Sensitivity of Coastal Rainfall Simulation to the Assimilation of Wind Profiles Near the Marine Boundary Layer Jet: An Observing System Simulation Experiment

Heavy rainfall that occurred at the south coast of China on 10–11 May 2014 was associated with a synoptic-system-related low-level jet (SLLJ) and a boundary layer jet (BLJ). To clarify the role of the double low-level jets in convection initiation (CI), we perform convective-permitting simulations using a nonhydrostatic mesoscale model. The simulations reproduce the occurrence location and mesoscale evolution of new convective cells as well as their small-scale wavelike structures at the elevated layers, which are generally consistent with radar observations despite some differences in their orientation. The nighttime BLJ over the northern South China Sea strengthens the convergence at ~950 hPa near the coast where the BLJ’s northern terminus reaches the coastal terrain. Meanwhile, the SLLJ to the south of the inland cold front provides divergence at ~700 hPa near the SLLJ’s entrance region. Such low-level convergence and midlevel divergence collectively produce strong mesoscale lifting for CI at the coast. In addition to the enhanced mesoscale lifting, the double low-level jets also provide favorable conditions for the superimposed small-scale disturbances that can serve as effective moistening mechanisms of the lower troposphere during CI. In a sensitivity experiment with coastal terrain removed, CI still occurs near the coast but is delayed and weaker compared to the control run. This latter experiment suggests that double low-level jets and their coupling indeed exert key effects on CI, while the BLJ colliding with terrain may enhance coastal convergence for amplifying CI. These findings provide new insights into the occurrence of coastal heavy rainfall in the warm sector far ahead of the fronts.

The sensitivity of planetary boundary layer (PBL) parameterization schemes in a marine boundary layer jet and associated precipitation is investigated in this study. Six PBL parameterization schemes in the Weather Research and Forecasting Model, including YSU, MYJ, MYNN, ACM2, BouLac, and UW schemes, are examined in simulating a marine boundary layer jet (BLJ) over South China Sea and associated coastal precipitation during a warm-sector heavy rainfall event (19–20 May 2015) near the coast of South China. The results show that YSU, MYJ, MYNN, and BouLac schemes can generally reproduce the coastal warm-sector heavy rainfall with 6-h accumulated precipitation exceeding 50 mm, but not for the ACMs and UW schemes. No convection initiation occurs in the ACM2 run, while rainfall is located to further north with weaker intensity in the UW run. Meanwhile, weakest and strongest BLJs are simulated in the ACM2 and UW runs, respectively. In the ACM2 run, the weaker BLJ with the maximum wind speed less than 17 m s−1 induces weaker convergence and lifting in the upwind side of the coastal terrain as well as less water vapor transport to the coastal area, which thus inhibit convection initiation. On the contrary, the too strong BLJ in the UW run with large area of wind speed greater than 18 m s−1 causes the northward movement of convection along with cold pools, and rainfall moves further north accordingly. The differences in BLJs’ strength among PBL schemes are attributed to varying simulated low-level vortex on the northern side of the BLJ through veering ageostrophic winds. The intensity of the simulated low-level vortex is affected by variations in boundary layer mixing over land and associated vertical temperature stratification under different PBL schemes.

