Stratospheric profiles of the mean age of air estimated from cryogenic air samples acquired during a field campaign over Indonesia, the Coordinated Upper-Troposphere-to-Stratosphere Balloon Experiment in Biak, were investigated using the boundary impulse evolving response (BIER) method and Lagrangian backward trajectories, with the aid of an atmospheric general circulation model-based chemistry transport model (ACTM). The ACTM provides realistic meteorological fields at 1-hour intervals by nudging toward the European Centre for Medium- Range Weather Forecasts Reanalysis-Interim. Since the BIER method is capable of taking unresolved diffusive processes into account, while the Lagrangian method can distinguish the pathways the air parcels took before reaching the sample site, the application of the two methods to the common transport field simulated by the ACTM is useful in assessing the CO2- and SF6-derived mean ages. The reliability of the simulated transport field has been verified by the reproducibility of the observed CO2, SF6, and water vapor profiles using the Lagrangian method. The profile of CO2 age is reproduced reasonably well by the Lagrangian method with a small young bias being consistent with the termination of trajectories in finite length of time, whereas the BIER method overestimates the CO2 age above the altitude of 25 km, possibly due to high diffusivity in the transport model. In contrast, the SF6 age is only reproducible in the lower stratosphere, and far exceeds the estimates from the Lagrangian method above the altitude of 25 km. As air parcels of mesospheric origin are excluded in the Lagrangian age estimation, this discrepancy, together with the fact that the observed SF6 mole fractions are much lower than the trajectory-derived values in this height region, supports the idea that the stratospheric air samples are mixed with SF6-depleted mesospheric air, leading to overestimation of the mean age.
This study proposes an algorithm detecting atmospheric blocking by extracting topological features of geopotential height data at 500 hPa. The algorithm uses topological flow data analysis (TFDA) providing a unique symbolic representation, named the partially cyclically ordered rooted tree (COT) representation, and a discrete graph structure, called a Reeb graph, to each structurally stable Hamiltonian vector field based on the mathematical theory of topological classifications for streamline patterns. It recognizes blocking events more simply and effectively using fewer meteorological parameters than conventional algorithms. Furthermore, the algorithm can determine morphological types of blocking events, an Omega shape or a dipole pattern, whereas no effective algorithm has been available so far. The identified blocking events and their morphological types are consistent with synopticians' subjective judgments.
An unprecedented cold wave swept through most parts of East Asia in January 2016, leading to record-breaking low temperatures and widespread snowstorms in several regions. Our analysis indicated that this East Asian cold wave was triggered by a series of consecutive extreme events in the Northern Hemisphere from late 2015 to early 2016. (1) On 28 December 2015, a severe cyclonic storm emerged in the North Atlantic, and a downstream blocking high formed in Europe through the downstream development process. The strong southerly jet stream between the storm and its downstream blocking high steered the storm into the Arctic Circle, transported enormous warm and moist air masses, establishing warm conveyor belts, which led to an extraordinary Arctic warming event in late 2015. (2) This Arctic warming event in late 2015 resulted in a distinct Arctic dipole pattern resembling the negative phase of the Arctic Oscillation in early–mid-January 2016. (3) The dipole pattern induced eastward propagation of the Rossby wave and led to the occupation of two downstream blocking highs in the Urals and western North America. These two blocking highs, together with the low pressure between them, resulted in an inverted omega-shaped circulation pattern (IOCP) over the Siberia–North Pacific region. Additionally, the IOCP distinctly modulated the meridional circulation cell along the East Asia–Siberia regions, which generated negative vorticity and anticyclonic advection to the Siberian region, ultimately intensifying the Siberian high. The IOCP and the associated enhanced Siberian high eventually induced the outbreak of a mega-cold wave in East Asia on 21–25 January 2016.
A non-Gaussian probability distribution function (PDF) and a new displacement correction method using PDF pseudo-regimes for precipitation were introduced to the dual-scale neighboring ensemble-based variational assimilation scheme (EnVar) to achieve improved assimilation of all-sky microwave imager (MWI) brightness temperatures (TBs) into a cloud-resolving model (CRM).
We evaluated the fits of the precipitation forecast perturbations of various disturbances with the existing non-Gaussian PDF models and selected a mixed lognormal distribution for the precipitation PDF model. Then, we introduced rain-free and rainy PDF regimes to EnVar. We developed a new method for correcting precipitation displacement that introduces pseudo rain-free, rainy, and heavy-rain regimes and approximated their PDFs as regional averages of the PDFs around the target point. We estimated the horizontal scales of averaging based on the similarity of precipitation forecast perturbations. These methods improved the bias and normality of TB differences between observation and the first guess.
We conducted assimilation experiments using all-sky MWI TB observations for Typhoon Etau (T1518). Results show that the precipitation analysis using the EnVar employed in this study was more similar to the global satellite map for precipitation (GSMaP) retrievals than those using a conventional EnVar. The introduction of a mixed lognormal PDF strengthened the precipitation analysis of heavy-rain areas around the typhoon and near a front. Using PDF pseudo-regimes considerably reduced the precipitation displacement error of the analysis. The EnVar employed in this study improved the CRM forecasts for precipitation distribution up to 12 h and the typhoon position and central surface pressure for > 24 h. The forecast analysis cycle of EnVar improved the CRM forecasts for heavy rain around the typhoon center up to 6 h and a heavy-rain band associated with the typhoon for > 24 h when compared with the EnVar using a single-time TB observation.
