Peak Precipitation Research Articles

Meso‐ and microscale response to variation in cloudiness at three forested sites in the Maritime Continent

AbstractThis study examines the relationships between the diurnal precipitation cycle, local morning insolation, and local and large‐scale moisture availability using local radiative flux and precipitation observations from three forested tropical sites, together with ERA5 reanalysis data. Our results suggest that, away from the coast, there is a weak positive correlation between local morning insolation and the magnitude of the afternoon precipitation peak, and a weak negative relationship between locally observed surface moisture in the morning and the magnitude of the afternoon precipitation peak. There is little relationship between large‐scale column moisture and the local afternoon precipitation peak. We examine analogous results in convection‐permitting simulations. Results suggest that the simulated afternoon precipitation rate shows almost no dependence on surface radiative forcing or moisture availability, while the observed afternoon precipitation rate is dominated by radiative forcing. Simulated and observed satellite precipitation and cloud estimates for a single case study suggest a diversity of surface‐flux–precipitation relationships amongst model configurations.

Evaluation of Asian summer precipitation in different configurations of a high-resolution general circulation model in a range of decision-relevant spatial scales

Abstract. High-resolution general circulation models (GCMs) can provide new insights into the simulated distribution of global precipitation. We evaluate how summer precipitation is represented over Asia in global simulations with a grid length of 14 km. Three simulations were performed: one with a convection parametrization, one with convection represented explicitly by the model's dynamics, and a hybrid simulation with only shallow and mid-level convection parametrized. We evaluate the mean simulated precipitation and the diurnal cycle of the amount, frequency, and intensity of the precipitation against satellite observations of precipitation from the Climate Prediction Center morphing method (CMORPH). We also compare the high-resolution simulations with coarser simulations that use parametrized convection. The simulated and observed precipitation is averaged over spatial scales defined by the hydrological catchment basins; these provide a natural spatial scale for performing decision-relevant analysis that is tied to the underlying regional physical geography. By selecting basins of different sizes, we evaluate the simulations as a function of the spatial scale. A new BAsin-Scale Model Assessment ToolkIt (BASMATI) is described, which facilitates this analysis. We find that there are strong wet biases (locally up to 72 mm d−1 at small spatial scales) in the mean precipitation over mountainous regions such as the Himalayas. The explicit convection simulation worsens existing wet and dry biases compared to the parametrized convection simulation. When the analysis is performed at different basin scales, the precipitation bias decreases as the spatial scales increase for all the simulations; the lowest-resolution simulation has the smallest root mean squared error compared to CMORPH. In the simulations, a positive mean precipitation bias over China is primarily found to be due to too frequent precipitation for the parametrized convection simulation and too intense precipitation for the explicit convection simulation. The simulated diurnal cycle of precipitation is strongly affected by the representation of convection: parametrized convection produces a peak in precipitation too close to midday over land, whereas explicit convection produces a peak that is closer to the late afternoon peak seen in observations. At increasing spatial scale, the representation of the diurnal cycle in the explicit and hybrid convection simulations improves when compared to CMORPH; this is not true for any of the parametrized simulations. Some of the strengths and weaknesses of simulated precipitation in a high-resolution GCM are found: the diurnal cycle is improved at all spatial scales with convection parametrization disabled, the interaction of the flow with orography exacerbates existing biases for mean precipitation in the high-resolution simulations, and parametrized simulations produce similar diurnal cycles regardless of their resolution. The need for tuning the high-resolution simulations is made clear. Our approach for evaluating simulated precipitation across a range of scales is widely applicable to other GCMs.

