Abstract This article presents the first long‐term characteristics of vertical air motion over the gateway of Indian summer monsoon using the Cochin stratosphere–troposphere radar (10.04°N, 76.33°E). It is seen that the lower troposphere (<4 km) generally presents a downward mean motion throughout the year. While centred around June–July–August (JJA), downdrafts below 12–13 km and updrafts above that (both generally <5 cm·s −1 in magnitude) are noted. It is ensured that the dataset considered does not suffer from any significant bias due to several possible factors, including possible jet contamination, and the results are thus representative of real atmospheric features. The structure of mean vertical velocity as seen in JJA represents a ‘two‐step process’ of transport of air mass from the lower troposphere to the lower stratosphere and vice versa. Hourly mean vertical motion shows that the clear two‐step structure persists during JJA with clear reversal around 12–13 km during 1000–1900 Indian Standard Time (IST = UTC + 0530). The consistent downdrafts can be called representative of the region and attributed to the gentle subsidence associated with intermittent yet widespread convective activity away from the location of observation. In contrast, like during December–January–February (DJF), such a mechanism of transport is not discernible.
Abstract The Northeast China Cold Vortex (NCCV) is the primary synoptic system causing severe weather in North China; however, forecasting first convection initiation (first‐CI) at its rear side remains highly challenging. This study investigates the underlying CI triggering mechanisms and the causes of low forecast skill in this regime. Using 2016–2021 mosaic reflectivity from seven operational Doppler radars and a modified convective tracking algorithm, we map first‐CI distributions and evaluate Numerical Weather Prediction model (NWP) simulations to pinpoint low‐forecasting‐skill regions. A representative case (8 June 2019) is then diagnosed using a high‐resolution rapid‐update (3‐km, 12‐minute) analysis assimilating radar and dense‐surface‐network observations. Results from the representative case show that CI is driven by complex interactions among a cold, dry northerly flow, a warm, moist southerly flow, and a modified sea breeze. Near 850 hPa, the converging northerly and southerly flows generate localized weak convergence and high humidity. Simultaneously, the topographically deflected, inland‐propagating sea breeze forces intense near‐surface convergence with the prevailing southerly flow. This combined dynamic forcing establishes a deep updraft channel that successfully initiates convection. Diagnostics reveal that NWP forecast failures primarily stem from inaccuracies in simulating nocturnal boundary‐layer wind fields. Although the model similarly captures synoptic‐scale forcing and the moisture environment, it produces an anomalously strong low‐layer northerly flow. This overestimation blocks the inland penetration of the sea breeze, thereby eliminating the critical near‐surface convergence zone. Absent this essential dynamic trigger, accumulated convective instability cannot be released, resulting in missed CI forecasts.
Abstract Quasi‐stationary band‐shaped precipitation systems (QBPSs) are the major cause of localized heavy‐rainfall events (HREs) in Japan. Using hourly radar/raingauge‐analysed precipitation amounts from 1989 to 2022, we statistically examined the occurrence characteristics of QBPSs and their associated environmental conditions. A total of 849 QBPSs were extracted during the 34‐year period, with the highest frequencies in western and southern Japan. Approximately 45% of them occurred in association with stationary fronts, and approximately 85% of them were oriented southwest–northeast or west–east. The annual occurrence frequency showed a significant increasing trend, with a regression indicating approximately a 3.4‐fold increase over the analysis period. QBPSs occurred most frequently between June and September, and their hourly frequency exhibited frequent occurrence between midnight and early morning. To elucidate the environmental characteristics favourable for QBPS formation, we analysed 225 events that occurred over Kyushu Island, western Japan, during June and July. Composite analyses using ERA5 revealed robust and coherent features: high low‐level equivalent potential temperature associated with warm and moist inflow from the East China Sea towards the Baiu frontal zone, large vertical wind shear with warm advection, synoptic‐scale ascent, low ‘level of free convection’, a deep moist layer extending into the midtroposphere, and a midlevel jet. These features were markedly pronounced relative to climatology and are consistent with previous studies on typical HREs along the Baiu frontal zone. Our results demonstrate that a combined evaluation of these environmental characteristics serves as an effective diagnostic indicator for QBPS occurrences.
Abstract This study investigates the joint effects of spatial thinning and observation error standard deviation inflation in satellite radiance assimilation, focusing on the common operational setting in which true observation errors are spatially correlated but are assumed to be uncorrelated. Idealized diagnostics are combined with near‐operational experiments from a global deterministic prediction system to reconcile theory and operational practice, providing a unified, physically based interpretation of thinning–inflation interactions and highlighting the underlying mechanisms governing their relationship. An idealized data assimilation framework, scaled to operationally relevant spatial characteristics, is used to diagnose how optimal inflation depends on thinning distance and on background and observation error correlation length scales. Results reveal a robust asymptotic behavior: when both background and observation error correlation length scales exceed the thinning distance, the optimal inflation approaches the square root of the ratio of the thinning distance. Spectral diagnostics explains this behavior and clarifies why the inflation saturates, providing a physical basis for commonly observed empirical tuning values. Conversely, when the observation error correlation length scale is short relative to the thinning distance, the optimal inflation remains close to unity. These theoretical results are evaluated in a near‐operational context using 15 experimental configurations over a 2.5‐month period, with thinning distances of 150 and 100 km and a range of inflation factors. Reducing thinning without inflation improves the analysis fit to radiances while degrading the fits to several other assimilated observation types and forecast performance, suggesting an excessive influence of the radiances. Increasing the inflation by factors of about 1.4–1.5 mitigates this behavior. Forecast scores demonstrate statistically significant improvements when thinning reduction is paired with appropriate inflation, particularly in the tropics and Southern Hemisphere. Overall, near‐operational results are broadly consistent with the mechanisms identified in the idealized framework, suggesting that these remain relevant in more realistic assimilation systems despite their greater complexity.
Abstract The afternoon–evening transition (AET) of canopy turbulence marks a fundamental shift from the daytime convective regime to nocturnal stable conditions. Despite its importance for canopy–atmosphere exchanges of momentum, heat, and scalars, this transition remains insufficiently understood, partly because resolving the coupled canopy–atmospheric boundary‐layer dynamics across evolving unstable‐to‐stable stratification remains challenging for large‐eddy simulations (LES). Here, LES are used to investigate the evolution of turbulence during the AET over an idealized forest of 32‐m height () subjected to a weak geostrophic forcing. Simulations revealed a progressive shift from daytime turbulence‐dominated to evening oscillatory flow dynamics, as surface cooling increases stability and near‐canopy winds weaken. The oscillatory motions, known as canopy waves, remained confined to the vicinity of the canopy top and exhibited wavelengths of approximately four to six canopy heights and periods consistent with the local buoyancy time‐scale (1–2 min). They propagated at a speed comparable with the mean wind velocity around , similar to canopy‐top coherent turbulent motions in near‐neutral conditions. Their contribution to vertical momentum and scalar transport is weak because vertical velocity and temperature fluctuations become increasingly out of phase. The transition toward oscillatory dynamics occurred progressively rather than abruptly, beginning approximately when the gradient Richardson number at the canopy top became positive and lower than . Although obtained under idealized conditions, these simulated wave characteristics were qualitatively consistent with observations from a field experiment conducted over a comparable forest canopy. These results provide new insight into the evolution of canopy‐scale motions during the AET and highlight the increasing role of wave dynamics as turbulence collapses under stable conditions.