Convection over the South China Sea (SCS) exhibits distinct quasi‐biweekly oscillations (QBWO). The QBWO_SCS modulates marine boundary layer jets (MBLJs) embedded in the southwesterly monsoon flow during June, the late Meiyu season. During the first half cycle from Phase 1 (P1) to Phase 2 (P2), convection is most suppressed over the SCS in P2, while enhanced convective activity is observed within the Meiyu trough over southeastern China. An anomalous low‐level anticyclonic circulation over the SCS leads to increased (decreased) MBLJ frequency over the northern (southern) SCS, and these anomalies reverse in Phase 4 (P4). In P2, QBWO_SCS‐intensified MBLJs near the southeastern coast of China create favorable conditions for heavy inland rainfall. First, strengthened southwesterly MBLJs enhance moisture transport from the SCS to inland southeast China and increase moisture convergence within the Meiyu trough. Second, warm, moist air advected inland meets continental cold air, enhancing baroclinicity and frontogenesis, which strongly correlates with inland rainfall maxima. Third, the topography of southeast China amplifies rainfall through orographic lifting as moisture‐laden MBLJs impinge on elevated terrain. In contrast, during P4, weakened MBLJs reduce moisture transport, convergence, and frontogenesis, leading to diminished inland rainfall. Meanwhile, QBWO_SCS‐induced convection‐coupled circulation in P4 supports monsoon trough development and enhances heavy precipitation over coastal south China, the northern SCS, and southern Taiwan. About 25% of QBWOs over SCS are positively correlated with low‐level cyclonic flow originating from QBWOs over the tropical western Pacific, 10% are weakly modulated by the boreal summer intraseasonal oscillation, and 65% are caused by other processes.

Two types of heavy rainfall, namely warm-sector and frontal heavy rainfall, coexist in South China during the presummer rainy season and manifest as varying mechanisms and features. They both exhibit close relationships with two types of low-level jets (LLJs): the boundary layer jet (BLJ) and synoptic-system-related low-level jet (SLLJ), but in different ways. The motivation of the present study is to elucidate the statistical relations between the two types of heavy rainfall and LLJs over South China using TRMM rainfall data and ERA5 reanalysis. Generally, warm-sector heavy rainfall mainly occurs over coastal areas and during the early morning, which is primarily caused by the interaction between the nocturnal BLJ and land breeze. In contrast, frontal heavy rainfall is mostly concentrated in inland regions and modulated by distinct diurnal forcings at different locations. Statistical analysis indicates that 76% (62%) of the warm-sector (frontal) heavy rainfall events are associated with LLJs. In the presence of heavy rainfall, low-level winds are often strengthened over Beibu Gulf, northern South China Sea, and the south side of fronts, corresponding to two branches of southerly BLJs at ~950 hPa over the ocean and the southwesterly SLLJs at ~850–700 hPa on land, respectively. Furthermore, BLJs are shown to be linked to both types of heavy rainfall and with the most frequent occurrences of rainfall in their exit region, whereas SLLJs are more closely associated with frontal heavy rainfall. The left side (entrance) of the SLLJ axis is favorable for frontal (warm-sector) heavy rainfall production. The regional rainfall distributions are affected by the structures and locations of LLJs.

This paper conducts an observing system simulation experiment (OSSE) to assess the impact of assimilating traditional sounding and surface data, along with dropsonde observations over the northern South China Sea (SCS) on heavy rain forecasts in Taiwan. Utilizing the hybrid ensemble transform Kalman filter (ETKF) and the three-dimensional variational (3DVAR) data assimilation (DA) system, this study focuses on an extreme precipitation event near Taiwan on 22 May 2020. The event was mainly influenced by strong southwesterly flow associated with an eastward-moving southwest vortex (SWV) from South China to the north of Taiwan. A nature run (NR) serves as the basis, generating virtual observations for radiosonde, surface, and dropsonde data. Three experiments—NODA (no DA), CTL (traditional observation DA), and T5D24 (additional dropsonde DA)—are configured for comparative analyses. The NODA experiment shows premature and weaker precipitation events across all regions compared with NR. The CTL experiment improved upon NODA’s forecasting capabilities, albeit with delayed onset but prolonged precipitation duration, particularly noticeable in southern Taiwan. The inclusion of dropsonde DA in the T5D24 experiment further enhanced precipitation forecasting, aligning more closely with NR, particularly in southern Taiwan. Investigations of DA impact reveal that assimilating traditional observations significantly enhances the SWV structure and wind fields, as well as the location of frontal systems, with improvements persisting for 40 to 65 h. However, low-level moisture field enhancements are moderate, leading to insufficient precipitation forecasts in southern Taiwan. Additional dropsonde DA over the northern SCS further refines low-level moisture and wind fields over the northern SCS, as well as the occurrence of frontal systems, extending positive impacts beyond 35 h and thus improving the rain forecast.