Factors Controlling the Hydraulic Efficiency of Green Roofs in the Metropolitan Area of Milan (Italy)

Green roofs (GRs) are considered sustainable solutions for the adaptation of urban water management to climate change. The use of GRs is particularly promising in urban environments like the Metropolitan Area of Milan, the most urbanized area in Italy. In this work, we evaluated the subsurface runoff coefficient at the event-time scale, for more than one year of observations, of 68 small-scale test beds comprising different configurations of green roofs (e.g., different vegetations, types and depths of growing media, and different slopes) installed in the Metropolitan Area of Milan. The objectives of this study are three-fold. Firstly, the controlling factors of the hydraulic have been assessed for efficiency. We calculated a mean drainage flow rate of 51%, finding that growing media play a significant role in determining the drainage flow during the spring, at the beginning of the vegetative period. During this season, water retention in fertilized beds increases significantly. At the beginning of the summer, the vegetation cover is able to significantly reduce the drainage flow, playing an even more crucial role with respect to the growing medium material. However, we found that the vegetation type (grass field and Sedum) does not play a significant role in the retention processes. Secondly, the delay of the peak flow rate was determined. We found a precipitation peak delay from 1 to 2 h, which would be sufficient to guarantee environmental benefits for urban drainage. Finally, the factors controlling the hydraulic efficiency of GRs for individual precipitation events were assessed. We found that soil moisture and cumulated precipitation are both significant factors determining the drainage flow rate. In conclusion, we point out that soil moisture is one of the main parameters characterizing GR drainage and should be further considered in future research efforts devoted to the analysis of GR performance.

Determining the heat treatment behaviour of metal injection moulded and wrought alloy 718 using differential scanning calorimetry

The heat treatment behaviour of metal injection moulded (MIM) and wrought alloy 718 was investigated using DSC, microscopy and hardness measurements. Reheating solutionized samples in the DSC resulted in a distinct exothermic peak around 725 °C, which was attributed to γ" precipitation. This was followed by a broad endothermic region (~750-900 °C), which was attributed to the dissolution of γ' and γ" precipitates. No DSC peak was observed corresponding to γ' precipitation upon reheat. The endothermic dissolution region (750–900 °C) ended with an abrupt exothermic shoulder around 900 °C, which was attributed to the solvus of the γ'/γ" precipitates and an acceleration of δ phase precipitation. Ageing the samples at progressively higher temperatures and for longer times resulted in a decrease in the 725 °C exothermic γ" precipitation peak and a deepening of the 750–900 °C endothermic dissolution region. Also, samples with higher Al content exhibited a deeper dissolution trough and a small bump in the exothermic shoulder at the end of this trough (~880 °C), which were attributed to the higher levels of γ' observed in the FESEM for these samples. A shallow endothermic trough peaking between 1000 and 1045 °C was observed in samples treated below the δ phase solvus and was attributed to dissolution of the δ phase. Hardness measurements showed an increase in hardness with the amount of γ' and γ" precipitated as expected. Differences in heat treatment behaviour appeared to be driven largely by differences in chemical composition rather than by processing route (wrought vs. MIM). However, the MIM material had smaller, more evenly distributed carbides and additional oxides not present in the wrought material which acted to restrict grain growth in samples solutionized up to 1232 °C.

The Process of High-Frequency Intraseasonal Oscillation Associated with a Persistent Rainfall Event over South China

AbstractIn the present study, the moist static energy (MSE) budget associated with a persistent rainfall event that occurred over South China in late August 2018 (PR1808) was analyzed using ERA-Interim data. The results revealed that the PR1808 event was closely related to a 12–30-day intraseasonal oscillation (ISO), with a significant westward-propagating mode. The recharge of MSE occurred before ISO deep convection, and the MSE was discharged during and after the peak precipitation. This result indicates that the recharge–discharge cycle of MSE played an important role in regulating the high-frequency ISO (HF-ISO) process during the PR1808 event. However, the midlevel MSE tendency, rather than the low-level tendency, controlled the column-integrated MSE change in this case, and there was no low-level MSE tendency ahead of HF-ISO convection, which is different from previous studies, implying that shallow convection was not a key factor in this case. The recharge of MSE related to the midlevel MSE change mode was mainly attributed to zonal advection, while the forcing related to radiative heating (longwave and shortwave radiation) and heat fluxes (latent and sensible heat fluxes) contributed little to the change in MSE. Furthermore, for the zonal advection of MSE, the main contribution originated from the advection by the low-frequency zonal flow across the low-frequency MSE gradient, and interactions between high- (low-) frequency zonal flow and low- (high-) frequency MSE. In addition, the disturbances from higher latitudes enhanced the persistent rainfall in this case over South China through the southward shift of baroclinic vorticity.