Precipitation statistics from Global Precipitation Measurement Core Observatory Dual-frequency Precipitation Radar (GPM DPR) are underestimated due to systematic bias depending on the scanning angle. Over five years of GPM DPR KuPR Version 06A data, the precipitation anomaly is −7 % and −2 % over land and ocean, respectively. This study improves the estimation of low-level precipitation-rate profiles and the detection of shallow storms (with top heights of ≤ 2.5 km), using reference datasets of near-nadir measurements.
First, the low-level precipitation profile (LPP) is updated using an a priori near-nadir database generated from structural-characteristics related variables of the precipitation and environmental parameters. The LPP correction increases precipitation over areas where downward-increasing precipitation profiles are dominant below 2 km, such as at high elevations and at middle and high latitudes. Globally, the LPP correction increases precipitation by 5 %. Second, the effect on precipitation data of missing shallow storms is estimated using the angle-bin difference in the detectability of storms with a top height of ≤ 2.5 km. The effect of the shallow-precipitation deficiency (SPD) is comparable in magnitude to that of the LPP correction. A priori lookup tables for the SPD correction, constrained by the clutter-free bottom level and spatially averaged shallow-precipitation fractions, are constructed so that the correction applies to gridded statistics at 0.1° and three-month scales. The SPD correction enhances precipitation by 50 % over specific low-rainfall oceans in the sub-tropics and at high latitudes, where shallow precipitation dominates. Based on these two corrections, precipitation increases by 8 % and 11 % over land and ocean, respectively. At latitudes between 60°N and 60°S, the difference in KuPR compared with satellite-gauge blended products is reduced from −17 % to −9 %, whereas with gauge-based products is reduced from −19 % to −15 % over land.
The Global Precipitation Measurement (GPM) Dual-frequency Precipitation Radar (DPR) has operated in full scan (FS) mode in both the Ku-band and Ka-band since May 2018. This FS mode can enable about 245 km full swath dual-frequency processing for the first time, whereas previous algorithms enabled dual-frequency processing in the narrower inner swath, having a width of approximately 125 km. This paper describes the classification (CSF) module in newly developed DPR level 2 V06X experimental algorithms corresponding to the FS mode. The CSF module classifies precipitation as three major types, stratiform, convective, and other, and provides bright-band (BB) information.
One-month statistics show that a significant jump occurs in the Ka-band precipitation type counts at the edges of the inner swath due to the different sensitivity of Ka-band radar in Ka-band Matched Scan (Ka-MS) mode compared with that in Ka-band High Sensitivity (Ka-HS) mode. However, precipitation type counts indicate that dual-frequency processing works well not only in the inner swath but also in the outer swath. Statistics on BB counts show a significant improvement in BB detection, especially in the outer swath, when dual-frequency processing is performed.
In addition, the V06X Ku-band algorithm solves two problems related to the CSF module: (a) appearance of a very large near-surface precipitation rate of stratiform precipitation reclassified by the slope method and (b) a rare case of misjudging BB peak as an upper part of surface echo.
The data structures of GPM DPR algorithms were drastically changed in V06X. The new data structures introduced in V06X will be used in V07A and later. In this regard, the V06X CSF module outlined in this paper is a prototype of future versions of each CSF algorithm.
We analyze a multi-model ensemble at a convection-resolving resolution based on the DYAMOND models and a resolution ensemble based on the limited-area model COSMO over 40 days to study how tropical and subtropical marine low clouds are represented at a kilometer-scale resolution.
The analyzed simulations produce low cloud fields that look in general realistic in comparison with satellite images. The evaluation of the radiative balance, however, reveals substantial inter-model differences and an underestimated low cloud cover in most models. Models that simulate increased low cloud cover are found to have a deeper marine boundary layer (MBL), stronger entrainment, and an enhanced latent heat flux. These findings demonstrate that some of the fundamental relations of the MBL are systematically represented by the model ensemble, which implies that the relevant dynamical processes start to become resolved on the model grid at a kilometer-scale resolution. A sensitivity experiment with the COSMO model suggests that differences in the strength of turbulent vertical mixing may contribute to the inter-model spread in cloud cover.
