Peak Precipitation Research Articles

Early morning peaks in the diurnal cycle of precipitation over the northern coast of West Java and possible influencing factors

Abstract. The diurnal cycles of precipitation over the northern coast of West Java have been studied using the Tropical Rainfall Measuring Mission (TRMM) Real Time Multi-satellite Precipitation Analyses (TMPA-RT) products with records spanning from 2000 to 2016, with emphasis on the occurrences of early morning precipitation peaks. Diurnal precipitation over the study area during November to March is basically characterized by precipitation peaks that occur in the afternoon to evening time (15:00–21:00 LT), but secondary peaks in the night to morning time (01:00–07:00 LT) are also pronounced in January and February. The harmonic analysis method was then applied to data of January and February to objectively determine the diurnal phase and classify the timing of precipitation for each day into three categories, i.e. afternoon-to-evening precipitation (AEP), early morning precipitation (EMP), and late morning precipitation (LMP) with peaks that occur in the time windows of 13:00–24:00, 01:00–04:00, and 05:00–12:00 LT, respectively. In terms of frequency of occurrence, AEP, EMP, and LMP constitute 55 %, 18.9 %, and 26.1 % of total samples of precipitation events. In spite of the smallest percentage, EMP events are characterized by seaward (as well as landward) propagation, flat phase distribution, and large mean amplitudes. The propagating characteristics of EMP are more prominent, with indications of stronger connectivity between precipitation systems over land and ocean, when data are composited by taking the 99th percentile values in each grid to represent extreme precipitation events. The flat phase distribution of EMP events suggests that the timing of coastal precipitation is not necessarily locked to the phase of land–sea breezes, thus allowing precipitation to occur more randomly. Furthermore, the role of the South China Sea cold tongue (SCS-CT) and cross-equatorial northerly surge (CENS) as influencing factors for the occurrences of the EMP event have also been investigated. In agreement with previous studies, we confirmed that the SCS-CT generally prevails in January and February, and morning precipitation events over the northern coast of West Java mainly occurred when there was more enhanced sea surface temperature (SST) cooling in the South China Sea. Additionally, we found that CENS is the most differential factor with regard to the phase of coastal precipitation. In this case, CENS is positively correlated with SCS-CT and, when associated with EMP events, concurrent enhancement of CENS and SCS-CT is connected to a narrow channelling of strong surface northerly wind anomalies just offshore of Indochina and the Malay Peninsula.

Comparative Evaluation of the GPM IMERG Early, Late, and Final Hourly Precipitation Products Using the CMPA Data over Sichuan Basin of China

The Global Precipitation Measurement (GPM) mission has generated global precipitation products of improved accuracy and coverage that are promising for advanced hydrological and meteorological studies. This study evaluates three Integrated Multi-satellitE Retrievals for GPM (IMERG) Hourly products, including the Early-, Late-, and Final-run products (IMERG-HE, IMERG-HL, and IMERG-HF, respectively), over Sichuan Basin of China. This highly complex terrain of the steep mountainous region offers further scrutiny on the quality and applicability of the data. The China Meteorological Precipitation Analysis (CMPA) data from January 2016 to December 2018 are used as the reference for the evaluation. Results show that: (1) At grid scale, IMERG-HL and IMERG-HF outperform IMERG-HE in terms of correlation coefficient (CC) and root-mean-square error (RMSE), but IMERG-HL has smaller relative bias (RB) than that of the IMERG-HF (by 21.16%). IMERG-HF presents the highest probability of detection (POD = 0.52) and critical success index (CSI = 0.32), except for high false alarm ratio (FAR) for light precipitation. (2) At regional scale, IMERG-HF outperforms IMERG-HE and IMERG-HL in annual evaluation in all the metrics except for the serious overestimation as shown in RB (20.18%, 3.84%, and 4.97%, respectively). Its accumulative precipitation deviation mainly comes from moderate precipitation events (1–10 mm/h), while better detection capability is seen in light precipitation (<1 mm/h). Seasonally, IMERG-HF performs the best in winter, while IMERG-HL performs the best in the other seasons. (3) IMERG-HF captures the peak precipitation more accurately in all seasons. In reproducing the diurnal cycle, IMERG-HF performs better in winter, while IMERG-HL performs better in summer and autumn, and IMERG-HE in spring. However, all three products overestimate the early morning precipitation (01:00–08:00 local standard time) of the diurnal cycle in spring, summer, and autumn.