Study of Substorm‐Related Auroral Absorption: Latitudinal Width and Factors Affecting the Peak Intensity of Energetic Electron Precipitation

AbstractPreviously, suggesting a scenario of “injected and drifting electron cloud” to describe the enhancements and precipitations of energetic electrons during substorms we used the linear prediction filter (LPF) method to build the empirical dynamical model of auroral absorption in the middle of auroral zone, driven by the midlatitude positive bay (MPB) index time series. In this paper, to understand better the relationship between magnetic dipolarization, injection, and energetic electron precipitation, we quantitatively explore correlations between dipolarization proxies (MPB and SML indices, substorm current wedge (SCW) intensity, location, and size) and precipitation‐related auroral absorption in the morning maximum region for 148 isolated substorms. We confirm good correlation of precipitation peak values with dipolarization proxies and show that the absorption amplitude is most strongly controlled by total SCW current and its azimuthal size, and is also influenced by solar wind‐dependent background energetic electron flux in the conjugate plasma sheet. This result confirms the adequacy of our starting assumption that there is a close relationship between the intensities and size of substorm dipolarizations and energetic particle injections. To extend the latitudinal coverage, we computed the LPF response functions for individual riometers of NORSTAR array. Finding similar shapes and amplitudes of their response at latitudes 63°–69° in the regions where maximal precipitation is statistically observed, we suggest, test, and approve that the auroral absorption in the belt 63°–69° may be predicted using a set of LPFs determined empirically in the center of auroral zone for various magnetic local times.

Spatial and temporal patterns of planetary boundary layer height during 1979–2018 over the Tibetan Plateau using ERA5

AbstractPlanetary boundary layer height (PBLH) is a key meteorological parameter that is affected by and influences exchanges of energy, momentum, and mass between the surface and the atmosphere, but there are few comprehensive studies on the PBLH climatology over the Tibetan Plateau (TP). This study presents a multi‐decadal climatology of PBLH over the TP by using high‐resolution reanalysis data (ERA5) for the period 1979–2018. The ERA5 PBLH was first evaluated against radiosonde‐derived PBLH from four sites. For most of the observation periods and sites, ERA5 captured the diurnal cycles reasonably well, although the highest peaks in the radiosonde‐derived PBLH were often not captured. Next, long‐term means and trends, seasonality and diurnal cycles of the PBLHs were examined. The results show that average PBLHs in the central and southwestern TP are higher than those in the west and east. On an interannual time scale, the PBLH is positively correlated with the North Atlantic Oscillation in western and southeastern TP in winter, eastern TP in summer, and central TP in autumn, while a negative correlation is seen for northeastern TP in winter. The summertime PBLH in the central and southwestern TP is negatively correlated to the Indian summer monsoon. As for the seasonal cycle, the PBLH in the western TP reaches its peak in August, while the PBLH in the eastern TP exhibits an earlier summer peak in June. In the central TP, the PBLH is the highest in April and displays a minimum during summer, corresponding to a precipitation peak and a dip in surface sensible heat flux. The climatological diurnal cycles of PBLH are generally affected by the insolation and reach their peaks in the afternoon when a large range in PBLH variation is also found.