Upper-tropospheric aircraft reconnaissance was carried out for Typhoon Lan (2017) using a civil jet with a newly developed dropsonde system. This was the first time a Japanese research group observed the inner core of an intense typhoon using dropsondes. This paper describes the warm-core structure in the eye and the associated thermodynamic and kinematic features of the eyewall. During two days of reconnaissance, the typhoon preserved its peak intensity in an environment with strengthening vertical shear. The dropsondes captured a double warm-core structure with a higher perturbation temperature in the middle and upper troposphere, which persisted between the two missions. The two warm cores showed a difference in equivalent potential temperature (θe) of more than 10 K, suggesting different air origins. Saturation-point analysis suggests that the air observed in the upper warm core was entrained from the eyewall. The eyewall updraft in the left-of-shear semicircle had a two-layer structure with a higher θe and lower absolute angular momentum on the inner side of the updraft core. Analyses of the saturation point and parcel method suggest that the warmer air with a θe exceeding 370 K on the inner side of the updrafts originated from the eye boundary layer and was eventually transported into the upper warm core. These results led us to hypothesize that the vertical transport of high-θe air from the eye boundary layer contributed to the continuous eye warming in the upper troposphere against the negative effect of strengthening environmental wind shear on storm intensity. This study demonstrates the significance of eyewallpenetrating upper-tropospheric reconnaissance for monitoring the warm-core structure in the present situation, where accurate measurements of both humidity and temperature for calculating θe can only be made with dropsonde-type expendables.
The central pressure fall in a typhoon is associated with the development of the warm core and mass divergence in the eye. Trajectory analyses were utilized to investigate the origins of air moving into the warm core and the paths of air parcels leaving the eye. First, developing Typhoon Wipha (2007) was simulated using a high-resolution (2 km) cloud-resolving model to represent the central pressure fall and axisymmetric structures such as the warm core in the upper levels of the eye, the eyewall, and the secondary circulation. Then, using the model output data, backward trajectories were calculated from the eye; the results show that the air parcels comprising the warm core originated from the lower troposphere and the lower stratosphere. Those originating from the lower troposphere, whose equivalent potential temperature (θe) is increased by the latent heat flux from the sea, ascend through the eyewall and move inward in the upper troposphere. Those originating in the lower stratosphere, which have high potential temperature (θ), descend from the lower stratosphere to the upper troposphere. Hence, the warm core comprises high-θe air from the lower troposphere and high-θ air from the lower stratosphere. Next, forward trajectories were calculated to examine the paths of air parcels leaving the eye; the results show that air parcels leave the eye through the eyewall throughout the troposphere, particularly at heights below 2 km and between 9 km and 12 km, which ultimately results in a central pressure fall.
This study presents the impacts of evaporative cooling from raindrops on precipitation over western Japan associated with the Baiu front during a heavy rainfall event from 5 to 8 July 2018. We conducted analyses of the dynamic and thermodynamic features of the stationary Baiu front using the Japanese 55-year reanalysis. During this period, large amounts of water vapor were continuously transported to the stationary Baiu front, supporting the record-breaking rainfall. The 299K isentropic surface was identified as the frontal surface. Along the isentropic surface, warm moist air adiabatically ascended, became saturated at an altitude of approximately 500 m, and initiated active precipitation systems. We found that the diabatic cooling near the tip of the frontal surface played an important role in retaining the position of the frontal surface and stalling its northward retreat. Additionally, numerical sensitivity experiments were conducted to examine the impacts of evaporative cooling and topography on heavy rainfall formation using a cloud-resolving non-hydrostatic numerical model (the Japan Meteorological Agency Non-Hydrostatic Model: JMA-NHM) with a horizontal resolution of 3 km. A heavy precipitation area extending from the Chugoku region to central Kinki was simulated regardless of whether the terrain was flattened or not. Precipitation formed primarily as a result of updrafts above a frontal surface at a potential temperature of 300 K. This precipitation area shifted northward by more than 100 km when raindrop evaporation was excluded from the experiment. Raindrop evaporation suppressed the northward retreat of the frontal surface by maintaining cold airmass amounts below the frontal surface.
The dual peak semidiurnal variation in surface rainfall rate over the tropics, simulated using a 3.5-km-mesh Nonhydrostatic Icosahedral Atmospheric Model (NICAM) for 26–31 December 2006, is analyzed and compared with data from the 17 year winter precipitation climatology of Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), Precipitation Radar (PR), and the same 6 day data of Global Satellite Mapping of Precipitation, as well as infrared data from geostationary satellites.
We focus on land areas including southern Africa and the Amazon. Over these land areas, the NICAM simulation captures the primary peak in the afternoon and the secondary peak in the early morning, at similar times to those captured using TRMM data. In the PR observation, the primary peak of rainfall is mainly due to convective rain, whereas the secondary peak is due to stratiform rain. In the NICAM simulation, if a simple method is used for the classification of convective/stratiform rain, convective rain is dominant all day long, and the rainfall rate is generally higher than in the PR observation. Nevertheless, an analysis of deep convection (DC) areas indicates consistency between the observation and NICAM; the primary peak of rainfall rate occurs at the mature stage of the number of DC areas, whereas the secondary peak occurs when the mean size of DC areas is almost at its highest point. However, in the NICAM simulation, the relative magnitudes of the two peaks are not represented well, and the contribution of the stratiform rain is underestimated.
The present study indicates that a high-resolution global nonhydrostatic model like NICAM has the potential to overcome the limitations of coarse-resolution general circulation models by reproducing the semidiurnal variation of DC, although there is room for improvement.