Numerical Sensitivity Experiments Research Articles

Impact of Western Ghats orography on the simulation of extreme precipitation over Kerala, India during 14–17 August 2018

The state of Kerala, located in southwest peninsular India, has experienced a series of extremely heavy rainfall events in the past few years, resulting in severe flooding and landslides. Among these events, an extremely heavy rainfall episode (50 to 480 mm/day) occurred during 14–17 August 2018 had led to devastating floods along the southwest coast of India. In this study, we have chosen this event as a case study to analyze the role of orography, using a high-resolution (1-km) cloud-resolving WRF model configuration. We conducted four numerical sensitivity experiments to examine the impact of different topographic configurations, ranging from no-mountain to plateau, moderate, and steep mountain terrains, on the simulation of the extreme rainfall event. The results of the mountain sensitivity experiments indicate that as mountain height increases, simulated low-level wind decelerates quickly. This flow blocking is also manifested in parameters such as reduced upper-air wind intensity, increased dynamical convergence, and increase of vertical velocities at the Western Ghat (WG) mountain barriers, which lead to the variations in moist instability and cloud microphysical processes influencing the horizontal and vertical distribution of solid-phase hydrometeors. Furthermore, the results also show that a reduction in the orography of the WGs decreases the vertical extension of low-level jet winds to upper levels, weakens the tropical easterly flow, and decreases the wind shears between the lower and upper troposphere. In addition, the results suggest that large-scale variations in mountain orography in numerical simulations can affect the simulated magnitude and direction of moisture transport, which may have played a role in determining the magnitude and distribution of simulated rainfall over Kerala.

Effects of a preexisting cold pool on the initiation of an extreme rainfall in southern Xinjiang

On June 15, 2021, the Hotan region of southern Xinjiang was hit by an extreme rainfall event. A numerical simulation of the storm process, with a resolution of 3 km, was conducted to analyze the initiation mechanism of the rainfall. The results show that lifting conditions play a crucial role in triggering convection, compared to moisture and stratification conditions. The easterly jet changes the direction in the western part of the Tarim Basin and speeds up, creating a zone of high wind speeds in front of Kunlun Mountain's northern slope, which provides favorable conditions for convection initiation. A remarkable feature during this process is the effect of a preexisting distant cold pool on the southern slope of Tianshan Mountain. The cold pool impedes the easterly development of the easterly airflow in the basin, causing it to turn northward, and accelerates the mountain front airflow in front of Kunlun Mountain. The inner dynamic mechanism is a strengthening of the low-level horizontal pressure gradient between the cold pool and the low-pressure belt along Kunlun Mountain. The numerical sensitivity experiments also highlight the key role the distant cold pool plays in the development of the high-speed wind belt ahead of the mountain and causes convection to start 4 h earlier. These results will serve as a valuable reference for future studies on the formation process of convection and improve the understanding of precipitation forecast in arid and semi-arid regions.

Wave–Tide Interaction by Typhoon Ampil on Wave and Storm Surge in the Changjiang River Estuary and Its Adjacent Coastal Areas

The study used the SCHISM ocean model combined with the WWM III wind wave model to quantify the interaction between wind waves and tides in the coastal zone of the Changjiang River Estuary and its adjacent areas. The wave and storm surge during Typhoon Ampil, which made landfall in July 2018 in Shanghai, were simulated by using the Climate Forecast System Version 2 (CFSv2) and Medium-Range Weather Forecasts (ECMWF) latest reanalysis (ERA5) wind dataset from 1 July to 31 July. Model results with CFSv2 forcing show better performance in terms of significant wave height and storm surge than those with ERA5 forcing. To investigate the interactions between waves, water levels, currents, and their combined effects on significant wave and surge variations, six numerical sensitivity experiments were designed according to the different coupling methods between SCHISM and WWMIII. The research shows that in coastal areas with water depths of less than 10 m, waves are affected by water levels and currents. The differences in the effect on significant wave height between wave-tide coupling and one-way coupling with water levels and currents are negligible. Wave setup is an important physical term which cannot be ignored during the variations of storm surge caused by Typhoon Ampil. The contributions of wave set up were concentrated in coastal areas with water depths less than 10 m. The peak wave setup occurred in the Changjiang River Estuary, reaching 0.15 m. In Xiangshan Bay and Sanmen Bay, wave radiation stress makes the proportion of wave setup to the total surge reached more than 30%. The consideration of wave-tide interaction can effectively improve the accuracy of numerical wave and storm surge simulations, which can provide more accurate hindcasts of wave and storm surge variations in the Changjiang River Estuary and its adjacent coastal areas.