Multiscale Simulation of Precipitation Over East Asia by Variable Resolution CAM‐MPAS

AbstractThis study evaluates the precipitation over East Asia simulated by the variable resolution CAM‐MPAS featuring the Model for Prediction Across Scales (MPAS) atmospheric dynamical core coupled with the physics parameterizations of the Community Atmosphere Model (CAM) version 5.4. Two CAM‐MPAS experiments, one with a global quasi‐uniform resolution mesh of 120 km (MPAS‐UR) and the other with a variable resolution mesh of 30–120 km refined over East Asia (MPAS‐VR), are conducted from 1989 to 2005 following the Atmospheric Model Intercomparison Project protocol. Two regional climate model simulations at ∼25 km resolution from the Coordinated Regional Downscaling Experiment East Asia second plan are also analyzed for comparison. Results show that CAM‐MPAS performs better than the selected regional models in simulating the precipitation climatology over East Asia. Compared with MPAS‐UR, MPAS‐VR with refinement over East Asia better simulates the precipitation over and around the Tibetan Plateau (TP), the upper‐level circulation, the precipitation frequency and intensity in various regions of China, and the interannual precipitation pattern associated with the East Asian summer monsoon. Both decreased grid spacing and the use of finer‐scale terrain information contribute to the improvements in MPAS‐VR relative to MPAS‐UR. However, decreased grid spacing is more essential for simulating the precipitation magnitude over TP and the location of peak precipitation south of TP, likely due to the better resolved physical and dynamical processes associated with orographic precipitation. The 30–120 km CAM‐MPAS model is shown to be a promising tool for precipitation simulation over East Asia.

Comparison of the impacts due to biomass burning, anthropogenic, and biogenic aerosol emissions on the mature phase of an urban heavy precipitation event over the Pearl River Delta

The study is aimed to discuss the differences in the aerosol impacts on the precipitation in mature phase due to three different aerosol emission sources, the biomass burning emission (BB), the anthropogenic emission (AE), and the biogenic emission (BE). Four simulations were conducted using WRF-Chem version 4.1.2 on the heavy urban precipitation event over the Pearl River Delta (PRD) region, which occurred on 21 February 2019, with 0800–0900 UTC 21 February as the mature phase. The inclusion of aerosol emissions improved the simulation performance on the temporal variations of the precipitation event and the peak precipitation. BB, AE, and BE contributed to the aerosol loading by around 6.8%, 88.1%, and 5.1%, respectively; and weakened the precipitation 28.0%, 31.0%, and 14.1%, respectively. The BE impact occurred the latest during the developing phase. The weakening effect was mainly over the convective precipitation. The aerosol indirect effect of BE strengthened the stratiform precipitation. The weakened convective precipitation was contributed by the weakened cloud microphysical processes above the zero-isotherm line. The BB impact induced the smallest change in cloud microphysical latent heating and weakened the precipitation more likely via its aerosol direct effect. The AE impact was the largest in precipitation, convection, and the cloud microphysical latent heating.

Precipitation Enhancement in Squall Lines Moving over Mountainous Coastal Regions

AbstractA mechanism for precipitation enhancement in squall lines moving over mountainous coastal regions is quantified through idealized numerical simulations. Storm intensity and precipitation peak over the sloping terrain as storms descend from an elevated plateau toward the coastline and encounter the marine atmospheric boundary layer (MABL). Storms are most intense as they encounter the deepest MABLs. As the descending storm outflow collides with a moving MABL (sea breeze), surface and low-level air parcels initially accelerate upward, though their ultimate trajectory is governed by the magnitude of the negative nonhydrostatic inertial pressure perturbation behind the cold pool leading edge. For shallow MABLs, the baroclinic gradient across the gust front generates large horizontal vorticity, a low-level negative pressure perturbation, and thus a downward acceleration of air parcels following their initial ascent. A deep MABL reduces the baroclinically generated vorticity, leading to a weaker pressure perturbation and minimal downward acceleration, allowing air to accelerate into a storm’s updraft. Once storms move away from the terrain base and over the full depth of the MABLs, storms over the deepest MABLs decay most rapidly, while those over the shallowest MABLs initially intensify. Though elevated ascent exists above all MABLs, the deepest MABLs substantially reduce the depth of the high-θelayer above the MABLs and limit instability. This relationship is insensitive to MABL temperature, even though surface-based ascent is present for the less cold MABLs, the MABL thermal deficit is smaller, and convective available potential energy (CAPE) is higher.

