In Japan, localized heavy rainfall events producing accumulated three-hour precipitation amounts larger than 200 mm are often observed to cause severe landslides and floods. Such events are mainly brought by quasistationary band-shaped precipitation systems, named “senjo-kousuitai” in Japanese. Senjo-kousuitai is defined as a band-shaped heavy rainfall area with a length of 50–300 km and a width of 20 - 50 km, produced by successively formed and developed convective cells, lining up to organize multi-cell clusters, and passing or stagnating at almost the same place for a few hours. The formation processes of senjo-kousuitai are categorized mainly into two types; the broken line type in which convective cells simultaneously form on a quasi-stationary local front by the inflow of warm and humid air, and the back building type in which new convective cells successively forming on the upstream side of low-level winds linearly organize with pre-existing cells.
In this study, previous studies of band-shaped precipitation systems are reviewed, and the numerical reproducibility of senjo-kousuitai events and the favorable conditions for their occurrence are examined. In a case of Hiroshima heavy rainfall observed in western Japan on 20 August 2014, the reproduction of the senjo-kousuitai requires a horizontal resolution of at least 2 km, which is sufficient to roughly resolve the formation and development processes of convective cells, while a resolution of 250–500 m is necessary to accurately reproduce their inner core structures. The 2-km model quantitatively reproduced the Hiroshima case when initial conditions 10 hours before the event were used, but the predicted amounts of maximum accumulated precipitation were considerably reduced as the initial time became closer to the occurrence time of the senjo-kousuitai. This reduction was brought from the excessive inflow of low-level dry air that shifted occurrence areas of new multi-cell clusters.
Six favorable occurrence conditions of senjo-kousuitai events for their diagnostic forecasts were statistically constructed from environmental atmospheric fields in previous localized heavy rainfall events. Two conditions of (1) large water vapor flux amounts (> 150 g m−2 s−1) and (2) short distances to the level of free convection (< 1000 m) were chosen representatively for the low-level water vapor field that is judged based on 500-m height data. Four other favorable conditions are selected; (3) high relative humidity at midlevels (> 60 % at 500 hPa and 700 hPa), (4) large vertical shear estimated from the storm relative environmental helicity (> 100 m2 s−2), (5) synoptic-scale ascending areas (400 km mean field at 700 hPa), and (6) the exclusion of warm air advection frequently appearing at 700–850 hPa and inhibiting the development of convection (i.e., an equilibrium level > 3000 m).
The accurate estimation of precipitation is an important objective for the Dual-frequency Precipitation Radar (DPR), which is located on board the Global Precipitation Measurement (GPM) satellite core observatory. In this study, a Bayesian correction (BC) approach is proposed to improve the DPR's instantaneous rainfall rate product. Ground dual-polarization radar (GR) observations are used as references, and a log-transformed Gaussian distribution is assumed as the instantaneous rainfall process. Additionally, a generalized regression model is adopted in the BC algorithm. Rainfall intensities such as light, moderate, and heavy rain and their variable influences on the model's performance are considered. The BC approach quantifies the predictive uncertainties associated with the Bayesian-corrected DPR (DPR_BC) rainfall rate estimates. To demonstrate the concepts developed in this study, data from the GPM overpasses of the Weather Service Surveillance Radar (WSR-88D), KHGX, in Houston, Texas, between April 2014 and June 2018 are used. Observation errors in the DPR instantaneous rainfall rate estimates are analyzed as a function of rainfall intensity. Moreover, the best-performing BC model is implemented in three GPM-overpass cases with heavy rainfall records across the southeastern United States. The results show that the DPR_BC rainfall rate estimates have superior skill scores and are in better agreement with the GR references than with the DPR estimates. This study demonstrates the potential of the proposed BC algorithm for enhancing the instantaneous rainfall rate product from spaceborne radar equipment.