Observed interannual variability of the Kuroshio and Luzon Undercurrent associated with tropical and subtropical wind forcing

The interannual variability of the Kuroshio and Luzon Undercurrent (LUC) east of the Luzon Island is examined using observations from a moorings during December 2010–October 2012 and three moorings during January 2018–May 2020 deployed at 18°N. Measurements from acoustic Doppler current profiler (ADCP) equipped on the moorings indicate that the interannual variations of upper-layer Kuroshio and lower-layer LUC were in phase. The interannual variability is strong in the Kuroshio but weakens substantially in the LUC. A proxy of the Kuroshio and LUC based on sea level slope is developed and validated by comparing the ADCP and satellite altimeter measurements. The proxy during 1993–2020 shows that the interannual sea level slope across the Kuroshio is primarily associated with sea level fluctuations on its eastern flank. Diagnostic analysis and numerical sensitivity experiments using a simplified ocean model indicate that the wind forcing in the subtropical northwestern Pacific Ocean played an essential role in the interannual variability of Kuroshio and LUC’s strength and water mass properties during the observation periods in 2011 and 2018–2020, although the tropical winds dominated the interannual variability of the two boundary currents during most time of the past three decades (1993–2020). The mooring observations show that the subsurface water mass in the LUC at the depth of about 650–800 m experienced a considerable freshening event from July 2019 until at least January 2020 due to abnormally strong subtropical wind forcing.

Tide-Induced Upwelling and Its Three-Dimensional Balance of the Vertical Component of Vorticity in the Wider Area of the Bohai Strait

Upwelling is a widespread phenomenon in the ocean and plays key roles in the marine environment, marine fishery and air–sea carbon exchange. In coastal regions, the upwelling is usually modulated by tides and complex topography, but the dynamical mechanism is still unclear and yet to be quantified. In this study, a three-dimensional (3D) regional ocean model is used to investigate tide-induced upwelling and its mechanisms quantitatively in the mouth of a semi-closed bay, the Bohai Strait, which is a tide-dominated coastal region. The results show that the upwelling mainly occurs near the tidal front in the north of the Laotieshan Channel and the southern region of the front, with the most active upwelling existing off promontories and small islands. Numerical sensitivity experiments indicate that the upwelling in the study area is mainly caused by tides, accounting for approximately 86% of the total. The 3D balance of the vertical component of the vorticity based on the model results quantifies the dynamic processes of the upwelling and reveals that tides induce the upwelling through tidal mixing and nonlinear effects. In the tidal front zone, the upwelling is mainly caused by baroclinic processes related to tidal mixing. Off promontories and small islands, we first reveal that the upwelling is driven by both the tidal mixing and nonlinear effect related to centrifugal force rather than just one of the two mechanisms, and the latter plays a dominant role in producing the upwelling. The strong nonlinear effect is attributed to the periodic movement of barotropic tidal currents rather than the mean flow.

Subsequent tropical cyclogenesis in the South China Sea induced by the pre-existing tropical cyclone over the western North Pacific: a case study

Mechanisms of tropical cyclogenesis have been studied for decades. A new one in the South China Sea, namely, PTC-STC is proposed. A subsequent tropical cyclone (STC) in the South China Sea can be induced by a pre-existing tropical cyclone (PTC) over the western North Pacific. The observations, reanalysis, and numerical sensitivity experiments suggest that the terrain of the Philippines (especially Luzon) is geographically essential to the tropical cyclogenesis and development of STC, whereas the intensity and track of PTC are conditionally decisive. If the terrain of the Philippines is replaced by sea, no STC forms. The steep mountain range of Luzon provides static blocking effect that can 1) enhance the upward motion; 2) accumulate warm moist air mass from the westerly and PTC; and 3) constrain the advection of vorticity from the PTC. Meanwhile, the northeasterly from the PTC climbs over the terrains, increases the adiabatic heating, and warms the proximity in the leeside of the mountains. These processes show that the interactions between the PTC and the terrain of the Philippines could provide favorable dynamic and thermodynamic conditions for the tropical cyclogenesis of STC in the low-to-mid troposphere of the South China Sea. Whereas, if the PTC is too strong, it could move into the South China Sea, suppressing the standalone favorable conditions for the tropical cyclogenesis of STC in the South China Sea.