The Role of Free‐Tropospheric Moisture Convergence for Summertime Heavy Rainfall in Western Japan

AbstractThe role of vertically integrated water vapor flux convergence (IVFC) in heavy rainfall events in western Japan is investigated by dividing the atmosphere into a boundary layer and free troposphere using the Japan Meteorological Agency's mesoscale gridded analysis product and reanalysis product. Rainfall events are defined by peaks of area‐averaged precipitation. The results show an increase in the free‐tropospheric IVFC more than 15 h before rainfall events, contributing to atmosphere moistening. This preceding moistening is favorable for mesoscale convective systems (MCSs) with slantwise ascending deep inflow layers (layer lifting) that produce large precipitation amounts. Appearance frequency of the moist absolutely unstable layer (MAUL) increases around the precipitation peak times, which is especially notable for larger peaks. These results indicate that the free‐tropospheric IVFC contributes to heavy rainfall in western Japan by providing environments favorable for producing and maintaining MAUL, enhancing the organization of MCSs.

Ensemble Projection of Extreme Precipitation Over China Based on Three Dynamical Downscaling Simulations

Based on the outputs of the global climate models (GCMs) HadGEM2-ES, NorESM1-M and MPI-ESM-LR from Coupled Model Intercomparison Project Phase 5 (CMIP5) and the downscaling results with the regional climate model (RCM) REMO, the ability of the climate models to reproduce the extreme precipitation in China during the current period (1986–2005) is evaluated. Then, the future extreme precipitation in the mid (2036–2065) and the late 21st century (2066–2095) is projected under the RCP8.5 scenario. The results show that the RCM simulations have great improvements compared with the GCMs, and the ensemble mean of the RCM results (ensR) outperforms each single RCM simulation. The annual precipitation of the RCM simulations is more consistent with the observation than that of the GCMs, with the overestimation of the peak precipitation reduced, and the ensR further reduces the bias. For the extreme precipitation, the RCM simulations significantly decrease the underestimation of intensity in the GCMs. The RCM simulations and the ensR can greatly improve the simulations of Rx5day and CWD compared with the GCMs, decreasing the wet bias in North China and Northwest China. In the future, the consecutive dry days (CDD) will decrease in the northern arid regions, especially in North China and Northeast China. However, the southern regions will experience longer dry period. Both the amount and the intensity of precipitation will increase in various regions of China. The number of wet days will decrease in the south and increase in the north area. The significantly greater Rx5day and R95t indicate more intensive extreme precipitation in the future, and the intensity in the late 21st century will be stronger than that in the middle. Attribution analysis indicates that the extreme precipitation indices especially the R95t have significant positive temporal and spatial correlations with the water vapor flux.

Eliminating the “hook” in Precipitation-Temperature Scaling

AbstractObservational studies of extreme daily and subdaily precipitation-temperature sensitivities (apparent scaling) aim to provide evidence and improved understanding of how extreme precipitation will respond to a warming climate. However, interpretation of apparent scaling results is hindered by large variations in derived scaling rates and divergence from theoretical and modelled projections of systematic increases in extreme precipitation intensities (climate scaling). In warmer climatic regions, rainfall intensity has been reported to increase with temperature to a maximum before decreasing, creating a second order discontinuity or “hook” like structure. Here we investigate spatial and temporal discrepancies in apparent scaling results by isolating rainfall events and conditioning event precipitation on duration. We find that previously reported negative apparent scaling at higher temperatures which creates the hook structure, is the result of a decrease in the duration of the precipitation event, and not to the decrease in precipitation rate. We introduce standardized pooling using long records of Australian station data across climate zones, to show average precipitation intensities and 1-h peak precipitation intensities increase with temperature across all event durations and locations investigated. For shorter duration events (< 6-h), average precipitation intensity scaling is in line with the expected Clausius- Clapeyron (CC) relation at ~7 %/°C, and this decreases with increasing duration, down to 2 %/°C at 24-h duration. Consistent with climate scaling derived from model projections, 1-h peak precipitation intensities are found to increase with temperature at elevated rates compared to average precipitation intensities, with super-CC scaling (10 – 14 %/°C) found for short-duration events in tropical climates.

