Water Vapor Conditions Research Articles

Interdecadal Variability of Spring Eurasian Snowmelt and Its Impact on Eastern China Summer Precipitation

In this study, the interdecadal variability of Eurasian spring snowmelt and its relationship with Eastern China summer precipitation (ECSP) are investigated based on observations and reanalysis data. Results show that the second mode of the empirical orthogonal function (EOF) analysis of the spring snowmelt, featured as a west-east dipole pattern, displays two interdecadal changes near the late-1970s and in the mid-2000s. The increased spring snowmelt over the Western Siberia (WSI) and the opposite situation over the Eastern Europe (EEU) are significantly linked to a meridional quadrupole summertime rainfall pattern on interdecadal time scales, with excessive rainfall over the regions of southern China (SC) and the Huang-Huai River (HHR) and deficient rainfall over the middle and lower reaches of Yangtze River Valley (YRV) and Inner Mongolia-northeastern China (IMNC). Besides, the possible mechanisms are discussed from the perspective of the hydrological effect related to snowmelt-induced soil moisture. Increased spring snowmelt can produce more water inflowing into the soil, leading to anomalous soil moisture, which can persist into summer. Excessive (deficient) snowmelt-related soil moisture anomalies over the WSI (EEU) in summer will change local land thermal conditions and thus generate cyclonic (anticyclonic) circulations with a quasi-barotropic structure. Therefore, the eastward-propagating wave-activity flux (WAF) could be strengthened over the WSI and EEU in the mid- and upper-troposphere and further propagate downstream. Combining the local response of atmospheric circulations to summer surface heating induced by local snowmelt-related SM anomalies over Mongolia, an anomalous anticyclonic (cyclonic) circulation near the Lake Baikal (SC) and the related strong descending (ascending) motion over the IMNC (SC) are founded. Besides, strong positive (negative) upper-level divergence appears over the YRV (HHR), which is the right (left) side of the exit region of upper-level jet. The upper-level divergence could be attributed to the convergence (divergence) of meridional winds on the right side of the anticyclonic circulation, thus the uniform descending (ascending) motion also appears over the YRV (HHR) through the collocation of upper-level convergence (divergence) and the compensatory lower-level circulations. Consequently, a quadrupole pattern of secondary circulation over Eastern China is founded, thereby causing the anomalously distributed ECSP under favorable water vapor conditions.

Understanding the Hydration Process of Salts: The Relation between Surface Mobility and Metastability

In this work, the relation between mobility and metastability is investigated. This is done under pure water vapor vacuum conditions. First, the properties of the metastable zone under pure water vapor conditions are investigated and compared to those under atmospheric conditions. Second, mobility is investigated using electrochemical impedance spectroscopy, whereby we differentiate between bulk and surface mobility. Using surface mobility, we showed that mobility is a prerequisite for the nucleation of K2CO3. Sufficient mobility must be introduced for the diffusion of ions and water, whereby an increase of almost two orders of magnitude (800 times) is observed on a hydrated sample. It has been shown that the metastable zone is both a consequence of the energy barrier for nucleation and the low ion mobility at low vapor pressures.

Comparative Analysis on Two Severe Convective Processes in Hunan Province in 2020