Low-level jets (LLJs) play a key role in driving early summer rainfall in South and East China (SEC) by supplying essential moisture and energy for convective processes. LLJs in South China (SC) are classified into two distinct types: synoptic-system-related LLJ (SLLJ) and boundary layer jet (BLJ), each exerting distinct influences on rainfall mechanisms. This study investigates the individual influence of SLLJ and BLJ, as well as their interplay, on early summer rainfall over SEC, with a focus on moisture flux dynamics. The findings reveal the independent yet equally significant contributions of SLLJs and BLJs in shaping the pattern of early summer rainfall in SEC. Rainfall intensity over SEC correlates with the intensity of BLJs. Strong BLJs result in pronounced boundary layer convergence and upward moisture transport, and thus favorable for enhanced rainfall over SEC. The location of rainfall is closely associated with the pattern of SLLJs. Southwesterly SLLJs along the coast of SC enhances local low-level convergence and upward motions, leading to rainfall mainly confined to SC. By contrast, a quasi-meridional-oriented SLLJ promotes the low-level northward advection of moisture, which is elevated from the boundary layer by topography in SC. This part of moisture, along with the northward-transported moisture within the boundary layer, is elevated to upper layers at the frontal zone in East China (EC), jointly contributing to enhanced rainfall in EC. Overall, the moisture for rainfall in SC is mainly contributed by the transport of BLJs, whereas the moisture for rainfall in EC is jointly attributed to the northward moisture advection by both BLJs and SLLJs. Significance Statement Low-level jets (LLJs) are frequently observed in South China (SC) during warm seasons and play a key role in early summer rainfall over the region by providing the moisture and energy necessary for convective processes. LLJs in the coastal south and continental China are classified into two types: synoptic-system-related LLJ (SLLJ) and boundary layer jet (BLJ), each exerting distinct influences on rainfall mechanisms. Despite their distinct characteristics and impacts on rainfall, the influences of SLLJs and BLJs on rainfall pattern are interconnected with each other through their interactions. In this work, an in-depth study considering the boundary layer and low level as a whole is conducted to fully understand not only the individual role of SLLJs and BLJs but also their interaction in shaping the spatial pattern of early summer rainfall in South and East China (SEC). The results reveal the distinct yet equally crucial roles of SLLJs and BLJs in determining the location and intensity of early summer rainfall over SEC in a climatological manner and underscore the need for further investigation to quantitatively understand the individual and interactive influences of SLLJs and BLJs on early summer rainfall across various SEC regions.

Convection initiation (CI) and the subsequent upscale convective growth (UCG) at the coast of South China in a warm-sector heavy rainfall event are shown to be closely linked to a varying marine boundary layer jet (MBLJ) over the northern South China Sea (NSCS). To elucidate the dynamic and thermodynamic roles of the MBLJ in CI and UCG, we conducted and analyzed convection-permitting numerical simulations and observations. Compared to radar observations, the simulations captured CI locations and the following southwest–northeast-oriented convection development. The nocturnal MBLJ peaks at 950 hPa and significantly intensifies with turning from southwesterly to nearly southerly by inertial oscillation. The strengthened MBLJ promotes mesoscale ascent on its northwestern edge and terminus where enhanced convergence zones occur. Located directly downstream of the MBLJ, the coastal CI and UCG are dynamically supported by mesoscale ascent. From a thermodynamic perspective, a warm moist tongue over the NSCS is strengthened by the MBLJ-driven mesoscale ascent as well as by a high sea surface temperature. The warm moist tongue farther extends northeastward by horizontal transport and arrives at the coast where CI and UCG occur. Near the CI location, rapid development of a low-level saturated layer is mainly attributed to the mesoscale ascent and low-level moistening associated with the MBLJ. In addition, subsequent CI happens on either side of the original CI along the coast due to the delay of low-level moistening, which partly contributes to linear convective growth. Furthermore, ensemble simulations confirmed that a stronger MBLJ is more favorable to CI and UCG near the coast.