Safety Study on Wax Deposition in Crude Oil Pipeline

The Shunbei crude oil pipeline is prepared to use the unheated transportation process to transport waxy crudes. However, the wax formation in the pipeline is unknown. In order to predict the wax deposition of the pipeline, the physical property experiment of Shunbei crude oil was carried out through field sampling. The density, freezing point, hydrocarbon composition, and viscosity–temperature characteristics of crude oil are obtained. The cloud point and wax precipitation characteristics of the crude oil were obtained using the differential scanning calorimetry (DSC) thermal analysis method. Then, the wax deposition rate of the pipeline was predicted by two methods: OLGA software and wax deposition kinetic model. Finally, the optimal pigging cycle of the pipeline was calculated on this basis. The results show that: Shunbei crude oil is a light crude oil with low wax content, a low freezing point, and a high cloud point. Comparing the OLGA simulation results with the calculation results of the Huang Qiyu model, the development trend of wax deposition along the pipeline was the same under different working conditions. The relative error of the maximum wax layer thickness was 6%, proving that it is feasible for OLGA to simulate wax deposition in long-distance crude oil pipelines. Affected by the wax precipitation characteristics of Shunbei crude oil, there was a peak of wax precipitation between the pipeline section where crude oil temperature was 9.31–13.31 °C and the recommended pigging cycle at the lowest throughput was 34 days in winter and 51 days in spring and autumn.

Soil Moisture Responses to Rainfall: Implications for Runoff Generation

AbstractSoil moisture is a key control on runoff generation and biogeochemical processes on hillslopes. Precipitation events can evoke different soil moisture responses with depth through the soil profile, and responses can differ among landscape positions along a hillslope. We sought to elucidate the nature of these responses by estimating changes in water content, response time between peak precipitation and peak soil moisture, and wetting front velocities for 43 storms at 45 locations on three adjacent hillslopes within a headwater catchment of the southern Appalachian Mountains (NC, USA). We used a multivariate modeling approach to quantify the relative influences and the predictability of soil moisture responses by a combination of landscape and storm characteristics. We quantified the lag correlations between hillslope mean soil moisture and catchment runoff to demonstrate how storm properties and hillslope‐scale characteristics may influence runoff at the catchment outlet. Soil moisture responses varied widely, and no consistent patterns were observed among response metrics laterally or vertically along hillslopes. In contrast to other studies, we found that the relative influence of hillslope properties and storm characteristics varied with soil moisture responses and during storms. Antecedent conditions and storm depths influenced the strength of lag correlations between soil moisture and runoff, whereas storm mean intensity was correlated with the lag times. These results highlight the utility of intensive observations for characterizing heterogeneity in soil moisture responses, suggesting, among other things, a need for better representation of the subsurface processes in rainfall‐runoff models. Identifying the relative importance of drivers can be beneficial in building parsimonious hydrological models.

Sensitivity of WRF-derived hydrometeorological extremes to sea surface temperatures in regions with complex topography and diverse climate