In this study, two severe convective weather processes in late March 2020 are compared and analyzed based on the surface observation data, Doppler weather radar data, satellite black body temperature data and the reanalysis data from the National Centers for Environmental Prediction. The results show that the two severe convective weather processes both can be divided into two stages, warm advection forcing stage and baroclinic frontogenesis stage. For the second process the warm advection forcing is not significant. In terms of dynamic conditions, in both processes there is low-level convergence, high-level divergence, middle-low-level southwesterly jet, and the intrusion of cold air. The upward motion in the second process is more intense than that in the first process. In both processes, the upward motion in the warm advection forcing stage is weaker than in the baroclinic frontogenesis stage. In the aspect of water vapor conditions, the divergence of water vapor flux at 850 hPa in the second process is larger than that in the first process. The water vapor convergence center rises to the level above 800 hPa and lasts for a long time. Through the comparative analysis in different stages, it is found that the water vapor convergence in the two processes in the baroclinic frontogenesis stage is stronger than that in the warm advection forcing stage. For the thermal conditions, the cold-dry air at high levels is superimposed over the warm-wet air at low levels, presenting a thermal unstable stratification. In the first process, the intensity and vertical extension of the low-level warm advection is larger than that in the second process. In both processes, the unstable energy is accumulated obviously in the warm advection forcing stage, and is released to a certain extent before the baroclinic frontogenesis stage. In the first process, there are 3 supercells within 100 km from ChangDe radar. In both processes, there are hails from 1900 BJT (Beijing Time) to 2000 BJT in Guanzhuang Town. The radar echoes both present the typical characteristics of hail. In the first process, the maximum reflectivity factor is larger, the intensity and vertical extension of mesocyclone are larger, the increase of vertical integrated liquid is more significant, and the high-level divergence is more intensive. Thus, the diameter of hail in the first process is larger.

Risk Assessment of Artifact Degradation in a Museum, Based on Indoor Climate Monitoring—Case Study of “Poni-Cernătescu” Museum from Iași City

Preservation of the cultural heritage of museums includes measures to prevent degrading effects induced by air temperature and humidity factors which are difficult to control. The present paper includes a synopsis of values of air temperature and relative humidity characterizing the indoor climate of the “Poni-Cernătescu” Museum of Iași, Romania for a period of one year. The objective of this research was to describe the museum microclimate and to identify and analyze the degradation risk of museum artifacts in order to study the impact of hygrothermal indoor and outdoor loads on indoor microclimate parameters. To achieve the objective, the following activities were carried out: acquisition of data on the relative humidity and the temperature of indoor and outdoor air; analysis of data with climate analysis tools and statistical methods; and transformation of data into quantitative and qualitative numerical measures of collection decay risks. The collected data enabled us to accurately describe the indoor climate conditions of the analyzed building. The main conclusions of the assessment were that the May–July period represented the interval with the highest degradation risk for all types of cultural assets (wood, leather, photos and paintings); this occurred because of the combination of a high amount of water vapor and high air temperature conditions. Based on charts and tabular data, this study presents the evolution of two parameters of internal microclimate, air temperature and relative humidity, and their correlation with external climate factors. The structural and functional parameters of the museum, the working levels of heating and air conditioning systems, the arrangement, the load, and the typological complexity of the artifacts displayed, were also considered in the analysis. The results obtained enabled us to develop useful recommendations to stabilize climate conditions inside the museum. Specific measures to mitigate the detrimental impact of the analyzed environmental factors are proposed. The results obtained show that in the basement, favorable conditions for mycelium growth occurred. In the summer months, across the entire museum space, the preservation indices were the lowest, from 20 to 25, so suitable conditions for storing the artifacts were not met.

Characteristics of rainstorm in Fujian induced by typhoon passing through Taiwan Island

Based on the typhoon track and intensity data and the precipitation data of typhoon in China during 1961–2020, the overall characteristics of the rainstorm in Fujian caused by typhoon passing though Taiwan Island were studied. More than 80 percent of typhoons passing though the Taiwan Island can bring heavy rain to Fujian. There are 1.5 events of typhoon rainstorm in Fujian every year, and the average annual impact days are 3.0. In terms of spatial distribution, the frequency and intensity of cross-island typhoon rainstorm decrease rapidly from the coastal areas of Fujian to the inland areas, and Zherong, Changle and Jiu xianshan stations in the coastal areas are the high value centers. The typhoon paths of cross-island typhoon rainstorm in Fujian are mainly divided into three categories: landing-Fujian type (including landing-Fujian northeast turning, landing-Fujian middle northbound and landing-Fujian south westbound), landing-Guangdong and Zhejiang type and offshore turning type, among which landing-Fujian type typhoon has the most significant influence(only the landing-Fujian type appears the rainstorm of ≥50 mm·(24 h)−1), and the rainstorm intensity, influence range and asymmetrical structure of the rainstorm are the strongest, the most extensive and the most significant in the landing-Fujian middle northbound path. Based on the NCEP reanalysis data, the comparative analysis of the environmental fields causing the difference of precipitation intensity between the two typhoons landing-Fujian middle northbound and landing-Fujian south westbound shows that: To the landing-Fujian middle northbound track, strong wind speed area on the north side of the typhoon center leads to strong onshore winds, in the role of mountain terrain, piedmont has better convergence and very strong deep vertical upward movement, with better moisture conditions, it can send low high-energy water vapor to the middle, the precipitation dynamics and water vapor conditions are significantly stronger than the landing-Fujian south westbound track, resulting in more typhoon heavy rain.