Phase inconsistency as a major source of error in NGFS forecast

South Asian monsoon exhibits multiscale spatiotemporal variability. Analyzing the nature and behavior of numerical weather forecast error associated with these space-time heterogeneities will eventually help in improving the models. We investigate the spatiotemporal error characteristics of the National Centre for Medium-Range Weather Forecasting (NCMRWF) Global Forecast System (NGFS) model over South Asian land and ocean separately. Although error grows with lead-time, it saturates within 3–5 days of forecast initiation. The saturated error is only about 15–25% higher than that of day-1, indicating that most of the error accumulates within first 24-h of forecast. Increase in error over oceanic regions is due to an increase in the area with high error at all precipitation ranges with large day-to-day variability. However, over land error growth is primarily confined at locations of high mean precipitation. Decomposition of error arising due to intensity and phase variations reveals that about 90% of it arises from the model’s inability to capture phase of precipitation at various timescales. We show that NGFS cannot capture synoptic scale variations ($$<10$$ day) after day-2. Both the high-frequency (10–20 day) and low-frequency (30–60 day) intraseasonal variations are reasonably predicted up to day-3. At diurnal timescale, NGFS forecasts show a peak in precipitation about 3–6 h prior to that observed, both over land and ocean. Surprisingly, this error does not change with lead-time. Lastly, we show that major error characteristics do not depend on the seasonal mean monsoon rainfall.

Characteristics of Ice Cloud–Precipitation of Warm Season Mesoscale Convective Systems over the Great Plains

AbstractIn this study, the mesoscale convective systems (MCSs) are tracked using high-resolution radar and satellite observations over the U.S. Great Plains during April–August from 2010 to 2012. The spatiotemporal variability of MCS precipitation is then characterized using the Stage IV product. We found that the spatial variability and nocturnal peaks of MCS precipitation are primarily driven by the MCS occurrence rather than the precipitation intensity. The tracked MCSs are further classified into convective core (CC), stratiform rain (SR), and anvil clouds regions. The spatial variability and diurnal cycle of precipitation in the SR regions of MCSs are not as significant as those of MCS precipitation. In the SR regions, the high-resolution, long-term ice cloud microphysical properties [ice water content (IWC) and ice water paths (IWPs)] are provided. The IWCs generally decrease with height. Spatially, the IWC, IWP, and precipitation are all higher over the southern Great Plains than over the northern Great Plains. Seasonally, those ice and precipitation properties are all higher in summer than in spring. Comparing the peak timings of MCS precipitation and IWPs from the diurnal cycles and their composite evolutions, it is found that when using the peak timing of IWPSR as a reference, the heaviest precipitation in the MCS convective core occurs earlier, while the strongest SR precipitation occurs later. The shift of peak timings could be explained by the stratiform precipitation formation process. The IWP and precipitation relationships are different at MCS genesis, mature, and decay stages. The relationships and the transition processes from ice particles to precipitation also depend on the low-level humidity.

A Case Study of the Physical Processes Associated with the Atmospheric River Initial-Condition Sensitivity from an Adjoint Model

Abstract Analysis of a strong landfalling atmospheric river is presented that compares the evolution of a control simulation with that of an adjoint-derived perturbed simulation using the Coupled Ocean–Atmosphere Mesoscale Prediction System. The initial-condition sensitivities are optimized for all state variables to maximize the accumulated precipitation within the majority of California. The water vapor transport is found to be substantially enhanced at the California coast in the perturbed simulation during the time of peak precipitation, demonstrating a strengthened role of the orographic precipitation forcing. Similarly, moisture convergence and vertical velocities derived from the transverse circulation are found to be substantially enhanced during the time of peak precipitation, also demonstrating a strengthened role of the dynamic component of the precipitation. Importantly, both components of precipitation are associated with enhanced latent heating by which (i) a stronger diabatically driven low-level potential vorticity anomaly strengthens the low-level wind (and thereby the orographic precipitation forcing), and (ii) greater moist diabatic forcing enhances the Sawyer–Eliassen transverse circulation and thereby increases ascent and dynamic precipitation. A Lagrangian parcel trajectory analysis demonstrates that a positive moisture perturbation within the atmospheric river increases the moisture transport into the warm conveyor belt offshore, which enhances latent heating in the perturbed simulation. These results suggest that the precipitation forecast in this case is particularly sensitive to the initial moisture content within the atmospheric river due to its role in enhancing both the orographic precipitation forcing and the dynamic component of precipitation.

