An ocean wind-wave climatology for the Southern Brazilian Shelf. Part I: Problem presentation and model validation

Geomorphological features and sparse visual wave measurements, presented in previous works, point out to the possible existence of alongshore wave energy gradients over the Southern Brazilian Shelf (SBS). After describing the two numerical models utilized and the model grids and settings, the present study evaluated the proposed validation of the basin-scale results against orbital altimetry and the regional-scale results against the two available wave-buoy data, discussing the extent to which the model reproduced local reality. The basin-scale model results of significant wave height could be considered as in good agreement with observations, presenting remarkable similarities with observed altimetry. Regarding the regional-scale modeling, both significant wave height and peak wave periods were considered as in good agreement with observations. The peak wave directions, however, were classified as in poor agreement, once the ESE waves were erroneously reproduced as E. Increased spatial resolution certainly played a role, but it was the lowering of bottom friction that represented the major improvement in the coastal grid simulations.     

A Case Study on the Effects of Heterogeneous Soil Moisture on Mesoscale Boundary-Layer Structure in the Southern Great Plains, U.S.A. Part II: Mesoscale Modelling

The importance of soil moisture inputs and improved model physics in the prediction of the daytime boundary-layer structure during the Southern Great Plains Hydrology Experiment 1997 (SGP97) is investigated using the non-hydrostatic fifth-generation Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model MM5. This is Part II of a two-part study examining the relationship of surface heterogeneity to observed boundary-layer structure. Part I focuses on observations and utilizes a simple model while Part II uses observations and MM5 modelling. Soil moisture inputs tested include a lookup table based on soil type and season, output from an offline land-surface model (LSM) forced by atmospheric observations, and high-resolution ( 800 m) airborne microwave remotely sensed data. Model physics improvements are investigated by comparing an LSM directly coupled with the MM5 to a simpler force-restore method at the surface. The scale of land surface heterogeneities is compared to the scale of their effects on boundary-layer structure.The use of more detailed soil moisture fields allowed the MM5 to better represent the large-scale (hundreds of km) and small-scale (tens of km) horizontal gradients in surface-layer weather and, to a lesser degree, the atmospheric boundary-layer (ABL) height, which was evaluated against observations measured by differential absorption lidar (DIAL). The benefits of coupling an LSM to the MM5 were not readily evident in this summertime case, with the model having particular difficulty simulating the timing of maximum surface fluxes while underestimating the depth of the mixed layer.     

Variability and trends of sub-continental scale surface climate in the twentieth century. Part II: AOGCM simulations

This work presents an analysis of simulated temperature and precipitation variability and trends throughout the twentieth century over 22 land regions of sub-continental scale in the HADCM3 and HADCM2 (two realizations) coupled models. Regional temperature biases in the HADCM3 and HADCM2 are mostly in the range of -5 K to +3 K for the seasonal averages and -3 K to +2 K for the annual average. Seasonal precipitation biases are mostly in the range of -50% to 75% of present day precipitation, with a tendency in both models to overpredict cold season precipitation. Except for cold season temperature in mid- and high-latitude Northern Hemisphere regions, the average climatology of the HADCM2 and HADCM3 is of comparable quality despite the lack of an ocean flux adjustment in the HADCM3. Both models show warming trends of magnitude in line with observations, although the observed inter-regional patterns of warming trend are not well reproduced. Measures of temperature and precipitation interannual to interdecadal variability in the models are in general agreement with observations except for Northern Hemisphere summer temperature variability, which is overestimated. The models somewhat underestimate the inter-decadal variations in interannual variability measures observed during the century and overestimate the range of anomalies. Both models tend to overpredict the occurrences of short persistences (1-3 years) and underpredict the occurrence and maximum length of long persistences (greater than three years), which is an indication of a deficiency in the simulation of long-lived anomaly regimes. Compared to observations, the models produce a higher magnitude of temporal anomaly correlation across regions and correlation between temperature and precipitation anomalies for a given region. This suggests that local processes that may be effective in decoupling the observed regional anomalies are not captured well. Overall, the variability measures in the HADCM2 and HADCM3 are of similar quality, indicating that the use of a flux correction in the HADCM2 does not strongly affect the regional variability characteristics of the model.     

A Model Simulation of Future Oceanic Conditions along the British Columbia Continental Shelf. Part II: Results and Analyses

An ocean circulation model for the British Columbia continental shelf is run with future initial conditions and forcing fields downscaled from the North American Regional Climate Change Assessment Program archive. Average seasonal sea surface temperatures for the period 2065 to 2078 are projected to increase by between 0.5° and 2.0°C with respect to analogous averages from 1995 to 2008. Seasonal sea surface salinities are projected to decrease by as much as 2.0 over the same period, though there are some regions where and periods when small increases are projected. Though stronger winter winds result in larger Haida Eddies, slightly stronger summer winds along the western Vancouver Island shelf do not result in appreciable changes to either the cross-shelf upwelling or to the magnitude of Juan de Fuca Eddies or the timing of their formation. However, increased flows are projected in some seasons for the Rose Spit, Middle Bank, and Goose Island Bank eddies. More precipitation over the watersheds emptying into coastal waters produces larger freshwater discharges and, in particular, a stronger estuarine flow in Juan de Fuca Strait and a stronger Vancouver Island Coastal Current. Generally increasing winds and decreasing density mean that the winter minus summer range of sea surface heights is projected to increase all along the coast.     