Impact of Arctic Sea Ice Interannual Variation on Nonmonsoonal Winter Precipitation over the Eurasian Continent

Abstract The present study analyzes the impact of interannual variation in autumn (September–November) Arctic sea ice concentration (SIC) on early winter (November–January) precipitation over nonmonsoonal Eurasian (NME) regions based on both observations and numerical model experiments. An energy budget analysis shows that negative autumn SIC anomalies in the Beaufort–Chukchi–East Siberian Seas (BCES) induce heating in the overlying atmospheric column, which excites a Rossby wave that propagates from the BCES through the Atlantic Ocean to the mid- and high-latitude Eurasian continent. This Rossby wave obtains energy from the mean flow by both baroclinic and barotropic energy conversions. In comparison, the baroclinic energy conversion is more important than the barotropic energy conversion. The low-level anomalous cyclone and anticyclone of the Rossby wave dominate the eastern North Atlantic Ocean–southern Europe and western Russia, respectively. Anomalous westerly wind along the south flank of the anomalous cyclone transports moisture from the North Atlantic Ocean to the continent, resulting in a water vapor flux convergence and positive precipitation anomaly over southern Europe in early winter. Pronounced anomalous northerly winds in the eastern part of the western Russian anticyclone cause negative precipitation anomalies over vast regions of central Asia and the west Siberian plain in early winter. The Rossby wave ray tracing experiment and numerical sensitivity experiments support the above BCES SIC–NME precipitation connection.

Enhanced Influence of Tropical Pacific Sea Surface Temperature Anomalies on Spring Extreme Heat Events Over Mid‐High Latitude Eurasia

AbstractThis study presents an interdecadal enhancement in the interannual relationship between spring extreme heat events (EHEs) over mid‐high latitude Eurasia and winter sea surface temperature (SST) over the tropical central‐eastern Pacific (TECP) after the late 1990s. Comparative analyses of the two subperiods before and after the late 1990s show that the interdecadal changes in the winter TECP SST could be responsible for such changeable relationship. After the late 1990s, the winter TECP SST anomalies (SSTA) feature a central Pacific (CP)‐type pattern. The CP‐type SSTA cause an anticyclone anomaly over the North Atlantic through the Pacific‐North American‐like teleconnection, further leading to a dipole SST pattern over the tropical to mid‐latitude North Atlantic. The CP‐type SSTA and North Atlantic dipole SSTA persist well from winter to spring, and the North Atlantic dipole SSTA favor the propagation of the CP‐type SST‐excited Rossby wave train to Eurasia in spring. Therefore, the TCEP SSTA have a significant interannual relationship with the spring Eurasian atmospheric circulations and EHEs after the late 1990s. The above physical processes are further confirmed by the SSTA sensitivity numerical experiments. In contrast, before the late 1990s, the winter TECP SSTA show an eastern Pacific‐type pattern, which is related to weak and insignificant North Atlantic SSTA and spring Eurasian atmospheric circulations; consequently, the TCEP SSTA have a weak relationship with the EHEs variations during this period. The TCEP SSTA with one season leading can provide valuable information for the prediction of spring Eurasian EHEs variations in the recent two decades.

Influences of the Mid-Level Vortex on the Formation of Tropical Cyclone Toraji (2013)

This study analyzes the influences of the mid-level vortex on the formation of Tropical Cyclone Toraji (2013). A rare case of a tropical cyclone that formed near Taiwan involved a mid-level vortex that was a remnant of Tropical Cyclone Kong-Rey (2013). The piecewise potential vorticity inversion method is applied to examine the contribution of the mid-level vortex to the low-level wind field under quasi-balanced conditions. Numerical sensitivity experiments are conducted to quantify the importance of the mid-level vortex on Toraji formation, in which the mid-level vortex is removed with different removing factors (percentages) from the initial field. The results indicate that mid-level positive potential vorticity anomalies significantly contribute to the low-level positive vorticity before Toraji formation. Furthermore, when the removing factors increase in the sensitivity experiments, either the intensity of the simulated low-level vortex or the development trend of pre-Toraji decreases. However, there is no significant relationship between the convection’s magnitude and the intensity of the mid-level vortex. The main difference comes from the mid-level vortex’s intensity, which would result in a greater high-level warm core structure and cause stronger vertical mass flux. In summary, the mid-level vortex plays a critical role in the formation of Toraji. It provides a favorable environment for forming the pre-Toraji vortex by maintaining a high-level warm-core structure, leading to the formation of Toraji.