Cloud Resolving WRF Simulations of Precipitation and Soil Moisture Over the Central Tibetan Plateau: An Assessment of Various Physics Options

AbstractThe evaluation of the regional climate model is of great importance for model's developments and applications. We assessed the performance of Weather Research and Forecasting (WRF) cloud resolving simulations with various physics options in terms of precipitation and soil moisture over the central Tibetan Plateau (TP) for a 2‐month simulation from July to August in 2015. The simulated precipitation is most sensitive to the microphysics scheme, followed by the land surface model, which plays a vital role in the soil moisture simulation, while the planetary boundary layer and radiation schemes have relatively minor impacts on the precipitation and soil moisture. Specifically, the heavy precipitation event has a close relationship with the land surface model. Among the different WRF schemes, the new Thompson microphysics scheme, the Noah land surface model, the GFDL radiation scheme, and the Mellor–Yamada planetary boundary layer scheme perform relatively better than other options over the central TP. In contrast, the Lin and WRF Single‐Moment 6‐class microphysics schemes tend to simulate an earlier precipitation peak in the diurnal cycle, excessively higher intensities, and greater frequencies for high precipitation events. The Rapid Update Cycle model performs the worst in the spatiotemporal pattern of precipitation and markedly exaggerates the diurnal variation of soil moisture. These results can provide valuable guidance for further fine‐scale simulation studies of land–atmosphere interaction over the TP.

MJO and CCEW modulation on South China Sea and Maritime Continent boreal winter subseasonal peak precipitation

MJO and CCEW modulation on South China Sea and Maritime Continent boreal winter subseasonal peak precipitation

Clausius-Clapeyron Scaling of Extremely Heavy Precipitations: Case Studies of the July 2017 and July 2018 Heavy Rainfall Events over Japan

The usefulness of Clausius–Clapeyron (CC) scaling in explaining extremely heavy precipitations is explored in the present-day climate and in pseudo-global-warming (PGW) conditions. This is analyzed by conducting regional-scale numerical simulations at 1-km grid resolution for two recent extreme rainfall events that occurred in Japan: the case in northern Kyushu during 5–6 July 2017 and the case in Shikoku Island during 5–8 July 2018. The Weather Research and Forecasting (WRF) model was used for the simulation, and the data samples were collected at each grid point individually for each hour over the two regions. We found that the frequency and intensity of extremely heavy precipitation associated with the two events are increased under PGW conditions. The extremely heavy precipitations (> 50 mm h−1) followed CC scaling for the temperatures up to 22°C in the present-day climate, while those under the PGW conditions followed CC-scaling up to 24°C. The peak intensity of the extremely heavy precipitations in the precipitation-temperature relationship is found as ∼ 140 mm h−1 at 25°C in the present-day climate, while the same with PGW conditions is projected as ∼ 160 mm h−1 at 27°C. The increasing rate of the extremely heavy precipitations in the present-climate condition is noticed as ∼ 3 % °C−1 and that under the PGW conditions is anticipated as ∼ 3.5 % °C−1. The increase in peak precipitation intensity and the rate of precipitation increase against temperature in future warming climate are attributed to the decrease in temperature lapse rate and increase in atmospheric water vapor and convective available potential energy. To our knowledge, this is one of the first quantitative investigations of CC scaling of extremely heavy precipitations based on case studies.

