Journal of the Meteorological Society of Japan. Ser. II Advance Publications

 A working group is studying the feasibility of a future Japanese space-borne coherent Doppler wind lidar (CDWL) for global wind profile observation. This study is composed of two companion papers: an instrumental overview of the space-borne CDWL for global wind profile observation (Part 1) and the wind measurement performance (error and bias) investigated using a full-fledged space-borne CDWL simulator (Part 2). The objective of this paper is to describe the future space-borne CDWL in terms of technical points and observation user requirements. The future mission concept is designed to have two looks for vector wind measurement with vertical resolutions of 0.5 (lower troposphere: 0-3 km), 1(middle troposphere: 3-8 km), and 2 km (upper troposphere: 8-20 km) and horizontal resolution of < 100 km along a satellite. The altitude and orbit of the satellite are discussed from a scientific viewpoint. The candidate altitude and orbit of the satellite are 220 km and an inclination angle of 96.4° (polar orbit) or 35.1° (low-inclination-angle orbit), respectively. The technical requirements of the space-borne CDWL are a single-frequency 2-μm pulse laser with an average laser power of 3.75 W, two effective 40-cm-diameter afocal telescopes, a wide-bandwidth (> 3.4 GHz) detector, a high-speed AD converter, and a systematic lidar efficiency of 0.08. The space-borne CDWL looks at two locations at a nadir angle of 35° at two azimuth angles of 45° and 135° (225° and 305°) along the satellite track. The future space-borne CDWL wind profile observation will fill the gap of the current global wind observing systems and contribute to the improvement of the initial conditions for NWP, the prediction of typhoons and heavy rain, and various meteorological studies.

 A feasibility study of tropospheric wind measurements by a coherent Doppler lidar aboard a super-low-altitude satellite is being conducted in Japan. The considered lidar uses a 2.05 μ m laser light source of 3.75 W. In order to assess the measurement performances, simulations of wind measurements were conducted. The mission definition is presented in a companion paper (Part 1) while, in this paper, we describe the measurement simulator and characterize the errors on the retrieved line-of-sight (LOS) winds. Winds are retrieved from the Doppler-shift of the noisy backscattered signal with a horizontal resolution of 100 km along the orbit track and a vertical resolution between 0.5 and 2 km. Cloud and wind fields are the pseudo-truth of an Observing System Simulation Experiment while aerosol data are from the Model-of-Aerosol-Species-IN-the-Global-AtmospheRe (MASINGAR) constrained with the pseudo-truth wind. We present the results of the analysis of a full month of data in summer time for a near-polar orbiting satellite and a LOS nadir angle of 35°. Below ≈ 8 km, the ratio of good retrievals is 30—55 % and the median LOS wind error is better than 0.6 m s^-1 (1.04 m s^-1 for the horizontal wind). In the upper troposphere, the ratio is less than 15 % in the southern hemisphere and high-latitudes. However the ratio is still 35 % in the northern Tropics and mid-latitudes where ice-clouds frequently occur. The upper-tropospheric median LOS-wind measurement error is between 1-2 m s^-1 depending on the latitude (1.74-3.5 m s^-1 for the horizontal wind). These errors are dominated by uncertainties induced by spatial atmospheric inhomogeneities.

 Northern Xinjiang (NX), China, located at middle latitude of Northern Hemisphere, has abundant snowfall and a long period of snow cover. To assess the impacts of climate change in this region and to provide scientific knowledge for the resources and contingency plans, analysis of the spatial and temporal variations in extreme snowfall events (ESEs) in NX was carried out based on five defined ESE indices in this study, i.e. days of heavy snowfall, maximum 1-day snowfall amounts, maximum 1-event snowfall amounts, maximum consecutive snowfall days, and frequency of heavy snowfall events. To reconstruct the snowfall dataset, the relationship between air temperature and snowfall events were compared, and it was found that daily minimum air temperature below 0℃ is the best indicator to select snowfall days. ESEs in NX were taking an increasing proportion in snow events, though snowfall days were decreasing. Consistent increasing trends in all ESE indices were found for the whole NX, while different changes in these ESE indices were found for subregions. With high increasing trends in these ESE indices in most of subregions, Daxigou-Xiaoquzi area and Qitai area were the hot spots for ESEs. Since these hotspots are likely to be influenced by airflow from the Arctic Ocean, the changes in Arctic Ocean and associated atmospheric circulation as a consequence of climate change might be the main reason for the detected increasing trends of the ESEs in NX.

 Mechanisms related to the diurnal cycle of tropical deep convection over a complex terrain were investigated in the Bandung basin, West Java, Indonesia. Observational data was analyzed from X-band radar, Global Navigation Satellite System (GNSS) receivers, and radiosondes, with high-resolution numerical model data.