Boundary-layer jets (BLJs) in the South China Sea play an important role in heavy rainfall in South China, yet observations in maritime locations are still limited. This study examines the vertical structures and temporal evolutions of BLJs in the northern South China Sea using intensive radiosonde observations from a research vessel from 15 to 18 June 2022 and evaluates the performance of various reanalysis datasets in capturing these features. Observations identified BLJs with jet cores at altitudes of approximately 500–700 m. Wind speeds slightly decreased from 15 to 16 June and then significantly increased after 17 June, showing double peaks on 17 June below 1 km at altitudes of 250 and 700 m. Among the reanalysis datasets, ERA5 exhibited more accurate results on average, followed by MERRA2, both of which outperformed JRA55 and FNL. ERA5 and MERRA2 had mixed performances in depicting BLJ characteristics. ERA5 accurately captured the initial decrease in wind speeds and their subsequent enhancement, while MERRA2 initially faltered but improved later. On the diurnal scale, neither MERRA2 nor ERA5 accurately represented the wind speed peaks observed at 2300 and 1100 LST, whereas ERA5 roughly reflected the nocturnal acceleration of the BLJs. During the observation period, the intensification of BLJs in the northern SCS, influenced by an eastward-moving high-pressure system and a southward-moving low-pressure vortex, led to enhanced precipitation in South China that gradually moved northward from the coastline to inland regions. This study provides new insights into the detailed characteristics of marine BLJs based on direct observations.

Low-level jets (LLJs) play a critical role as carriers of moisture, significantly influencing heavy rainfall. In South China, three distinct LLJ branches are identified: the boundary layer jets in the Beibu Gulf (BLJ-BG) and in the northern South China Sea (BLJ-SCS), as well as the synoptic-system-related LLJs in South China (SLLJ-SC). This study investigates their varying moisture transport, sources, and sinks utilizing the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), employing both backward and forward trace simulations. As the presummer rainy season progresses, the primary moisture sources of LLJs gradually shift southwestward from the South China Sea to the Indian Ocean. BLJ-BG predominantly directs moisture trajectories to cross the Indochina Peninsula, facilitating more moisture transport from the Indian Ocean (∼42.7% after the onset of the South China Sea summer monsoon) and strengthening monsoonal moisture transport compared to non-BLJ-BG events. Conversely, BLJ-SCS consistently draws moisture from the South China Sea (∼44.5%), with trajectories circumventing the Indochina Peninsula. Notably, BLJ-SCS transports even less moisture from the Indian Ocean than nonevent periods. After the monsoon onset, SLLJ-SC shares a common southwestern moisture channel with BLJ-BG and exhibits a pronounced tendency to transport moisture from the northern side (∼10.3% moisture trajectories) compared to other LLJs, though still less than the non-SLLJ-SC events. Moisture transported by BLJ-BG (BLJ-SCS) is released in northeastern Guangxi (central Guangdong), coinciding with intensified inland maximum rainfall centers. SLLJ-SC transports more moisture further north into northern Jiangxi, aligning with the formation of rainbands further north. Significance Statement Different types of low-level jets (LLJs), including boundary layer jets and synoptic-weather-related LLJs occur frequently in South China during the presummer rainy season. This study distinguishes their respective roles in moisture transport from the perspectives of moisture paths, sources, and sinks. These LLJs do not uniformly enhance the southwesterly monsoonal moisture transport from distant oceanic regions. In particular, the boundary layer jets in the northern South China Sea predominantly draw moisture nearby. Moisture sinks associated with these LLJs align with areas where precipitation is intensified. This study provides valuable insights for forecasting extreme precipitation events in the region and establishes a connection between the origins and culmination of moisture in rainfall through the perspectives of LLJs as a crucial moisture carrier.