Seasonal variations in the lake-water oxygen isotope composition of four lakes in the East Asian summer monsoon region: Implications for the interpretation of paleo-isotope records

The scarcity of modern process studies of the relationship between climatic variables and lake-water oxygen isotope (δ18OLW) composition has restricted our understanding of the climatic significance of oxygen isotope (δ18O) records from lake sediments. We measured changes in δ18OLWat four lakes (Nanyihu, Gonghai, Mayinghai and Xihaizi) at various locations across the East Asian summer monsoon (EASM) region, from December 2016 to January 2018. The variations in δ18OLWat Lake Nanyihu, located in the lower reach of the Yangtze River, are controlled by precipitation oxygen isotope (δ18OP) composition, temperature and evaporation during the non-monsoon season. However, the variations in δ18OLWat the lakes close to the northern limit of the EASM (Gonghai, Mayinghai and Xihaizi) are affected not only by δ18OP, temperature and evaporation, but also by the replenishment of groundwater during the non-monsoon season. In general, during the non-monsoon season, the variation of δ18OLWexhibits a variety of patterns and the controlling factors across the EASM region are complex. During the monsoon season, although the effect of evaporation-induced isotopic enrichment still affects δ18OLW, the variations in δ18OLWare more influenced by the supply of abundant precipitation with lower δ18OPto the lakes, which caused a substantial decrease in δ18OLWacross the EASM region in this season. Therefore, the variations in δ18OLWmainly reflect the δ18OPof the EASM during the monsoon season. However, the timing of the peak in monsoon precipitation does not correspond well to minima in δ18OLW, and correlation analysis reveals that there is only a weak relationship between δ18OLWand precipitation/humidity at all four lakes during the monsoon season. These findings demonstrate that although δ18OLWcan represent the δ18OPof the EASM, it does not reflect precipitation/humidity during the monsoon season. Overall, our findings indicate that the seasonal variations of δ18OLWacross the EASM region are controlled by different climatic variables, which implies that the interpretation of lacustrine δ18O records across the EASM region as an indicator of regional effective precipitation/humidity needs to be reassessed.

Decadal long convection-permitting regional climate simulations over eastern China: evaluation of diurnal cycle of precipitation

With a decadal long period (1998–2010) climate simulation using the Weather Research and Forecasting model at convection-permitting resolution (4 km) (WRF_CPM), the diurnal cycles of precipitation amount (PA), frequency (PF) and intensity (PI) and their related large-scale atmospheric circulations over eastern China are analyzed. The simulations are further compared against the CN05.1, CMORPH v1.0 and the ECMWF Re-Analysis Interim (ERAIN). Results show that WRF_CPM can reasonably represent the observed seasonal rainfall and the atmospheric circulations. As for the features at a sub-daily scale, WRF_CPM is superior at reproducing the diurnal amplitude of PF that is similar to PA in terms of the spatial distribution. Moreover, the diurnal peak timing of summer PF and PA over the three sub-regions, i.e., North China (NC), Yangtze-Huaihe River basin (YHR) and South China (SC), can be properly reproduced by WRF_CPM. The observed precipitation systems exhibit obvious eastward propagation from the Plateau to its downstream, which may be due to the solenoid circulations associated with the low-level anomalous wind and moisture convergence. However, they are almost overestimated by WRF_CPM and in turn causing overestimated precipitation along YHR. The early morning precipitation in WRF_CPM has a larger fraction than CMORPH, which is related to the overestimated nocturnal low-level jet. Whereas, due to the solar heating and the land-sea breezes, the late-afternoon precipitation peak is mainly located along the coasts of eastern China, which matches well with the vertical motion in WRF_CPM.

Land–Sea Contrast in the Diurnal Variation of Precipitation from Landfalling Tropical Cyclones

AbstractRecent composite analysis of landfalling tropical cyclones (TCs) suggests a rain rate peak in the early morning, which contradicts the typically observed peak in convective precipitation over land seen in the late afternoon to early evening. We conducted a set of idealized simulations of TCs and analyzed observational data from TC Bebinca (2018), which stalled near the shoreline of southern China. We show a distinct land–sea contrast in the diurnal variation of TC precipitation and an 8–12 hr offset between the peak precipitation time over land compared with that over the sea in a TC that stalls at the shoreline. The highest land surface temperature and maximum low‐level buoyancy during the afternoon led to peak precipitation over land at this time. However, the peak precipitation over the sea in the early morning was generated by the increase in relative humidity caused by nighttime radiative cooling and enhanced instability.