Synergistic Effects of PDO and IOD on Water Vapor Transport in the Preflood Season over South China

It is urgent to improve the prediction accuracy of precipitation in the preflood season (PFS) over South China (SC) under the background of global warming, and thus the research of water vapor conditions is the key. For the period of 1960–2012, using the daily precipitation data from 60 meteorology stations in SC and National Centers for Environmental Prediction (NCEP) reanalysis data, the synergistic effect of PDO (the Pacific Decadal Oscillation) &IOD (the Indian Ocean Dipole Mode) on water vapor transport process to frontal/monsoon precipitation is revealed, based on the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT_4.9). For the frontal precipitation, the positive PDO phase (PDO+) compared with the negative PDO phase (PDO−), there is more water vapor over the West Pacific (WP), the northern South China Sea (SCS), and the Bay of Bengal (BOB). Water vapor for frontal precipitation mainly comes from WP and SCS. When PDO and IOD are in phase resonance, the water vapor transport tracks from the SCS, WP are shorter and westward, so more water vapor is transported to SC, the precipitation efficiency of water vapor to PFS precipitation is higher too. For the summer monsoon precipitation, the tropical Indian Ocean (IO)-BOB is rich in water vapor, especially for PDO−& IOD+. The main water vapor transport tracks are the cross-equatorial flows in the IO, BOB and SCS. The precipitation efficiency of water vapor from the IO-BOB is higher for the positive IOD phase (IOD+) than that for the negative IOD phase (IOD−); however, the precipitation efficiency of water vapor from SCS is higher for the IOD− than that for IOD+. Compared with frontal precipitation, the strong westerly anomaly in the northern IO increases the water vapor transport from the north IO, BOB to SC during monsoon precipitation. For the PDO+&IOD+, the stronger Indian Low and cyclonic anomaly in the WP increases the water vapor transported from the IO-BOB to SC, improving the precipitation efficiency of water vapor. Understanding the synergistic effect of the PDO and IOD on water vapor transport will help to improve the accuracy of precipitation prediction, and reduce the negative impact of drought and flood disasters.

Environmental effects on aerosol–cloud interaction in non-precipitating marine boundary layer (MBL) clouds over the eastern North Atlantic

Abstract. Over the eastern North Atlantic (ENA) ocean, a total of 20 non-precipitating single-layer marine boundary layer (MBL) stratus and stratocumulus cloud cases are selected to investigate the impacts of the environmental variables on the aerosol–cloud interaction (ACIr) using the ground-based measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the ENA site during 2016–2018. The ACIr represents the relative change in cloud droplet effective radius re with respect to the relative change in cloud condensation nuclei (CCN) number concentration at 0.2 % supersaturation (NCCN,0.2 %) in the stratified water vapor environment. The ACIr values vary from −0.01 to 0.22 with increasing sub-cloud boundary layer precipitable water vapor (PWVBL) conditions, indicating that re is more sensitive to the CCN loading under sufficient water vapor supply, owing to the combined effect of enhanced condensational growth and coalescence processes associated with higher Nc and PWVBL. The principal component analysis shows that the most pronounced pattern during the selected cases is the co-variations in the MBL conditions characterized by the vertical component of turbulence kinetic energy (TKEw), the decoupling index (Di), and PWVBL. The environmental effects on ACIr emerge after the data are stratified into different TKEw regimes. The ACIr values, under both lower and higher PWVBL conditions, more than double from the low-TKEw to high-TKEw regime. This can be explained by the fact that stronger boundary layer turbulence maintains a well-mixed MBL, strengthening the connection between cloud microphysical properties and the below-cloud CCN and moisture sources. With sufficient water vapor and low CCN loading, the active coalescence process broadens the cloud droplet size spectra and consequently results in an enlargement of re. The enhanced activation of CCN and the cloud droplet condensational growth induced by the higher below-cloud CCN loading can effectively decrease re, which jointly presents as the increased ACIr. This study examines the importance of environmental effects on the ACIr assessments and provides observational constraints to future model evaluations of aerosol–cloud interactions.