High spectral resolution (or hyperspectral) infrared (IR) sounders onboard low earth orbiting satellites provide high vertical resolution atmospheric information for numerical weather prediction (NWP) models. In contrast, imagers on geostationary (GEO) satellites provide high temporal and spatial resolution which are important for monitoring the moisture associated with severe weather systems, such as rapidly developing local severe storms (LSS). A hyperspectral IR sounder onboard a geostationary satellite would provide four-dimensional atmospheric temperature, moisture, and wind profiles that have both high vertical resolution and high temporal/spatial resolutions. In this work, the added-value from a GEO-hyperspectral IR sounder is studied and discussed using a hybrid Observing System Simulation Experiment (OSSE) method. A hybrid OSSE is distinctively different from the traditional OSSE in that, (a) only future sensors are simulated from the nature run and (b) the forecasts can be evaluated using real observations. This avoids simulating the complicated observation characteristics of the current systems (but not the new proposed system) and allows the impact to be assessed against real observations. The Cross-track Infrared Sounder (CrIS) full spectral resolution (FSR) is assumed to be onboard a GEO for the impact studies, and the GEO CrIS radiances are simulated from the ECMWF Reanalysis v5 (ERA5) with the hyperspectral IR all-sky radiative transfer model (HIRTM). The simulated GEO CrIS radiances are validated and the hybrid OSSE system is verified before the impact assessment. Two LSS cases from 2018 and 2019 are selected to evaluate the value-added impacts from the GEO CrIS-FSR data. The impact studies show improved atmospheric temperature, moisture, and precipitation forecasts, along with some improvements in the wind forecasts. An added-value, consisting of an overall 5% Root Mean Square Error (RMSE) reduction, was found when a GEO CrIS-FSR is used in replacement of LEO ones indicating the potential for applications of data from a GEO hyperspectral IR sounder to improve local severe storm forecasts.

An observing System Simulation Experiment (OSSE) is a method to evaluate impacts of hypothetical observing systems on analysis and forecast accuracy in numerical weather prediction (NWP) systems. Since OSSE requires simulations of hypothetical observations, uncertainty of OSSE results is generally larger than that of observing system experiments (OSEs). To reduce such uncertainty, OSSEs for existing observing systems are often carried out as calibration of the OSSE system. The purpose of this study is to achieve reliable OSSE results based on results of OSSEs with multiple methods. There are three types of OSSE methods. The first one is the sensitivity observing system experiment (SOSE) based OSSE (SOSEOSSE). The second one is the ensemble of data assimilation cycles (ENDA) based OSSE (ENDA-OSSE). The third one is the nature-run (NR) based OSSE (NR-OSSE). These three OSSE methods have very different properties. The NROSSE evaluates hypothetical observations in a virtual (hypothetical) world, NR. The ENDA-OSSE is very simple method but has a sampling error problem due to a small size ensemble. The SOSE-OSSE requires a very highly accurate analysis field as a pseudo truth of the real atmosphere. We construct these three types of OSSE methods in the Japan meteorological Agency (JMA) global 4D-Var experimental system. In the conference, we will present initial results of these OSSE systems and their comparisons.

The interaction between cold pools and low‐level vertical wind shear plays an important role in the initiation and development of convection. However, the impact of the boundary layer jets (BLJs), with their unique low‐level vertical wind shear structures and moisture transport properties, on cold pools remains incompletely understood. This study utilizes observations from the 356‐m high Shenzhen Meteorological Tower to compare cold pool characteristics under BLJ and non‐BLJ conditions during April to June from 2018 to 2020. A total of 54 cold pools are identified, with 26 influenced by BLJs. BLJ‐related cold pools typically exhibit weaker average temperature drops, more pronounced decreases in specific humidity, and higher turbulence transport compared to those unrelated to BLJs. The enhanced moisture transport by BLJs constrains rainfall evaporation, resulting in weaker cold pools. In addition, the strong wind shear associated with BLJs promotes upward energy transport and turbulence, further weakening the intensity of cold pool. These distinctions are particularly pronounced in cold pools associated with mesoscale convective systems rather than individual convective cells. These findings provide valuable insights into the complex interactions among cold pools, BLJs, and convection in South China.

