial spinup period is determined by how quickly the inner-core region becomes nearly saturated in the middle and lower troposphere, and thus, deep convection near the RMW is initiated and organized. Because of the larger volume and weaker Ekman pumping, the inner core of the initially larger vortex takes longer time to become saturated and thus experiences a longer initial spinup period. The vortex initially with the larger RMW (with the slower radial decay of tangential wind outside the RMW) has lower inertial stability inside the RMW (higher inertial stability outside the RMW), develops more active convection in the outer-core region and weaker boundary-layer inflow in the inner-core region, and thus experiences lower IR during the primary intensification stage.
The cloud variability and regime transition from stratocumulus to cumulus across the sea surface temperature front in the Kuroshio region over the East China Sea are important regional climate features and may affect the Earth's energy balance. However, because of large uncertainties among available cloud products, it is unclear which cloud datasets are more reliable for use in studying the regional cloud features and in validating cloud simulations in the region by climate models. In this study, the monthly low cloud amount (LCA) and total cloud amount (TCA) datasets in the region from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), Moderate-resolution Imaging Spectroradiometer (MODIS), and International Comprehensive Ocean–Atmosphere Data Set (ICOADS) are validated against the combined product of CloudSat + CALIPSO (CC) in terms of consistency and discrepancy in the climatologically mean, seasonal cycle, and interannual variation. The results show that LCA and TCA derived from MODIS and CALIPSO present relatively high consistency with CC data in the climatological annual mean and show similar behaviors in seasonal cycle. The consistency in LCA between the three datasets and the CC is generally good in cold seasons (winter, spring, and fall) but poor in summer. MODIS shows the best agreement with CC in fall, with a correlation coefficient of 0.77 at a confidence level over 99 %. CALIPSO and MODIS can provide a competitive description of TCA in all seasons, and ICOADS is good in terms of the climatological seasonal mean of TCA in winter only. Moreover, the interannual variation of LCA and TCA from all datasets is highly correlated with that from CC in both winter and spring with the Matching Score ranging between 2/3 and 1. Further analysis with long-term data suggests that both LCA and TCA from ICOADS and MODIS can be good references for studies of cloud interannual variability in the region.
The climatological features of surface air temperature variations on time scales of a few minutes to 1 h were examined using 1 min data spanning a 4-year period from 917 automated stations in Japan. The temperature time series was spectrally analyzed after the application of a Gaussian high-pass filter, and the variances with periods of 64 min or less were statistically analyzed as sub-hourly temperature variations. The result obtained shows that daytime temperature variation is observed throughout the country with relatively small regional differences. The amplitudes of daytime temperature variations were larger during spring and summer than during autumn and winter and were larger under high temperature and sunny weather than under low temperature, no sunshine, and precipitation. A cross-spectral analysis of temperature and wind speed reveals that temperature peaks tend to coincide with or lag behind wind speed minima. The variation is likely to correspond to the convective motion in the mixing layer. On the other hand, the intensity of nighttime temperature variation showed a large amount of scatter among stations, with exceptionally large variations during winter at some stations in northern and eastern Japan. Nighttime temperature variation tends to be in phase with wind speed variation, with longer periods than daytime temperature variation and with more intensity under low temperature and low wind speed than under high temperature, high wind speed, and precipitation. Stations with large winter nighttime temperature variations tend to be located on a col or a slope, where the surface inversion layer is likely to be easily disturbed by any kind of atmospheric motion.
Geographical distributions of heavy snowfall, especially in the Pacific Ocean side of Japan, have not been previously elucidated due to low occurrence frequency of heavy snowfall and limited number of snow observation points. This study investigates the characteristics of synoptic conditions for heavy daily snowfall from western to northeastern Japan in the present climate, analyzing high-resolution regional climate ensemble experiments with 5-km grid spacing. The Japanese 55-year Reanalysis (JRA-55) and the 10-ensemble members of the database for Policy Decision making for Future climate change (d4PDF) historical experiments are applied to the lateral boundary conditions of the regional climate model. Dynamical downscaling using d4PDF (d4PDF-DS) enabled us to evaluate much heavier snowfall events than those simulated by dynamical downscaling using JRA-55 (JRA55-DS).
Over the Sea of Japan side, heavy snowfall occurs due to cold air outbreaks, while over the Pacific Ocean side, heavy snowfall is brought about by extratropical cyclones passing along the Pacific Ocean coast. A comparison between JRA55-DS and d4PDF-DS indicated that heavier snowfall can occur due to more developed extratropical cyclones and enhanced cold air damming in the Tokyo metropolitan area. The geographical distributions of extremely heavy snowfall are different between two typical synoptic conditions, i.e., cold air outbreaks and extratropical cyclones. The difference is much clearer in the extremely heavy snowfall events than in all snowfall events. Heavy daily snowfall occurs in January and February on the Pacific Ocean side, in December and January on the Sea of Japan side, and in November and March in high mountainous areas. Saturated water vapor pressure is largest around 0°C under the snowing conditions. Synoptic conditions from late fall to winter are closely related to preferable conditions for heavy snowfall over the mountainous areas where the surface air temperature is much less than 0°C in the heavy snowfall events.
This study evaluated the impact of a future space-borne Doppler wind lidar (DWL) on a super-low-altitude orbit by using an observing system simulation experiment (OSSE) based on a sensitivity observing system experiment (SOSE) approach. Realistic atmospheric data, including wind and temperature, was provided as “pseudo-truth” (PT) to simulate DWL observations. Hourly aerosols and clouds that are consistent with PT winds were also created for the simulation. A full-scale lidar simulator, which is described in detail in the companion paper, simulated realistic line-of-sight wind measurements and observation quality information, such as signal-to-noise ratio (SNR)and measurement error. Quality control (QC) procedures in the data assimilation system were developed to select high-quality DWL observations on the basis of the averaged SNR from strong backscattering in the presence of aerosols or clouds. Furthermore, DWL observation errors used in the assimilation were calculated using the measurement error estimated by the lidar simulator.