Data from the continuous observations of four shallow snow events (echo top < 8 km) and two deep events (> 10 km) were obtained using the C-band vertically pointing radar with frequency-modulation continuous-wave technology with extremely high resolution during the winter of 2015–2016 in middle latitudes of China. Snow-generating cells (GCs) were found near the cloud top in each event. Reflectivity (Z), radial velocity (Vr), and the vertical gradients of Z (dZ/dh, where h is the vertical distance) and Vr (dVr/dh) showed different vertical distribution characteristics between the upper GC and lower stratiform regions (St regions). Fall streaks (FSs) associated with GCs were embedded in the St regions. In the deep events, the proportions of GC regions were slightly larger, but the average contributions to the growth of Z (33 %) were lower than those in the shallow events (42 %). The average d Z /dh values were usually two to three times larger inside GCs and FSs compared to outside. Bimodal Doppler spectra were used to establish the relationships between Z and the reflectivity-weighted particle fall speed (Vz) for the two regions. The vertical air velocity (Wa) and Vz were then retrieved, and the results showed that both the updraft and the downdraft were alternately observed in GC regions. GC locations were usually accompanied by strong upward air motion, with average speeds mostly distributed around 1.2 m s−1, whereas downward air motion often appeared between GCs. In the St regions, the speeds of Wa were mainly within 0.5 m s−1. The upper areas of the St regions consisted primarily of weak upward motion, whereas weak downward motion dominated the lower areas. There was no apparent difference in Wa inside and outside the FSs. The average Vz was slightly larger inside GCs and FSs compared to outside, with a difference of 0.1–0.3 m s−1 and 0.2–0.4 m s−1, respectively.
Global simulations with 1.45 km grid spacing are presented that were performed using the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). Simulations are uncoupled (without ocean, sea ice, or wave model), using 62 or 137 vertical levels and the full complexity of weather forecast simulations is presented, including recent date initial conditions, real-world topography, and state-of-the-art physical parametrizations, as well as diabatic forcing including shallow convection, turbulent diffusion, radiation and five categories for the water substance (vapor, liquid, ice, rain, and snow). Simulations are evaluated with regard to computational efficiency and model fidelity. Scaling results are presented, which were performed on the fastest supercomputer in Europe, Piz Daint (Top 500, November 2018). Important choices for the model configuration at this unprecedented resolution for the IFS are discussed such as the use of hydrostatic and non-hydrostatic equations or the time resolution of physical phenomena which is defined by the length of the time step.
Our simulations indicate that the IFS model—based on spectral transforms with a semi-implicit, semi-Lagrangian time stepping scheme in contrast to more local discretization techniques—can provide a meaningful baseline reference for O(1) km global simulations.
Variations in raindrop size distribution (DSD) during the southwest monsoon (SWM) season over different climatic regions in the Indian subcontinent and adjoining seas are studied in this paper using five years (2014–2018) of global precipitation measurement dual-frequency precipitation radar derived DSDs. The rain rate (R) stratified DSD measurements show clearly that land, sea, and orography differ in their mass-weighted mean diameter (Dm) values. Irrespective of R, Dm values of deep rain were found to be larger in continental rain than in maritime and orographic rain. However, for shallow storms, the Dm values were smaller for continental rain than for orographic and maritime rain. Based on the Dm values and their variations with R of the deep systems, the regions could be categorized into four groups, within which the Dm values were nearly equal: (1) the northwest India (NWI) and the southeast peninsular India (SEPI); (2) the foothills of the Himalayas (FHH) and the central India (CI); (3) the northeast India (NEI) and the Bay of Bengal (BOB); and (4) the Arabian Sea (AS), the Western Ghats (WG), and the Myanmar coast (MC). Compared to other geographical regions of the Indian subcontinent, the Dm values of the deep systems were the largest over NWI and SEPI and the smallest over the WG, MC, and AS; while for shallow systems, the Dm values were the largest over the BOB and AS and the smallest over the SEPI and NWI regions. Though the cloud drops were smaller over the continental regions, the raindrops were larger than in the maritime and orographic rain regions. The microphysical and dynamical processes that occur during precipitation play a vital role in altering the DSDs of continental rain.
In this study, an algorithm was developed to detect the spurious differential phase ΦDP and specific differential phase KDP in the rain for application after the removal of gate-to-gate ΦDP fluctuations. The algorithm is a threshold filter designed based on the empirical relationship between the KDP and radar reflectivity factor at horizontal polarization ZH for raindrops. The construction and validation of the algorithm were performed using the data observed by the C-band polarimetric radar on board the research vessel Mirai near Sumatra from 23 November to 17 December 2015, when a pilot field campaign of the Years of the Maritime Continent (YMC) project was conducted. Perturbations exist in the ΦDP and associated spurious values of KDP on a 10-km scale in the range direction, which are mainly induced by second-trip echoes and nonuniform beam filling. This new algorithm can efficiently detect these perturbed ΦDP values and the positively and negatively biased KDP values. The standard deviation of the KDP in areas with relatively low ZH is also significantly reduced by the application of the algorithm. Simultaneously, the estimation of the rain rate from the filtered KDP has been greatly improved. The results indicate that the algorithm developed in this study can efficiently manage the quality of the data observed not only in the open ocean but also in coastal areas of the Maritime Continent.