An advanced kinetic approach to the multistep thermal dehydration of calcium sulfate dihydrate under different heating and water vapor conditions: kinetic deconvolution and universal isoconversional analyses.

This study aims to identify the kinetic features of individual reaction steps of the multistep thermal dehydration of calcium sulfate dihydrate (CS-DH) to anhydride via a hemihydrate (CS-HH) intermediate by achieving the universal kinetic description of each reaction step under different heating and water vapor pressure (p(H2O)) conditions. The mass loss processes of the thermal dehydration of CS-DH were systematically traced via humidity-controlled thermogravimetry under isothermal and linear nonisothermal conditions at various atmospheric p(H2O) values. After reconfirming the variation in the thermal dehydration pathway from a single-step dehydration to anhydride to a multistep process via the CS-HH intermediate with an increase in the p(H2O) value, the kinetic curves for each component reaction step were obtained by separating each component process from the partially overlapping mass-loss curves by kinetic deconvolution analysis as required. The induction period (IP) and the mass-loss processes of the thermal dehydrations of CS-DH to anhydride and CS-HH intermediate were compared, wherein more significant retardation effects of water vapor were observed for the IP process followed by direct dehydration to anhydride and for the mass-loss process from CS-DH to the CS-HH intermediate. The universal kinetic behavior of the thermal dehydration of the CS-HH intermediate to anhydride was compared with that of the CS-HH reagent; thus, comparable universal kinetic behaviors were observed except the reaction geometry. Based on the universal kinetic results, the key kinetic phenomenon for regulating the variation of the thermal dehydration pathway of CS-DH was discussed.

Study on Diagnosis Weather Process and Flight Impact of Heavy Snowfall in Northeast China “11/2021”

Based on ERA5 reanalysis data, the characteristics of weather situation, water vapor condition, dynamic uplift condition, energy condition, ice accumulation environment and flight effect of aircraft in the heavy snowfall process in northeast China from November 5 to 12, 2021 are analyzed. The results show that the heavy snowfall process in Northeast China is caused by the combination of Northeast China Cold Vortex, trough, low level frontal cyclone and cold front. Through the analysis of the physical field, it is found that the sufficient water vapor transport is from the south and the southeast, the convergence rising in the lower layer, divergence “pumping” in the upper layer, air flow rising in the vertical plane and a large amount of convection effective potential energy are all contributing to the heavy snowfall. The impact of heavy snowfall on flight mainly includes low visibility and ice accumulation. Water vapor flux, water vapor flux divergence, vertical velocity, potential temperature and convective effective potential energy can all be used as the judging indexes of heavy snowfall forecast.

Development of SST Validation Methodology for Upcoming SSTM/EOS-06 Sensor: A Prelude Experiment on SLSTR/Sentinel-3A