 Significant diurnal variation of GNSS-derived precipitable water vapor (PWV), which peaked in the early evening, was observed from 13 to 19 March 2013. During this period, the X-band radar detected convective initiation at approximately 1200 Local Time (LT) over the southern slope of the basin. A 2 km mesh model successfully simulated the observed diurnal variations of PWV and rainfall, from 15 to 17 March 2013. In the model, moist air was present in the bottom of the basin during the early morning, which was transported to the southern slope of the basin by valley wind circulation after sunrise. In contrast, humidity was lower in the northern part of the basin due to a downward circulating valley wind. The valley wind decreased static stability around the southern slope of the basin by transporting moisture. It also caused low-level wind convergence, resulting in convective initiation on the southern slope of the basin. The GNSS receiver network also recorded this simulated water vapor variability associated with the valley wind.

 These results suggest that water vapor in the bottom of the basin during the morning, and its advection by valley wind, strongly influences convective initiation in Bandung.

 This study investigates future changes in atmospheric circulation during the Baiu in Japan using 20-km-mesh Atmospheric General Circulation Model (AGCM) simulations for the present-day (1979–2003) and the future (2075–2099) climates under the Representative Concentration Pathways 8.5 scenario. The simulated future climates include the outputs obtained with one control sea surface temperature (SST) and three different SST patterns. The Baiu frontal zone defined as the meridional gradient of equivalent potential temperature gradually moves northward during June-July-August in the present-day climate. In the future climate simulations using the control SST, the Baiu frontal zone is projected to stay to the south of Japan in June. Thus, precipitation is projected to increase over this region, while decreasing in the western part of Japan. The future changes in precipitation activity and atmospheric circulations in June are consistent across all four SST patterns. However, precipitation and atmospheric circulation in July and August in the future climate simulation depends on the SST patterns as follows: in non-El Nino-like SST pattern, the Baiu terminates in late July, similar to that of the present-day climate; a result with an El-Nino like SST pattern shows that sufficient amount moisture is transported to the Japan islands and leads in a delay of the Baiu termination until August; and in the SST pattern with strong warming in the western North Pacific, a sufficient amount of moisture is transported to the south of Japan from June until August. The difference in SST pattern leads to a variation in sea level pressure in the western North Pacific, and affects a variation of the Northern Pacific subtropical high around the Japanese islands in July and August.

 During the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS), many isolated convective storms developed in the southern Kanto Plain on August 17, 2012. The aim of this study was to clarify the dynamics leading to the convection initiation of one of them using different remote sensing instruments.

 Before the convection initiation, a southeasterly flow transported water vapor inland from Tokyo Bay and the well-mixed and a cumulus-cloud-topped convective boundary layer developed. A convergence line in the form of a sea breeze front (SBF) also moved inland from Tokyo Bay. A near-surface air parcel was lifted to its lifting condensation level (LCL) by an updraft in a convergence zone with a 3 km horizontal scale, which formed the west edge of the convergence line. The saturated air parcel at the LCL was then lifted to its level of free convection (LFC) by the updrafts associated with thermals below the cumulus cloud base. The first echo of hydrometeors was detected by a Ku-band radar about 6 minutes after the air parcel reached its LFC, then the convective cell developed rapidly. When an SBF arriving from Sagami Bay passed under the cell, the updraft over the nose of the SBF triggered a new precipitation cell, but no intensification of the preexisting cell was observed.

 In Part Ι of this study, the development processes of Baiu frontal depressions (BFDs) have been examined through case-study numerical experiments. The numerical simulations revealed that latent heating is dominant for the development of BFDs in the western part of the Baiu frontal zone (W-BFDs), west of roughly 140°E, while both latent heating and baroclinicity are important for the development of BFDs in the eastern part of the zone (E-BFDs), east of roughly 140°E. In this study, idealized numerical simulations with zonally homogeneous basic fields are conducted to obtain a more generalized perspective of the development processes of BFDs.

 The basic fields for the idealized simulations are made from the composites of the environments under which 28 W-BFDs and 43 E-BFDs developed. The idealized simulations successfully reproduce a realistic W-BFD and E-BFD. The W-BFD has a slightly westward-tilted vertical structure, which is modulated by latent heating at low levels of the atmosphere. In contrast, the E-BFD has a westward-tilted structure through the troposphere, which is similar to the well-known baroclinic wave structure. Results of available potential energy diagnosis for the effects of latent heating and baroclinicity on the BFD development are consistent with those in Part Ι. The W-BFD has a mechanism mainly driven by latent heating yielding strong convection, while the E-BFD develops through baroclinic instability in moist atmosphere.