Striking impacts of biomass burning on PM2.5 concentrations in Northeast China through the emission inventory improvement

Biomass burning exerts substantial influences on air quality and climate, which in turn to further aggravate air quality. The biomass burning emissions in particular of the agricultural burning may suffer large uncertainties which limits the understanding of their impact on air quality. Based on an improved emission inventory of the Visible Infrared Imaging Radiometer Suite (VIIRS) relative to commonly used Global Fire Emissions Database (GFED), we thoroughly evaluate the impact of biomass burning on air quality and climate during the episodes of November 2017 in Northeast China which is rich in agriculture burning. The results first indicate substantial underestimates in simulated PM2.5 concentrations without the inclusion of biomass burning emission inventory, based on a regional air quality model Weather Research and Forecasting model and Community Multiscale Air Quality model (WRF-CMAQ). The addition of biomass burning emissions from GFED then reduces the bias to a certain extent, which is further reduced by replacing the agricultural fires data in GFED with VIIRS. Numerical sensitivity experiments show that based on the improved emission inventory, the contribution of biomass burning emissions to PM2.5 concentrations in Northeast China reaches 32%, contrasting to 15% based on GFED, during the episode from November 1 to 7, 2017. Aerosol direct radiative effects from biomass burning are finally elucidated, which not only reduce downward surface shortwave radiation and planetary boundary layer height, but also affect the vertical distribution of air temperature, wind speed and relative humidity, favorable to the accumulation of PM2.5. During November 1–7, 2017, the mean daily PM2.5 enhancement due to aerosol radiative effects from VIIRS_G is 16 μg m−3, a few times higher than that of 2.8 μg m−3 from GFED. The study stresses the critical role of biomass burning, particularly of small fires easily missed in the traditional low-resolution satellite products, on air quality.

Impacts of moisture supply from the subtropical western Pacific on the subtropical high and the atmospheric river during the heavy rain of 2020 in Japan

Our recent study suggested that moisture from the subtropical Western Pacific (WP) contributed the most to an atmospheric river (AR) event and the related heavy rainfall during the heavy rain of 2020 in Japan based on a Lagrangian approach. However, the actual role of moisture from the subtropical WP region in the AR and heavy rain formations remains unclear. To evaluate that, we conducted a set of numerical sensitivity experiments by adjusting the surface moisture supply over the subtropical WP region with factors of 0%, 50%, and 200%. The sensitivity experiments suggest that the reduced surface evaporation over the subtropical WP suppressed the local convective activity and decreased moisture content in the whole troposphere, leading to shallow and weak positive geopotential height anomalies. Although the slightly strengthened WP subtropical High (WPSH) and related anomalous anticyclonic circulation enhanced the southwesterly wind, convective activities along the Meiyu-Baiu front were still weakened due to the largely reduced moisture supply, resulting in another anomalous anticyclonic circulation over Japan but had much stronger and deeper structures. These two anomalous circulations and the reduced moisture modulated the AR over Japan, which eventually caused the weakened rainfalls and the northward migration of the rainband. By contrast, larger surface evaporation enhanced the local convective activities and weakened the dominant WPSH, resulting in the weakening and the southward migration of the AR. Overall, this study confirmed the large contributions of moisture supply from the subtropical WP region to the AR and related precipitation over Japan during the record-breaking Meiyu-Baiu season in 2020 via both dynamic and thermodynamic influences. In addition, it reveals that, although larger evaporation over the WP region would increase the total rainfalls but would not have led to more several rainfalls over certain regions, such as the relatively small Kyushu Island.