Radiometers aboard satellites provide an essential Sea Surface Temperature (SST) data archive, which is one of the key indicators of climate change. Since new radiometers are planned to fill the gap in the temporal data coverage, it is important to quantify the retrieval accuracy to utilize the data and seamlessly merge it into an existing SST data-archive. In this regard, we validate SST product from SLSTR/Sentinel-3A with in-situ measurements to create a reference frame for evaluating the new sensors products. We used NOAA <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> Quam SST dataset for validating satellite-retrievals, since it’s easily obtainable unlike skin-SST measurements. Several criteria have yielded a total of 30,353 high-quality cloud-free collocated data points between SLSTR and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> Quam SST datasets for 2017. Detailed evaluation of biases globally under different wind speeds and precipitable water vapor conditions indicates the fundamental difference between the satellite and in-situ SST, and recommendations are made for how datasets should be handled. SLSTR-retrieved skin-SST agrees with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> Quam subsurface-SST, irrespective of region and time of measurement, within -0.17±0.04K for all-day, -0.11±0.06K for daytime and -0.21±0.05K for nighttime datasets, with respective RMSE values of 0.62±0.10K, 0.58±0.09K and 0.65±0.13K. These small differences are partly due to the difference between ocean skin-temperature and subsurface-temperature, measurement uncertainties and sampling mismatch. Comprehensive validation carried out for SLSTR-retrieved SST has direct implications for future validation studies and future blended infrared, microwave and in-situ SST products.

Analysis of a Haze Weather Process in Eastern China from November 11th to 17th, 2020

This study mainly introduces a large-scale heavy haze weather process in northern China from November 11 to 17, 2020, and analyzes the weather process. Methods such as comparative analysis and data analysis were used in this study. The main reasons for the formation of smog are as follows. First, the relative humidity is high and the water vapor content is relatively high. Second, the atmosphere is in a static and stable state, and suspended particulate matter in the atmosphere is not easy to diffuse and dilute, so it will gradually accumulate in urban areas and suburban areas. Third, there is a temperature inversion in the vertical direction, and the suspended particles in the air are difficult to drift to high altitudes and are blocked at low altitudes and near the ground. Fourth, there must be cooling conditions. Meteorological factors are the external causes for the formation of smog. And inversion intensity has a negative correlation with visibility, which has some indicating importance in fog and haze predictions. Large-scale haze weather mainly occurs in the large-scale atmospheric circulation with weak cold air and good water vapor conditions, and wind near the ground is small. Due to the high humidity in haze weather, fog droplets provide adsorption and reaction sites, accelerating the conversion of gaseous pollutants to liquid particles. At the same time, particles are also easy to act as condensation nucleus to accelerate the generation of haze. When the two interact, the pollutants are not easy to diffuse outward, resulting in agglomeration effect, and the pollution is getting heavier.

Enhanced chemical stability and electrochemical performance of BaCe0.8Y0.1Ni0.04Sm0.06O3-δ perovskite electrolytes as proton conductors

Although doped BaCeO3 electrolytes are proton conductors that undergo the fast conduction processes of inter-mediate temperature solid oxide fuel cells (IT-SOFCs), their wide application is impeded by their poor chemical stability under water vapour or CO2 conditions. In this work, we selected the metallic cations with suitable electronegativity to improve the perovskite crystal structures of BaCeO3 to enhance the chemical stability and electrochemical performance. We utilized Ni and Sm cations to improve the chemical stability of BaCeO3 electrolytes in water vapour and CO2 via a novel synthesis method, and the results revealed that the BaCe0.8Y0.1Ni0.04Sm0.06O3-δ electrolyte, -with high electrochemical performance and chemical stability-, is a promising electrolyte material for conduction in IT-SOFCs. The optimum result offers new insights into synthesizing the membrane of IT-SOFCs for their application.

Terrain Effects on Regional Precipitation in a Warm Season over Qinling-Daba Mountains in Central China