Simulation of the responses of rainstorm in the Yangtze River Middle Reaches to changes in anthropogenic aerosol emissions

The model WRF-Chem sensitivity simulation experiments with changing intensity of anthropogenic emissions sources were applied to simulate a rainstorm process in the Yangtze River Middle Reaches (YRMR) during June 18–19, 2018 to study the responses of clouds and precipitation in the rainstorm to changes in aerosol concentrations in this region. The simulation experiments revealed that the aerosol-cloud interaction during low and high emission phases tended to inhibit and to enhance the precipitation process with the precipitation peak lagging 1–2 h. In the later period of rainstorm, high concentrations of aerosols improved precipitation efficiency significantly, resulting in more centralized clusters of intense precipitation. The cloud droplet number concentrations and cloud water contents demonstrated an increasing logarithmic relationship with increasing PM2.5 concentrations. The PM2.5 concentration of about 25 μg/m3 was estimated as the response threshold of cloud droplet number concentrations from sharp to smooth changes. Before and after the peak precipitation, the relationship between the average precipitation rates and PM2.5 concentrations presented an inverse power function. Aerosol-induced precipitation changes were sensitive to ambient relative humidity (RH). When 80% ≤ RH < 85%, the response of precipitation to aerosol emissions was in equilibrium. When RH < 80% or RH > 85% increasing anthropogenic aerosol emissions tended to inhibit or enhance precipitation, especially in the case of low (high) aerosol emissions.

Land Surface Phenologies and Seasonalities in the US Prairie Pothole Region Coupling AMSR Passive Microwave Data with the USDA Cropland Data Layer

Land surface phenologies and seasonalities in the US Prairie Pothole Region (PPR) were characterized using land surface variables derived from the coarse spatial resolution (25 km) Advanced Microwave Scanning Radiometer (AMSR) blended data for 2003 to 2016 linked with the optically based USDA NASS Crop Data Layer (CDL) at a much finer spatial resolution. Two transects of AMSR pixels—one in east-central North Dakota and the other in eastern South Dakota—were selected for analysis. The AMSR data were grouped earlier (2003–2005, 2007) and later (2013–2016) to emphasize temporal change and to avoid data discontinuity in 2011–2012 and a major drought in 2006. The nonparametric Mann-Kendall trend test on the CDL data revealed that area in grasslands and wetlands strongly decreased in both transects, while corn and soybean coverage strongly increased. In crop-dominated sites, the AMSR vegetation optical depth (VOD) time series caught the early spring growth, ploughing, and crop growth and senescence. In contrast, the VOD time series at grassland dominated sites exhibited a lower peak but extended growth period. Crop-dominated sites had significantly higher amplitude VODs in both periods and transects. Based on the paired two-sample t-test, neither the peak VOD amplitude nor the peak VOD timing measured in accumulated growing degree-days was significantly different between temporal groups in the North Dakota transect. In contrast, in South Dakota, both the peak VOD amplitude and its timing were significantly different with shifts to later peak timing during the 2013–2016 period. In addition, in South Dakota but not North Dakota, there were significantly earlier shifts in the timing of peak growing degree-days and peak precipitation water vapor. Both spatial and temporal changes in AMSR land surface variables are linked to shifts in land cover along the South Dakota transect as revealed in the CDL data. More research is required to understand the dynamics evident in the passive microwave time series.

Improvement of elevated-temperature strength and recrystallization resistance via Mn-containing dispersoid strengthening in Al-Mg-Si 6082 alloys

The precipitation behavior of Mn-containing dispersoids in Al-Mg-Si 6082 alloys with different Mn contents (0, 0.5 and 1.0 wt%) during various heat treatments (300–500 °C) was investigated. The effects of dispersoids on elevated-temperature strength and recrystallization resistance during hot-rolling and post-rolling annealing were evaluated. The results showed that the dispersoids in the Mn-containing alloys (0.5 and 1.0%) began to precipitate at 350 °C and reached the optimum conditions after 2–4 h at 400 °C. However, the dispersoids coarsened with increasing holding time at temperatures above 450 °C. After the peak precipitation treatment at 400 °C for 2 h, the yield strength at 300 °C increased from 28 MPa (base alloy free of Mn) to 55 MPa (alloy with 0.5% Mn) and 70 MPa (alloy with 1% Mn), respectively, demonstrating a significant dispersoid strengthening effect at elevated temperature. In addition, the dispersoids were thermally stable at 300 °C for up to 1000 h holding owing to its relative high precipitation temperature (350–400 °C), leading to the superior constant mechanical performance at elevated temperature during the long service life. During hot rolling and post-rolling annealing, the presence of a large amount of dispersoids results in the higher Zener drag PZ compared with base alloy and then significantly improved the recrystallization resistance. The alloy containing 0.5% Mn exhibited the highest recrystallization resistance among three experimental alloys studied during the post-rolling process, likely resulted from the lower coarsening rate of dispersoids and the lower dispersoids free zone.