Proxy‐Based Preformed Phosphate Estimates Point to Increased Biological Pump Efficiency as Primary Cause of Last Glacial Maximum CO2 Drawdown

AbstractUpwelling deep waters in the Southern Ocean release biologically sequestered carbon into the atmosphere, contributing to the relatively high atmospheric CO2 levels during interglacial climate periods. Paleoceanographic evidence suggests this “CO2 leak” was lessened during the last glacial maximum (LGM), potentially due to increased stratification, weaker and equatorward‐shifted winds, and/or enhanced biological carbon export. The collective influences of these mechanisms on the ocean's biological pump efficiency and amount of atmospheric CO2 can be quantified by determining preformed phosphate of deep waters. We quantify preformed PO4 (Ppre,AOU) and preformed () of LGM bottom waters using a compilation of published paleo‐temperature, nutrient and oxygen estimates from benthic foraminifera. Our results show that preformed phosphate of the Pacific and Indian deep oceans was reduced by about −0.53 ± 0.13 μM and suggest that much (64 ± 28 ppmv) of the Glacial‐Interglacial CO2 drawdown resulted from changes in the ocean's biological pump efficiency. Once carbonate compensation is accounted for, this can explain the entire CO2 drawdown (87 ± 40 ppmv). Preformed shows similar results. The reconstructed LGM Ppre,AOU and oxygen are qualitatively consistent with the changes produced by a suite of numerical sensitivity experiments that roughly simulate three proposed mechanisms for an increase in LGM biological pump efficiency: an increase in biological activity, a decrease in wind‐driven upwelling, and an increase in stratification in the Southern Ocean.

Mesoscale Simulation of Open Cellular Convection: Roles of Model Resolutions and Physics Parameterizations

The Open Cellular Convection (OCC) associated with cold air outbreaks is a common phenomenon over the North Sea where a large number of wind parks are presented. Thus, reliable numerical simulations of OCC events have great importance for offshore wind energy. We investigate the ability to simulate the OCC events using the Weather Research and Forecast (WRF) model with the ERA5 reanalysis data as initial and lateral boundary conditions and the OSITA data as the sea surface temperature. The domains were nested from 9 km as the outermost domain to 1 km as the innermost domain surrounding the Teesside wind farm located in the North Sea off the northeast coast of England. We simulated an OCC event in 2015 with three series of sensitivity numerical experiments of planetary boundary layer, microphysics, and radiation parameterizations. The model outputs were validated against the wind observation at the Teesside’s meteorological mast. The results suggest that the planetary boundary layer schemes are the most sensitive during the events compared to other parameterization schemes. Futher more, a convective-resolved resolution is necessary for simulating the OCC variation properly. The paper also discuss the verification methods for such short time-scale events like the OCC.

Numerical Simulations of Cloud Number Concentration and Ice Nuclei Influence on Cloud Processes and Seeding Effects

Aerosols, through cloud condensation nuclei (CCN) or ice nuclei (IN), affect cloud microphysics. With increasing concentrations of aerosols, it is important to consider the impact of IN along with CCN on clouds and precipitation in numerical simulations; further, aerosols may also affect the weather-modification seeding effect. On the basis of the observation of natural IN concentration and cloud-drop number concentrations, numerical sensitivity experiments for a snowfall case were designed to study the effects of parameters of IN and cloud number concentrations at the cloud base to consider the CCN effects on clouds and precipitation as well as weather-modification seeding effects. Generally, with smaller cloud-drop number concentration, the mass contents were much lower. With more ice nuclei, more ice crystals were able to nucleate, and additional snow particles were generated through ice crystals. Cloud-drop number concentrations heavily affected the location and amount of snowfall. During the 1e9 test, 2.4 mm was the highest reduction in the amount of snowfall; additionally, the amount of snowfall from the combined impacts of increased IN and cloud-drop number decreased in wide areas, and its maximum precipitation reduction exceeded 2.7 mm as well as up to 15% of the daily amount of snowfall. More IN reduced the artificial seeding effect, lowered the increase in snowfall in the center of the seeding, and lowered the reduction of snowfall in the reduction center of the seeding. With more IN, the seeding effect was able to shift approximately 0.6% from the 3.9% seeding effect of the control simulation.