This study investigates the impact of sea surface temperature (SST) describing the lower boundary of Weather Research and Forecasting (WRF) model on the extreme weather events that occurred on the Mediterranean (MED) and the Eastern Black Sea (EBS) regions of Turkey. For each region, one extreme event case is selected and characterized as a summer convective system in EBS and as a winter synoptic system in MED region. The SST impact on the WRF model forecasts of these events is accomplished in two ways. First, the SST analysis is conducted by making a 10-days simulation for each event with and without activating the SST options (update and skin options) available in the model configuration. In these runs, the selected initial and boundary condition datasets, Global Forecasting System (GFS) for the MED region and the ERA5 for the EBS regions, provide corresponding internal SST data. Second, the prediction performance of the model is evaluated among the simulations of non-external, time-varying (GFS and ERA5 SST) and external, time–varying, high-resolution SST products (The Group for High Resolution Sea Surface Temperature; GHRSST, Medspiration, and NCEP) for each event. The results show that the WRF model simulations, even for short-term event predictions, are highly sensitive to time-variant SST options. The overestimation and high spreading feature of maximum and total precipitation are realistically reduced by time-variant SST products. The Medspiration and the NCEP sources for the MED region and the GHRSST and the Medspiration sources for the EBS region yielded warmer SSTs among the simulations. By establishing the amplified air-sea interactions, these sources provide more appropriate peak precipitation distributions in both regions. Furthermore, based on this study, the effect of using high-resolution SST (GHRSST and Medspiration) is more prominent in predicting convective event developed over complex topography of the EBS while the resolution effect is not much critical for synoptic system simulation even under the complex topography in MED. Lastly, the highest mean correlation of conditional hourly rain is calculated as 0.65 with the Medspiration product on MED and 0.48 with the GHRSST product on the EBS region.

Future projections of flooding characteristics in the Lancang-Mekong River Basin under climate change

Climate change intensifies hydrological cycle, bringing critical challenges to the global and regional socio-economic development. Here we used four global climate models to project future precipitation that drives hydrological modeling of future 30-year daily runoff in a large international river basin, the Lancang-Mekong River Basin (LMRB). We applied four probability distribution functions to fit the flood peak and maximum 3-day flood volume series at four major hydrologic stations. The copula function was used to establish the joint distribution between extreme precipitation and flood peak to estimate the impact of precipitation change on floods. Results show that annual maximum 30-day precipitation (AM30D) and flood peak at Pakse station are highly correlated. Future basin-averaged AM30D is projected to increase under most climate scenarios, although AM30D varies widely in spatial distribution. The extreme values of AM30D (e.g., AM1D and AM3D) in the future (2021–2050) are larger than those in historical period (1981–2004). The temporal distribution of future AM30D is more uneven, showing the “bimodal” distribution. For the Yun Jinghong station, large uncertainty is estimated in change of direction of annual maximum flood peak and maximum 3-day flood volume. As for the other three stations, they are all projected to have larger flood risks in spite of different magnitudes. The larger the return period, the larger the increase, the greater the impact of climate change. The change of basin-averaged AM30D precipitation in the future will lead to the increase of flood peak at Pakse station via the bivariate frequency analysis. From the perspective of water conservancy project safety, considering the “adverse principle”, flood control design according to the design results derived from the joint distribution of two variables between AM30D and flood peak can reduce the risk of flood disaster. This study can provide scientific reference and basis for flood control and disaster reduction and water resources cooperation policy development in LMRB.

Understanding the Roles of Convective Trigger Functions in the Diurnal Cycle of Precipitation in the NCAR CAM5

AbstractThe wrong diurnal cycle of precipitation is a common weakness of current global climate models (GCMs). To improve the simulation of the diurnal cycle of precipitation and understand what physical processes control it, we test a convective trigger function described in Xie et al. with additional optimizations in the NCAR Community Atmosphere Model version 5 (CAM5). The revised trigger function consists of three modifications: 1) replacing the convective available potential energy (CAPE) trigger with a dynamic CAPE (dCAPE) trigger, 2) allowing convection to originate above the top of planetary boundary layer [i.e., the unrestricted air parcel launch level (ULL)], and 3) optimizing the entrainment rate and threshold value of the dynamic CAPE generation rate for convection onset based on observations. Results from 1° resolution simulations show that the revised trigger can alleviate the long-standing GCM problem of too early maximum precipitation during the day and missing the nocturnal precipitation peak that is observed in many regions, including the U.S. southern Great Plains (SGP). The revised trigger also improves the simulation of the propagation of precipitation systems downstream of the Rockies and the Amazon region. A further composite analysis over the SGP unravels the mechanisms through which the revised trigger affects convection. Additional sensitivity tests show that both the peak time and the amplitude of the diurnal cycle of precipitation are sensitive to the entrainment rate and dCAPE threshold values.