Influence of multiphase microstructure evolution and Fe-rich phases on Cu uphill diffusion in Al-Si-Cu-Mg alloys

The evolution of multiphase microstructures in Al-Si-Cu-Mg alloys has been investigated by studying four types of alloys with different compositions and solidification rates. The experiment has been conducted with and without 0.6 wt% Zn addition by using the permanent mould and sand mould. Scanning electron microscope with energy dispersive spectroscopy, differential scanning calorimetry and metallographic statistics were applied to determine the evolution of the multiphase microstructure in the solution treatment. The texture and morphology of the Fe-rich phases on the uphill diffusion of Cu atoms was revealed. The results indicate that the existing forms of multiphase microstructures, especially the crystal texture and morphology of Fe-rich phases, were different with Zn addition and secondary dendrite arm spacing, which resulted from different solidification processes. In sand casting, large-sized acicular β-Fe was modified by Zn into short rod-like free β-Fe, while in the permanent mould, many π-Fe phases were formed with Zn addition. Unlike the decomposition of the π-Fe phase, the free β-Fe in the alloy 3DSZ can directly provide a low-energy grain boundary for the uphill diffusion of Cu atoms, which shortens the precipitation peak time of the Al2Cu. Two schematic diagrams were developed to describe the effect of decomposition of the π-Fe and free β-Fe phases on the uphill diffusion of Cu atoms.

Temporal relations between precipitable water vapour and precipitation during wet seasons based on nearly two decades of data from the Lhasa River valley, Tibetan Plateau

AbstractHourly precipitation and ground‐based Global Positioning System sensing precipitable water vapour (PW) measurements are analysed to better understand the relationships between PW and precipitation at small time scales. The measurements were made during the wet seasons—from May to September each year—between 2001 and 2017 at the Lhasa River Valley, a region representative over the Tibetan Plateau. In addition, we validate the power law expected from the critical phenomena in atmospheric precipitation using hourly PW and precipitation observations made in the region. The results show that peaks in PW, low cloud cover and precipitation values, and high frequencies of lightning usually occur under conditions of convective activities during the wet season which extends from May to September at the Lhasa River Valley. PW is found to have considerable autocorrelation periods that are twice longer as those in tropical regions. In addition, composite analysis results show that precipitation increases with PW. In the time range extending 36 to 7 hr prior to precipitation events, PW increases slowly. This phenomenon is likely related to synoptic‐scale atmospheric activities. From 6 to 0 hr prior to precipitation events, PW increases rapidly—a change that is likely to be associated with mesoscale convective activities. The peak of PW is about 1 hr before the peak of precipitation, indicating that PW increases stimulate precipitation occurrences. Furthermore, our results show that the power law represents well the relationship between PW and precipitation with a PW critical value of 21.5 mm for Lhasa River valley. Also, this critical value is a tipping point for the number distribution of PW measurements. When the PW is less than the critical value, the number of PW measurements increases gradually along with the growth of PW. Otherwise, it drops rapidly.

Improved representation of the diurnal variation of warm season precipitation by an atmospheric general circulation model at a 10\xa0km horizontal resolution