Sensitivity of boundary layer schemes in simulating the asymmetric rainfall of landfalling typhoon Lekima (2019)

A group of control (CTL) plus ten sensitivity numerical experiments have been conducted to investigate the sensitivity of planetary boundary layer (PBL) schemes in simulating the asymmetric precipitation distribution of typhoon Lekima (2019) during landfall. The simulated track and intensity are quite sensitive to the choice of the PBL scheme. In CTL that applies the Mellor–Yamada Nakanishi and Niino (MYNN) PBL and the surface layer scheme, the observed eyewall replacement 6 h prior to landfall and the asymmetric precipitation during landfall have been simulated well. However, in the PBL1 experiment that applies the Yonsei University (YSU) PBL scheme and the Revised Mesoscale Model version 5 (MM5) Monin–Obukhov surface layer scheme, no double eyewall is simulated. PBL1 and the other sensitivity experiments also simulate more axisymmetric precipitation distribution. PBL1 simulates intensification just before landfall, sustains intensity longer after landfall, but then dissipates quite rapidly. Such differences from CTL are due to larger enthalpy flux, higher PBL height (which is almost unchanged in CTL), and eddy diffusivity extending more into the free atmosphere in PBL1. These factors lead to outward expansion of the radius of maximum wind, larger radial inflow, larger axisymmetric tangential wind in the boundary layer, and larger updrafts in the eyewall. After landfall, larger momentum flux and larger friction velocity in PBL1 enable the more rapid dissipation. The intensification before landfall in PBL1 makes the axisymmetric component stronger. Asymmetry developed in the outer eyewall, and PBL1 was less successful in simulating the eyewall replacement that affects the degree of rainfall asymmetry. These results indicate that the model PBL schemes largely influence the simulated tropical cyclone (TC) intensity and structure including asymmetric rainfall distribution during landfall.

On the influences of the continental shelf bathymetry correction in Prydz Bay, East Antarctica

The bathymetry around Antarctica can govern the shelf sea circulations and play a key role in conditioning water masses. In Prydz Bay, the Prydz Bay Gyre and coastal currents are also determined by the continental shelf topography. However, due to the paucity of beam echo sounding data, the bathymetric datasets in Prydz Bay still have large uncertainties. With the aid of in situ hydrographic observations, this study focuses on the correction of an up-to-date bathymetric dataset and the resultant influences on the shelf circulation and the basal melting of the ice shelves. The corrected bathymetry mainly improves the biased shallow representations in the uncorrected bathymetric data set, with a maximum change of ~500 m deepening in the eastern flank of Prydz Bay. Sensitivity numerical experiments show that the bathymetric corrections in Prydz Bay have a significant impact on the circulation pattern and onshore warm water intrusions. In addition, the corrected bathymetry markedly decreases the heat transport towards the calving front of the Amery Ice Shelf. The onshore heat transport reduces by ~22.18% from ~5.23×1013 J s-1 to ~4.07×1013 J s-1 over the outer shelf. Over the inner shelf, the heat transport towards the Amery Ice Shelf reduces by ~18.15% from ~5.95×1013 J s-1 to ~4.87×1013 J s-1. Consequently, the temporally and spatially averaged basal melting rate of the Amery Ice Shelf reduces by ~13.04% from ~0.69 m yr-1 to ~0.60 m yr-1.

Causes for the increases in both evapotranspiration and water yield over vegetated mainland China during the last two decades

Quantifying the contributions of climate and vegetation to the dynamics of evapotranspiration (ET) and water yield (i.e., precipitation minus ET) will help us better understand the changes in the water budget. In this study, we identified the contributions of climate variables (including precipitation, radiation, temperature, and relative humidity), human-disturbed vegetation, and natural vegetation to the trends in annual ET and water yield over vegetated mainland China during 2001–2020, using a process-based terrestrial ecosystem model and a joint-solution method with multiple sensitivity numerical experiments. Results showed that 46% of the study area experienced significant (p<0.05) increases in ET, with an overall increase of 2.32 mm y−1. Meanwhile, the overall trend in water yield was 2.56 mm y−1 but insignificant. Spatially, vegetation and precipitation are the dominant factors for ET trends over 55% and 32% of vegetated mainland China, respectively. Over the regions where vegetation dominates the ET trends, nearly half of these regions are covered by human-disturbed vegetation (e.g., cropland or regions with land cover changes), suggesting that anthropogenic activities play a crucial role in the hydrological cycle there. Concerning the trends in water yield, precipitation is the dominant factor over 64% areas. Human-disturbed vegetation and natural vegetation play similar roles and combined can explain the water yield trends over 30% areas. Our study highlights the spatial variations in the mechanisms behind changes in the water budget over mainland China, particularly in regions covered by human-disturbed vegetation. This finding should be considered in the existing and future national ecological recovery policies to maximize its eco-hydrological benefits.