The terrain effects of Qinling–Daba Mountains on reginal precipitation during a warm season were investigated in a two-month day-to-day experiment using the Weather Research and Forecasting (WRF) model. According to the results from the terrain sensitivity experiment with lowered mountains, Qinling–Daba Mountains have been found to have an obvious effect on both the spatial-temporal distribution and diurnal cycle of reginal precipitation from July to August in 2019, where the Qinling Mountains mainly enhanced the precipitation around 34° N, and the Daba Mountains mainly enhanced it around 32° N at the time period of early morning and midnight. Horizontal distribution of water vapor and convective available potential energy (CAPE), as well as cross section of vertical velocity of wind and potential temperature has been studied to examine the key mechanisms for these two mountains’ effect. The existence of Qinling Mountains intercepted transportation of water vapor from South to North in the lower troposphere to across 34° N and caused an obvious enhancement of CAPE in the neighborhood, while the Daba Mountains intercepted the northward water vapor transportation to across 32° N and caused an enhanced CAPE nearby. The time period of the influence is in a good accordance with the diurnal cycle. In the cross-section, the existence of Qinling Mountains and Daba Mountains are found to stimulate the upward motion and unstable environment effectively at around 34° N and 32° N, separately. As a result, the existence of the two mountains lead to a favorable environment in water vapor, thermodynamic, and dynamic conditions for this warm season precipitation.

Mechanical-Chemical Coupling Effects on an Environmental Barrier Coating System under High-Temperature Water Vapour Conditions.

The degradation mechanisms for environmental barrier coatings (EBCs) under high-temperature water vapour conditions are vital for the service of aero-engine blades. This study proposes a theoretical model of high-temperature water vapour corrosion coupled with deformation, mass diffusion and chemical reaction based on the continuum thermodynamics and the actual water vapour corrosion mechanisms of an EBC system. The theoretical model is suitable for solving the stress and strain fields, water vapour concentration distribution and coating corrosion degree of an EBC system during the water vapour corrosion process. The results show that the thickness of the corrosion zone on the top of the EBC system depended on water vapour diffusion, which had the greatest influence on the corrosion process. The top corroded area of the rare-earth silicate EBC system was significantly evident, and there was a clear dividing line between the un-corroded and corroded regions.

Comparative analysis of two hazard rainstorm processes in Hunan affected by the Southwest China vortex

Based on surface rain-gauge data, Doppler radar products, the National Center for Environmental Prediction (NCEP) reanalysis data and other multi-source datasets, two regional rainstorm processes caused by the Southwest China vortex (SCV) in June, 2015 are diagnosed and analyzed from the aspects of water vapor condition, dynamic and thermodynamic mechanisms, and the causes of the regional rainstorms are discussed. The results show that the two processes belong to the SCV rainstorm, and there is the southwest jet in the middle and lower levels. The high-value area of water vapor flux and the water vapor convergence center can well indicate the rainstorm area. In the first process, two water-vapor-flux transport bands from the Bay of Bengal and the South China Sea converge. In the second process, the water vapor mainly comes from the Bay of Bengal. The extreme values of dynamic and thermodynamic factors all appear near 850 hPa in the rainstorm development stage, and the latitudes of the extreme values are close to the severe rainfall area. The first process has a typical rainstorm vorticity configuration with negative vorticity at upper layer and positive vorticity at lower layer. The second process is mainly driven by the strong positive vorticity at the lower layer. There are strong energy frontal zones near the ground, and the rainstorm area is consistent with the convergence region of cold and warm air. The characteristics of radar echo show that there are mid-level and low-level jet streams in the two rainstorm processes. The first process is dominated by the cumulus precipitation, and the adverse wind region promotes the development of the rainstorm. The second process is dominated by cumuliform-stratiform-mixed cloud precipitation, and the train effect is the main cause of the regional rainstorm.

Measurement and simulation of viscosity characteristics of coal ash slag under water vapor condition in coal gasification