Ozone loss pathways and their rates in the ozone quasi-biennial oscillation (QBO), which is simulated by a chemistry-climate model developed by the Meteorological Research Institute of Japan, are evaluated using an objective pathway analysis program (PAP). The analyzed chemical system contains catalytic cycles caused by NOx, HOx, ClOx, Ox, and BrOx. PAP quantified the rates of all significant catalytic ozone loss cycles, and evaluated the partitioning among these cycles. The QBO amplitude of the sum of all cycles amounts to about 4 and 14 % of the annual mean of the total ozone loss rate at 10 and 20 hPa, respectively. The contribution of catalytic cycles to the QBO of the ozone loss rate is found to be as follows: NOx cycles contribute the largest fraction (50–85 %) of the QBO amplitude of the total ozone loss rate; HOx cycles are the second-largest (20–30 %) below 30 hPa and the third-largest (about 10 %) above 20 hPa; Ox cycles rank third (5–20 %) below 30 hPa and second (about 20 %) above 20 hPa; ClOx cycles rank fourth (5–10 %); and BrOx cycles are almost negligible. The relative contribution of the NOx and Ox cycles to the QBO amplitude of ozone loss differs by up to 10 % and 20 %, respectively, from their contribution to the annual mean ozone loss rate. The ozone QBO at 20 hPa is mainly driven by ozone transport, which then alters the ozone loss rate. In contrast, the ozone QBO at 10 hPa is driven chemically by NOx and the temperature dependence of [O]/[O3], which results from the temperature dependence of the reaction O + O2 + M → O 3 + M. In addition, the ozone QBO at 10 hPa is influenced by the overhead ozone column, which affects [O]/[O3] (through ozone photolysis) and the ozone production rate (through oxygen photolysis).
We examined the processes of tropical cyclogenesis in strong monsoon trough pattern over the western North Pacific (WNP) using reanalysis data and numerical experiments. Composite analysis showed that more tropical cyclones are likely to form in the central WNP (130–165°E) and fewer tropical cyclones appear in the western (120–130°E) and eastern (165–180°E) WNP when monsoon trough extends southeastward. Numerical experiments with the same weak artificial vortices inserted into eight different regions of the monsoon trough showed that weak tropical disturbances tend to develop more rapidly in the central WNP near 140–160°E, particularly near 150–155°E when the monsoon trough extends eastward, whereas weak tropical disturbances tend to develop more slowly in the eastern WNP near 165–170°E and do not form in the western WNP near 120–137.5°E. Our modeling results are consistent with the observational analyses. The failure of tropical cyclogenesis in the western WNP is due to the decrease of the moisture and heat (including the sensible and latent heat) from the underlying ocean, whereas large vertical wind shear and dry conditions in the upper-level of the vortex reduce the gradient of intensification of tropical disturbances in the eastern WNP when the vortices have a similar initial intensity.
Atmospheric rivers (ARs), which are narrow water vapor transport bands over the mid-latitudes, often have great socio-economic impacts over East Asia. Although summertime AR activity over East Asia is strongly induced by preceding-winter El Niño development, the extent to which seasonal transitions of El Niño–Southern Oscillation (ENSO) from winter to summer affect the AR activity remains unclear. Here, we examine the relationship between the seasonal transitions of ENSO and the summertime AR activity over East Asia using an atmospheric reanalysis and high-resolution atmospheric general circulation model (AGCM) ensemble simulations. A rapid transition from preceding-winter El Niño to summertime La Niña results in more AR occurrence over northern East Asia via the northward expansion of an anomalous low-level anticyclone over the western North Pacific compared with sustained or decayed El Niño cases. The northward expansion of the anticyclone is consistent with a steady response of the atmosphere to the anomalous condensation heating over the Maritime Continent and equatorial Pacific. Meridional positions of the extratropical AR occurrence and circulation anomalies are different between the reanalysis and AGCM simulations, which is possibly contributed by a limited sample size and/or AGCM biases and suggests that the seasonal prediction of AR-related natural disaster risk over East Asia on a regional scale remains a challenge.