The forecast impacts of DWL onboard polar- and tropical-orbiting satellites were assessed using the operational global data assimilation system. Data assimilation experiments were conducted in January and August in 2010 to assess overall impact and seasonal dependence. It is found that DWL on either polar- or tropical-orbiting satellites is overall beneficial for wind and temperature forecasts, with greater impacts for the January experiments. The relative forecast error reduction reaches almost 2 % in the tropics. An exception is degradation in the southern hemisphere in August, thus suggesting a need to further refine observation error assignment and QC. A decisive conclusion cannot be drawn with regard to the superiority of polar- or tropical-orbiting satellites because of their mixed impacts. This is probably related to the characteristics of error growth in the tropics. The limitations and possible underestimation of the DWL impacts, for example, due to a simple observation error inflation setting, in the SOSE-OSSE are also discussed.
Cloud fractions were observed during research cruises onboard the research vessel (R/V) Shirase between Japan and Antarctica using a whole-sky camera and a ceilometer. The cruises, Japanese Antarctic Research Expeditions (JARE) 55 and 56, took place from November 2013 to April 2014 and from November 2014 to April 2015, respectively. Cloud fractions were estimated from the whole-sky camera based on the sky brightness and spectral characteristics, and the ceilometer recorded the cloud occurrence frequency. According to the comparison of daily-averaged cloud fractions from the whole-sky camera with the ceilometer observations over the open ocean between Japan and Antarctica, the correlation coefficients were 0.87 and 0.93 for JARE 55 and 56, respectively. Overall, the results from both observation methods were consistent over the open ocean. Nevertheless, it was necessary to take surface conditions into consideration, particularly for the estimated cloud fractions from the whole-sky camera, because the contrast in brightness and spectral properties between cloudy and clear skies was lower over the sea ice region, owing to the higher surface albedo. Hence, the classification parameter was expressed as a function of sun elevation over the sea ice region in this study. This parameter was determined from part of the data over the sea ice region during JARE 55 and then applied to JARE 56 as well as to the remaining data from JARE 55. As a result, the daily-averaged cloud fractions over the sea ice region were approximately 84 % and 57 % from JARE 55 and 56, respectively. The daily-averaged cloud fractions estimated from the whole-sky camera were also consistent with the ceilometer observations, where the correlation coefficients with the sea ice region were 0.93 and 0.96 for JARE 55 and 56, respectively.
The aerosol optical characteristics in the East Asian cities of Fukuoka and Beijing were measured from 2010 to 2014. These long-term season-crossing data were compared to understand the differences between the aerosol characteristics at a source and a downstream region. Previously, few long-term, season-crossing observations have been reported. Using a method developed by one of the present authors, the measurement data were analyzed so that the retrieved optical properties can be more accurate than those obtained in previous studies. Using these data, the aerosol characteristics and their frequency distributions were reliably obtained. In Fukuoka, the annual means of the extinction, scattering, and absorption coefficients Cext (525 nm), Csca (525 nm), and Cabs (520 nm) were 74.6, 66.1, and 8.1 Mm−1, respectively, whereas those in Beijing were 412.1, 367.2, and 42.4 Mm−1, respectively. The coefficients in Fukuoka were approximately one-fifth of those in Beijing. The single-scattering albedos ω0 (525 nm) in Fukuoka and Beijing were 0.877 and 0.868, respectively. The asymmetry factors G (525 nm) in the two cities were 0.599 and 0.656, respectively. The extinction Ångström exponents αext in the two cities were 1.555 and 0.855, respectively. The absorption Ångström exponents αabs in the two cities were 1.106 and 0.977, respectively. The fine- and coarse-mode volume fractions in Fukuoka were approximately 80 % and 20 %, and those in Beijing were both approximately 50 %, except in summer.
Cext, Csca, and Cabs showed a seasonal variation in both cities. Some other properties also showed a seasonal variation. In particular, the seasonal variation in αabs was clear in both cities; it tended to be small in summer and large in winter. The frequency distributions of various parameters were also investigated. The frequency of Cext > 500 Mm−1 in Fukuoka was very low, and large Cext values were recorded more frequently in spring than in other seasons. In Beijing, Cext > 1000 Mm−1 values were recorded more frequently, and the frequency of 10 Mm−1 ≤ Cabs ≤ 60 Mm−1 was high in spring and summer. Furthermore, αabs < 1.0 values were recorded frequently, which cannot be explained by the simple external mixture of absorbing aerosols.
To demonstrate the usefulness of the data obtained in this study, the relationships among αabs, αext, the volume size distribution, the imaginary part of the refractive index, and ω0 were investigated, and two characteristic cases in Beijing (winter) and Fukuoka (spring) were preliminarily analyzed.
A new recurrence formula to calculate the associated Legendre functions is proposed for efficient computation of the spherical harmonic transform. This new recurrence formula makes the best use of the fused multiply–add (FMA) operations implemented in modern computers. The computational speeds in calculating the spherical harmonic transform are compared between a numerical code in which the new recurrence formula is implemented and another code using the traditional recurrence formula. This comparison shows that implementation of the new recurrence formula contributes to a faster transform. Furthermore, a scheme to maintain the accuracy of the transform, even when the truncation wavenumber is huge, is also explained.