Orbital Influence on Precipitation, Fire, and Grass Community Composition From 1.87 to 1.38 Ma in the Turkana Basin, Kenya

The Turkana Basin in northern Kenya and southern Ethiopia has yielded hundreds of hominin fossils and is among the most important localities in the world for studying human origins. High resolution climate and vegetation reconstructions from this region can elucidate potential linkages between hominin evolution and environmental change. Microcharcoal and phytoliths were examined from a 216 m (1.87–1.38 Ma) drill core (WTK13), which targeted paleo-Lake Lorenyang sediments from the Nachukui Formation of the Turkana Basin. A total of 287 samples were analyzed at ∼32–96 cm intervals, providing millennial-scale temporal resolution. To better understand how basin sediments record fire and vegetation from the watershed, the paleorecord was compared with nine modern sediment samples collected from Lake Turkana along a transect of increasing distance from the 1978 to 1979 shoreline. This included vegetation surveys and phytolith production data for species from areas proximal to the basin. We found that phytolith and microcharcoal concentrations decreased predictably moving off shore. However, phytoliths from plants sourced in the Ethiopian Highlands increased moving off shore, likely the result of increased exposure to the Omo River sediment plume. In our down-core study, microcharcoal was well-preserved but phytolith preservation was poor below ∼60 m (∼1.50 Ma). Spectral analysis revealed that microcharcoal often varied at precessional (∼21 kyr) periodicities, and through a correlation with δDwax, linked orbitally forced peaks in precipitation with elevated fire on the landscape. Phytoliths revealed that alternating mesic C4 versus xeric C4 grass dominance likely varied at precessional periodicities as well, but that grass community composition was also mediated by basin geometry. Two high eccentricity intervals of particularly high amplitude and abrupt environmental change were centered at ∼1.72 and 1.50 Ma, with the intervening period experiencing high fire variability. With the switch from lacustrine to fluvial-deltaic deposition at the core site by 1.5 Ma, mesic C4 grasses dominated and fire activity was high. This upper interval correlated to the time interval from which Nariokotome Boy (Homo erectus/ergaster) was discovered 3 km east of our drill site. Phytoliths indicated a seasonally wet and open landscape dominated by xeric C4 grasses, sedges, and other herbaceous plants.

Model intercomparison of COSMO 5.0 and IFS 45r1 at kilometer-scale grid spacing

Abstract. The increase in computing power and recent model developments allow for the use of global kilometer-scale weather and climate models for routine forecasts. At these scales, deep convective processes can be partially resolved explicitly by the model dynamics. Next to horizontal resolution, other aspects such as the applied numerical methods, the use of the hydrostatic approximation, and time step size are factors that might influence a model's ability to resolve deep convective processes. In order to improve our understanding of the role of these factors, a model intercomparison between the nonhydrostatic COSMO model and the hydrostatic Integrated Forecast System (IFS) from ECMWF has been conducted. Both models have been run with different spatial and temporal resolutions in order to simulate 2 summer days over Europe with strong convection. The results are analyzed with a focus on vertical wind speed and precipitation. Results show that even at around 3 km horizontal grid spacing the effect of the hydrostatic approximation seems to be negligible. However, time step proves to be an important factor for deep convective processes, with a reduced time step generally allowing for higher updraft velocities and thus more energy in vertical velocity spectra, in particular for shorter wavelengths. A shorter time step is also causing an earlier onset and peak of the diurnal cycle. Furthermore, the amount of horizontal diffusion plays a crucial role for deep convection with more diffusion generally leading to larger convective cells and higher precipitation intensities. The study also shows that for both models the parameterization of deep convection leads to lower updraft and precipitation intensities and biases in the diurnal cycle with a precipitation peak which is too early.