This study investigates the diurnal variation of the warm season precipitation simulated by the Goddard Earth Observing System version 5 atmospheric general circulation model for 2 years (2005–2006) at a horizontal resolution of 10 km. The simulation was validated with the satellite-derived Tropical Rainfall Measuring Mission (TRMM) 3B42 precipitation data and the Modern-Era Retrospective analysis for Research and Applications atmospheric reanalysis for atmospheric winds and moisture. The simulation is compared with the coarse-resolution run in 50 km to examine the impacts driven by resolution change. Overall, the 10 km model tends to reproduce the important features of the observed diurnal variation, such as the amplitude and phase at which precipitation peaks in the evening on land and in the morning over the ocean, despite an excessive amplitude bias over land. The model also reproduces the realistic propagation patterns of precipitation in the vicinity of ocean coasts and major mountains. The regional characteristics of the diurnal precipitation over two regions, the Bay of Bengal and the Great Plains in North America, are examined in detail, where the observed diurnal cycle exhibits a systematic transition in the peak phase due to the development and propagation of regional-scale convective systems. The model is able to reproduce this pattern as well as the diurnal variation of low-level wind and moisture convergence; however, it is less effective at representing the nocturnal peak of precipitation over the Great Plains. The model results suggest that increasing the horizontal resolution of the model to 10 km substantially improves the representation of the diurnal precipitation cycle. However, intrinsic model deficiencies in topographical precipitation and the accurate representation of mesoscale convective systems remain a challenge.

Hydrological mass variations in the Nile River Basin from GRACE and hydrological models

Terrestrial water storage (TWS) change is a key component of the global water cycle and hydrologic cycle. Therefore, it is of significance to quantify TWS variations at large scale and understand how the climate changes affect in the water systems, particularly in the Nile River Basin. In this study, TWS variations in the Nile River Basin are estimated and investigated from January 2003 to August 2016 by using the Gravity Recovery and Climate Experiment (GRACE). Also, the Global Land Data Assimilation System (GLDAS) and Tropical Rainfall Measuring Mission (TRMM) are used to understand the causes of the TWS variations in the Nile River Basin (NRB). The peak of precipitation has happened in July and August. After one month the appearance of the influence of precipitation on soil moisture is clear. On the other hand, after two months the same effect of precipitation in TWS is clear. While the peak of soil moisture has occurred in August and September, the maximum of TWS has observed in September and October. The maximum annual TWS from GRACE measurements is 42.66 mm in NRB during September–November (autumn), and its minimum is −33.77 mm during March–May (spring). Also, the maximum annual soil moisture is 25.22 mm in NRB during September–November (autumn), and its minimum is −23.34 mm during March–May (spring) while the peak precipitation is 100.11 mm during June–August (summer) and its minimum is 14.62 mm during December–February (winter). The trend of TWS variations in NRB is 0.04 mm/yr from GRACE. Furthermore, the TWS variations are mainly dominated by the soil moisture in the Nile River Basin with the correlation coefficient of 0.88. In addition, the correlation factor between the non-seasonal TWS and ScPDSI is 0.60, indicating that GRACE-derived TWS well capture the most severe droughts occurred in 2006 and 2011.

Contribution of the main moisture sources to precipitation during extreme peak precipitation months

A worldwide study is presented to understand the role of major global moisture source regions in the occurrence of extreme monthly precipitation over the continents. The Lagrangian model FLEXPART v9.0 was used to analyse the moisture transport for precipitation, and the extreme precipitation was calculated for each grid point (at 0.25°) using the Multi-Source Weighted-Ensemble Precipitation (MSWEP) monthly precipitation dataset. Although the eventual aim of this work is to show the link between the contributions from the sources to their sinks during months of extreme rainfall, we provide, for the first time, much-needed information on these contributions at a monthly scale – previously this information was only available at an annual scale. We also provide maps showing global monthly moisture sources, together with the identification of the peak precipitation month (PPM) and of the preferred moisture source for precipitation in that month.

Simulation of the effects of sea‐salt aerosols on cloud ice and precipitation of a tropical cyclone

AbstractThis study used the Weather Research and Forecasting model with chemistry (WRF‐Chem) with a parameterization for the emissions of sea‐salt aerosols to assess the effects of sea‐salt aerosols on a tropical cyclone. A control simulation (Ctl) and a simulation with a sea‐salt emission flux that is one‐tenth of that in Ctl (Low) were conducted. Results show that sea‐salt aerosols enhance the condensation process, promoting vertical motion and precipitation in a tropical cyclone. Peak precipitation value in Ctl is 120% that in Low. Enhanced vertical motion in Ctl results in increase of water vapor transport to the upper atmosphere, which promotes deposition growth of cloud ice crystals. Therefore, cloud ice mixing ratio and stratiform precipitation in Ctl are larger than those in Low. Stratiform precipitation in the rain band is about 0.5 mm/h in Ctl and about 0.4 mm/h in Low.