The long term and safe operation of entrained gasifiers strongly depends on smooth and steady removal of the ash slag. Management of slag behavior for safe slag-tapping requires the well understanding of slag viscosity temperature behavior and reliable prediction model for the slag viscosity in presence of water vapor. Based on the ratio of acidic to basic oxides (A/B) in coal ash, the viscosity temperature characteristics were studied under Ar with water vapor, and the optimization model for predicting the slag viscosity as a function of T and slag composition, including the constituent H2O was established. When water vapor is present in Ar, the slag viscosities decreased due to the breaking of Si-O-Si bridge bonds for the slags with A/B < 0.8. As A/B ≥ 0.8, the slag viscosities increased which ascribed to the formation of free hydroxyl. The viscosity prediction model for the fully molten slags and partly crystallized slags was developed under water vapor atmosphere based on the modified viscosity models in Ar. The modified Urbain model has been used to predict the completely molten slag and the remaining liquid phase for partly crystallized slags. The viscosities of heterogeneous slags were predicted by modified Roscoe model. These models can predict well the slag viscosity and Tcv. This work enables the prediction of slag viscosity at high temperature under water vapor condition and will provide a theoretical basis for the operation of entrained flow gasification, especially for the coal water slurry gasification.

Estimating Rainfall with Multi-Resource Data over East Asia Based on Machine Learning

The lack of accurate estimation of intense precipitation is a universal limitation in precipitation retrieval. Therefore, a new rainfall retrieval technique based on the Random Forest (RF) algorithm is presented using the Advanced Himawari Imager-8 (Himawari-8/AHI) infrared spectrum data and the NCEP operational Global Forecast System (GFS) forecast information. And the gauge-calibrated rainfall estimates from the Global Precipitation Measurement (GPM) product served as the ground truth to train the model. The two-step RF classification model was established for (1) rain area delineation and (2) precipitation grades’ estimation to improve the accuracy of moderate rain and heavy rain. In view of the imbalance categories’ distribution in the datasets, the resampling technique including the Random Under-sampling algorithm and Synthetic Minority Over-sampling Technique (SMOTE) was implemented throughout the whole training process to fully learn the characteristics among the samples. Among the features used, the contributions of meteorological variables to the trained models were generally greater than those of infrared information; in particular, the contribution of precipitable water was the largest, indicating the sufficient necessity of water vapor conditions in rainfall forecasting. The simulation results by the RF model were compared with the GPM product pixel-by-pixel. To prove the universality of the model, we used independent validation sets which are not used for training and two independent testing sets with different periods from the training set. In addition, the algorithm was validated against independent rain gauge data and compared with GFS model rainfall. Consequently, the RF model identified rainfall areas with a Probability Of Detection (POD) of around 0.77 and a False-Alarm Ratio (FAR) of around 0.23 for validation, as well as a POD of 0.60–0.70 and a FAR of around 0.30 for testing. To estimate precipitation grades, the value of classification was 0.70 in validation and in testing the accuracy was 0.60 despite a certain overestimation. In summary, the performance on the validation and test data indicated the great adaptability and superiority of the RF algorithm in rainfall retrieval in East Asia. To a certain extent, our study provides a meaningful range division and powerful guidance for quantitative precipitation estimation.

A machine learning-based method to account for 3D Short-Wave radiative effects in 1D satellite observation operators

Data assimilation of Short-Wave (SW, ≤ 4μm) satellite radiance in cloudy regions could potentially improve cloud forecasts. Operational SW observation operators neglect cloud 3D radiative effects, which may cause nonnegligible errors in some circumstances, for example, with large solar zenith angles or for broken clouds. This study introduced a machine learning (ML)-based method to include some cloud 3D radiative effects in a 1D observation operator based on liquid water cloud simulations by the Weather Research and Forecasting model and radiative transfer simulations by the Spherical Harmonics Discrete Ordinate Method. The inputs of the ML correction method include the reflectance simulated by a 1D observation operator at the targeting column, cloud top height, cloud water content, and effective particle size at its adjacent columns, and other ancillary information (sun-viewing geometries, water vapor conditions, etc.). Preliminary results were presented for the channel 3 (0.75 ∼ 0.90µm) of the Advanced Geostationary Radiation Imager (AGRI) onboard FY-4A. Compared with 1D simulations, the ML correction method could reduce the biases from 6.9% ∼ 11.7% to 2.0% ∼ 4.7% and the root mean square errors from 22.3% ∼ 34.8% to 15.4% ∼ 27.4%. The elapsed CPU time for the ML correction method is approximately the same or two times that of the 1D observation operators, depending on whether atmospheric gases are included or not. In general, the ML correction method is computationally more efficient than traditional 3D radiative transfer solver, and could be used to correct current 1D observation operators’ simulations.