The Impacts of Midlevel Moisture on the Structure, Evolution, and Precipitation of Afternoon Thunderstorms: A Real-Case Modeling Study at Taipei on 14 June 2015

Abstract A severe afternoon thunderstorm (ATS) system developed within the Taipei basin on 14 June 2015, which produced intense rainfall (with a rainfall rate of 131 mm h−1) and urban-scale flooding. A control simulation (CNTL) using the Weather Research and Forecasting (WRF) Model with the horizontal grid size nested down to 500 m was performed to capture reasonably well the onset of the sea breeze, the merger of convective cells, and the evolution of the afternoon thunderstorm system. Four numerical sensitivity experiments with the increase or decrease of midlevel (700–500 hPa) relative humidity (RH) of 10% and 20% were conducted, and simulation results were compared with those from the CNTL. Although the response of convection to midlevel RH was somewhat nonlinear, sensitivity experiments showed that a dry layer at middle levels would result in stronger cold pool, more intense convection, stronger updraft, more graupel particles, stronger net latent heating above the melting level, and a much larger area of the potential flooding region [&gt;40 mm (30 min)−1]. The estimation of bulk entrainment rate provided evidence that the entrainment rate could be reduced by stronger cold pool and the widening of moist convection area. Three terrain-removal sensitivity experiments indicated that Taipei basin modulated the response of convection intensity to midlevel RH. The basin terrain confined the outflow associated with ATS and forced it to converge with the moist sea breeze continuously, providing a favorable dynamic and thermodynamic environment for subsequent convection development. This “basin confinement effect” may be crucial for short-duration rainfall extremes over complex terrain. Significance Statement This study has examined the impact of midlevel moisture on the structure, evolution, and precipitation of an afternoon thunderstorm system that produced intense rainfall at Taipei using eight numerical experiments based on high-resolution model outputs. Our findings explain how a drier layer at middle levels would produce a more intense thunderstorm system, although the response of convection intensity to midlevel moisture is somewhat nonlinear. In addition, it is found that terrain could modulate the response of convection to midlevel moisture, which is rarely discussed in previous studies.

Impacts of Long‐Term Urbanization on Summer Rainfall Climatology in Yangtze River Delta Agglomeration of China

AbstractIn this study, we investigated the urbanization‐induced summer rainfall changes in the Yangtze River Delta (YRD) by analyzing long‐term observations and numerical simulations. The observation‐based analysis showed that long‐term urbanization increased the region's summer rainfall, particularly through the intensification of heavy rainfall, which is noted as the urban rain island (URI) effect. A series of numerical sensitivity experiments with three historical land use and land cover scenarios (1990, 2000, and 2010) were designed to further understand the urbanization impacts on rainfall. The observed URI effect was well reproduced by the numerical simulations, and on average, urban expansion during 1990–2010 increased summer rainfall over urban areas by 51.91 mm. The URI effect slightly weakened in the late stage of urbanization (2000–2010) compared to the early stage (1990–2000). We conclude that the strengthening of precipitation‐inhibiting effects during the late period offset the precipitation‐enhancing effects, which led to the weakening of the URI effect.

Role of the amazon outflow on the barotropic tide on the amazonian shelf

The Amazon yields the largest freshwater discharge to the world ocean, outflowing on a shallow, macro-tidal shelf. Its plume dynamics is complex and the impact of the discharge on the tidal properties on the adjoining shelf remains poorly understood. Through a series of numerical modelling experiment, we investigate the interaction between the Amazon discharge and the tide over the Amazonian shelf. We find that the runoff is responsible for a rise of the M2 tidal amplitude of ten cm in the embayment between Cabo Norte and Cabo Cassipore. The runoff also induces a rise of the mean sea level there, of 10 cm as well. Such a one-to-one sensitivity of the tidal amplitude and of the mean sea level over this region is quite unique among the world coastline. The modulation of the bottom friction by the thickness of the water column as well as by the vertical density stratification in the Amazon plume appears to be the main forcing factor of this response. Our numerical sensitivity experiments suggest that the seasonal variability of the Amazon discharge has little impact on the temporal variability of the tidal characteristics over the Amazonian shelf.