Observing System Simulation Experiments (OSSEs) are a powerful tool used to assess the potential impact on numerical weather prediction skill from planned or hypothetical future observing systems. Over the last several years an international Joint OSSE collaboration has emerged centered on the use of NASA's and NOAA's data assimilation systems. A Nature Run provided by the European Centre for Medium Range Weather Forecasts (ECMWF) has undergone extensive validation, and a set of simulated reference observations have been subjected to a set of calibration experiments. One of the first candidate observing systems assessed by this system is a wind lidar based on the Global Wind Observing Sounder (GWOS) concept developed by NASA in response to the National Research Council (NRC) Decadal Survey. OSSEs were conducted at Joint Center for Satellite Data Assimilation (JCSDA) and positive impacts from GWOS on medium range weather forecast were demonstrated. For OSSEs, all major observations used for the data assimilation have to be simulated as a control observation in addition to the observations being tested by an OSSE. Simulation of control observations and OSSE calibration are the most significant initial investments for an OSSE before it can be used to evaluate the data impact of future instruments. The Nature Run data and control observation that were simulated at NOAA from the Nature Run are made available from a NASA portal and NCAR for international collaborative Joint OSSEs. Recent developments and plans for a JCSDA OSSE based on a 2012 observation system will be also described.

Ensembles of parallel 4D‐Var data assimilation cycles have been used to assess the impact of two observing systems: the existing network of radiosonde and wind profilers, and the future spaceborne ADM–Aeolus wind‐profiling LIDAR. We demonstrate that this new technique for impact assessment provides a practical alternative to the traditional observing system simulation experiments (OSSEs), with the particular advantage that real existing observations are assimilated exactly as in operational practice, and do not need to be simulated artificially. It is only the future observing system under test (ADM–Aeolus in our case) that is generated through simulation. Furthermore, in contrast with OSSEs, there is no need to generate an artificial reference atmosphere (‘proxy truth’ or ‘nature run’), and the problems normally associated with identical‐twin experiments are thus avoided. Our results, based on detailed simulation of the ADM–Aeolus wind‐measuring capabilities and expected data quality, show that ADM–Aeolus will provide benefits comparable to the radiosonde and wind‐profiler network, with analysis impact particularly over ocean and in the tropics. The impact is retained up to the medium range of forecast (around day 5). Our results for radiosonde and wind‐profiler impact agree qualitatively with those obtained with the well‐established observing system experiment (OSE) technique; this agreement gives some confidence in the usefulness of the ensemble‐based technique for impact assessment. Copyright © 2007 Royal Meteorological Society

Meteorological observing systems are continuously being developed to improve our knowledge of the atmosphere and our forecasting capabilities. Observing System Simulation Experiments (OSSEs) are a general technique to assess a priori the potential impact of future instruments, which is particularly important in the case of spaceborne systems. One crucial component of OSSEs is the Nature Run (NR), representing a virtual atmosphere from which observations can be simulated so that the impact of future instruments can be assessed. A community‐based, 13‐month T511 NR was designed in an international collaborative effort and was produced by the European Centre for Medium‐Range Weather Forecasts (ECMWF) to build a next‐generation OSSE capability. This new Joint OSSE NR is being analyzed with emphasis on tropical development over the western African monsoon region and the tropical Atlantic. The NR representation of the African Easterly Jet and the characteristics of African Easterly Waves including their propagation and development in tropical‐cyclone like vortices are investigated. This is the first NR that encompasses one entire Atlantic tropical cyclone season producing realistic tropical cyclone activity. As such it is a valuable tool to perform OSSEs to assess the possible impact of future instruments targeting hurricanes.