Multiscale Causes of Persistent Heavy Rainfall in the Meiyu Period over the Middle and Lower Reaches of the Yangtze River

Based on observation data supplied by the Chinese Meteorological Administration (CMA) and reanalysis datasets provided by the ECMWF, the multiscale causes of persistent heavy rainfall events (PHREs) that occurred from 1979 to 2018 during Meiyu periods over the middle and lower reaches of the Yangtze River (MLYR) are investigated. During Meiyu periods, precipitation shows obvious interannual variabilities. In PHRE years, the contribution rate of persistent heavy rainfall to the total precipitation is approximately 57%. Precipitation also shows significant synoptic-scale (less than 10 days) characteristics. Through the quantitative diagnosis of interactions among background-scale (greater than 30 days), quasi-biweekly-scale (10–30-days), and synoptic-scale variables, the possible causes of PHREs are explored. The results reveal that the difference in precipitation intensity between PHRE years and non-PHRE years is determined by the background water vapor, background wind and synoptic-scale wind conditions. In PHRE years, the prevailing background southwesterly winds from lower latitudes provide more background water vapor, and more mean kinetic energy is converted to perturbation energy. Moreover, the active synoptic-scale oscillations from higher latitudes and the convergence of Rossby wave disturbance energy over the MLYR could also cause the occurrence and maintenance of PHREs during Meiyu periods. The multiscale causes and corresponding circulation patterns in 2020 PHREs are similar to PHREs years.

A Comparative Study on the Vertical Structures and Microphysical Properties of Stratiform Precipitation over South China and the Tibetan Plateau

Under different water vapor and dynamic conditions, and the influence of topographies and atmospheric environments, stratiform precipitation over South China and the Tibetan Plateau can produce different features. In this study, stratiform precipitation vertical characteristics, bright-band (BB) microstructures, and the vertical variations of the raindrop size distribution (DSD) over a low-altitude site (Longmen site, 86 m) in South China and a high-altitude site (Nagqu site, 4507 m) on the Tibetan Plateau were comprehensively investigated and compared using measurements from a Ka-band millimeter-wave cloud radar (CR), a K-band microrain radar (MRR), and a Parsivel disdrometer (disdrometer). A reliable BB identification scheme was proposed on the basis of CR variables and used for stratiform precipitation sample selection and further statistics and analysis. Results indicate that melting layers over the Longmen are much higher and slightly thicker than those over the Nagqu due to significant differences in atmospheric conditions. For stratiform precipitation, vertical air motions and radar variables over the two sites show different variation trends from cloud top to the ground. Vertical air motions are very weak in the stratiform precipitation over the Longmen, whereas updrafts are more active over the Nagqu. Above the melting layer, radar equivalent reflectivity factor Ze (mean Doppler velocity VM) gradually increases (decreases) as height decreases over the two sites, but the aggregation rate for ice particles over the Longmen can be faster. In the melting layer, Ze (VM) at the BB bottom/center over the Longmen is larger (smaller) than those over the Nagqu for the reason that melted raindrops in the melting layers over the Longmen are larger than those over the Nagqu. Below the melting layer, profiles of radar variables and DSDs show completely different behaviors over the two sites, which reflects that the collision, coalescence, evaporation, and breakup processes of raindrops are different between the two sites. Over the Longmen, collision and coalescence dominate the precipitation properties; in particular, from 2.0–2.8 km, the breakup process competes with collision–coalescence processes but later is overpowered. In contrast, due to the lower BB heights over the Nagqu, collision and coalescence dominate raindrop properties. Comparisons of raindrop spectra suggest that the concentration of small (medium-to-large) raindrops over the Nagqu is much higher (slightly lower) than that over the Longmen. Therefore, the mass-weighted mean diameter Dm (the generalized intercept parameter Nw) over the Nagqu is smaller (larger) than that over the Longmen.