A finite element-spherical harmonics model for radiative transfer in inhomogeneous clouds.: Part II. Some applications

EVENT has been used to examine the effects of 3D cloud structure, distribution, and inhomogeneity on the scattering of visible solar radiation and the resulting 3D radiation field. Large eddy simulation and aircraft measurements are used to create realistic cloud fields which are continuous or broken with smooth or uneven tops. The values, patterns and variance in the resulting downwelling and upwelling radiation from incident visible solar radiation at different angles are then examined and compared to measurements. The results from EVENT confirm that 3D cloud structure is important in determining the visible radiation field, and that these results are strongly influenced by the solar zenith angle. The results match those from other models using visible solar radiation, and are supported by aircraft measurements of visible radiation, providing confidence in the new model.     

The Role of Land-sea Distribution and Orography in the Asian Monsoon. Part II： Orography

The role of various mountains in the Asian monsoon system is investigated by AGCM simulations with different mountains. The comparison of the simulation with Asian mountains (MAsia run) with the simulation without mountains (NM run) reveals that the presence of the Asian mountains results in a stronger South Asian summer monsoon (SASM), characterized by enhanced lower-tropospheric westerly winds, upper-tropospheric easterly winds, and stronger water vapor convergence. In East Asia, the southerly winds and water vapor convergence are significantly strengthened in association with the intensified zonal pressure gradient between the East Asian continent and the Pacific Ocean. Both the dynamical and thermodynamic forcing of the Tibetan Plateau play important role in strengthening the Asian summer monsoon. In winter, the presence of Asian mountains significantly strengthens the continental high, which leads to a stronger Asian winter monsoon. The presence of African--Arabian mountains helps to intensify the exchange of mass between the Southern Hemisphere and Northern Hemisphere by strengthening the cross equatorial flows in the lower and upper troposphere over East Africa. Asian mountains also play a crucial role in the seasonal evolution of Asian monsoons. In comparison with the NM run, the earlier onset and later withdrawal of lower-tropospheric westerly winds can be found over South Asia in the MAsia run, indicating a longer SASM period. The African--Arabian mountains also moderately contribute to the seasonal variation of the South Asian monsoon. In East Asia, the clear south-to-north march of the southerly winds and subtropical rainfall starts to occur in early summer when the effects of Asian mountains are considered.     

A Regional Ensemble Forecast System for Stratiform Precipitation Events in the Northern China Region. Part II: Seasonal Evaluation for Summer 2010

In this study,the Institute of Atmospheric Physics,Chinese Academy of Sciences-regional ensemble forecast system(IAP-REFS) described in Part I was further validated through a 65-day experiment using the summer season of 2010.The verification results show that IAP-REFS is skillful for quantitative precipitation forecasts(QPF) and probabilistic QPF,but it has a systematic bias in forecasting near-surface variables.Applying a 7-day running mean bias correction to the forecasts of near-surface variables remarkably improved the reliability of the forecasts.In this study,the perturbation extraction and inflation method(proposed with the single case study in Part I) was further applied to the full season with different inflation factors.This method increased the ensemble spread and improved the accuracy of forecasts of precipitation and near-surface variables.The seasonal mean profiles of the IAP-REFS ensemble indicate good spread among ensemble members and some model biases at certain vertical levels.     

Predictability of Mediterranean climate variables from oceanic variability. Part II: Statistical models for monthly precipitation and temperature in the Mediterranean area

The objective of this study is to investigate the predictability of monthly climate variables in the Mediterranean area by using statistical models. It is a well-known fact that the future state of the atmosphere is sensitive to preceding conditions of the slowly varying ocean component with lead times being sufficiently long for predictive assessments. Sea surface temperatures (SSTs) are therefore regarded as one of the best variables to be used in seasonal climate predictions. In the present study, SST-regimes which have been derived and discussed in detail in Part I of this paper, are used with regard to monthly climate predictions for the Mediterranean area. Thus, cross-correlations with time lags from 0 up to 12?months and ensuing multiple regression analyses between the large-scale SST-regimes and monthly precipitation and temperature for Mediterranean sub-regions have been performed for the period 1950?C2003. Statistical hindcast ensembles of Mediterranean precipitation including categorical forecast skill can be identified only for some months in different seasons and for some individual regions of the Mediterranean area. Major predictors are the tropical Atlantic Ocean and the North Atlantic Ocean SST-regimes, but significant relationships can also be found with tropical Pacific and North Pacific SST-regimes. Statistical hindcast ensembles of Mediterranean temperature with some categorical forecast skill can be determined primarily for the Western Mediterranean and the North African regions throughout the year. As for precipitation the major predictors for temperature are located in the tropical Atlantic Ocean and the North Atlantic Ocean, but some connections also exist with the Pacific SST variations.     

Parameterization of Sheared Entrainment in a Well-developed CBL.Part Ⅱ:A Simple Model for Predicting the Growth Rate of the CBL

Following the parameterization of sheared entrainment obtained in the companion paper, Liu et al. (2016), the present study aims to further investigate the characteristics of entrainment, and develop a simple model for predicting the growth rate of a well-developed and sheared CBL. The relative stratification, defined as the ratio of the stratification in the free atmosphere to that in the entrainment zone, is found to be a function of entrainment flux ratio (A _e). This leads to a simple expression of the entrainment rate, in which A _e needs to be parameterized. According to the results in Liu et al. (2016), A _e can be simply expressed as the ratio of the convective velocity scale in the sheared CBL to that in the shear-free CBL. The parameterization of the convective velocity scale in the sheared CBL is obtained by analytically solving the bulk model with several assumptions and approximations. Results indicate that the entrainment process is influenced by the dynamic effect, the interaction between mean shear and environmental stratification, and one other term that includes the Coriolis effect. These three parameterizations constitute a simple model for predicting the growth rate of a well-developed and sheared CBL. This model is validated by outputs of LESs, and the results show that it performs satisfactorily. Compared with bulk models, this model does not need to solve a set of equations for the CBL. It is more convenient to apply in numerical models.     

Possible Sources of Forecast Errors Generated by the Global/Regional Assimilation and Prediction System for Landfalling Tropical Cyclones. Part Ⅱ: Model Uncertainty

This paper investigates the possible sources of errors associated with tropical cyclone (TC) tracks forecasted using the Global/Regional Assimilation and Prediction System (GRAPES). In Part I, it is shown that the model error of GRAPES may be the main cause of poor forecasts of landfalling TCs. Thus, a further examination of the model error is the focus of Part II. Considering model error as a type of forcing, the model error can be represented by the combination of good forecasts and bad forecasts. Results show that there are systematic model errors. The model error of the geopotential height component has periodic features, with a period of 24 h and a global pattern of wavenumber 2 from west to east located between 60°S and 60°N. This periodic model error presents similar features as the atmospheric semidiurnal tide, which reflect signals from tropical diabatic heating, indicating that the parameter errors related to the tropical diabatic heating may be the source of the periodic model error. The above model errors are subtracted from the forecast equation and a series of new forecasts are made. The average forecasting capability using the rectified model is improved compared to simply improving the initial conditions of the original GRAPES model. This confirms the strong impact of the periodic model error on landfalling TC track forecasts. Besides, if the model error used to rectify the model is obtained from an examination of additional TCs, the forecasting capabilities of the corresponding rectified model will be improved.     

Numerical Simulation of a Torrential Rain Event in the Northeast of Huaihe Basin. Part I: Model Verification and Analysis of MCSs

A torrential rain event accompanying Typhoon Prapiroon occurred in 2000, with 24-h rainfall amount reaching 800mm near the typhoon center. This event is simulated by the nonhydrostatic mesoscale model ARPS (V5.2), with thriple one-way nested-grids. Grid spacings of 45, 15, and 5 km are chosen for the three nested domains. The corresponding grid sizes are 75×75, 140×140, and 180×180, respectively. The NCEP/NCAR reanalysis data, radar echoes, and GMS-5 satellite images are assimilated with the ARPS model initially using a 3-D data assimilation system--ADAS. The simple ice phase scheme and the Kain-Fritsch cumulus parameterization scheme are used. There are 35 layers in the vertical, with a vertical grid spacing of about 625 m. The integration is performed up to 48 h from 0800 BT 29 to 0800 BT 31 August 2000. Compared with radar echoes, GMS-5 satellite images, and intensive surface observations, the results show that the heavy rain area down between the 500-hPa trough and the subtropical high in the left-front of Prapiroon is well simulated by the model ARPS, and the simulated rainfall centers are consistent with observations. A comparison of the radar echoes with these retrieved from the simulated hydrometeors reveals that there are meso-β scale convective systems that exhibit distinctive characteristics, and there are four convective belts converging in the vicinity of Xiangshui, where the maximum rainfall is observed. A further comparision of skew T-lgp diagrams from simulated and observed data demonstrates significant instability in this torrential rain process. The persistent vertical wind shear provides kinetic energy for the development of the MCSs, hence promoting the baroclinic development of convective cells, and the concentration of heavy rain at the specific location. The consistency between model results and observations encourages a further study of the torrential rain event using the simulation data.     

多台国内外先进天气现象仪对比分析

业务降水现象仪作为一种降水滴谱测量设备,可以用于天气现象判断,其包含的降水滴谱信息更是反映了诸多降水微物理特征,而不同设备的观测特性和精度存在一定的差异,了解不同设备的差异,对其资料的后续应用有重要的意义。本文利用2018年湖北省咸宁市黑山观测站8次液态降水过程中地面降水观测、DSG5型业务降水现象仪与同址的Parsivel型降水滴谱仪测量的降水粒子谱信息及其反演的物理参量,共1 503对样本开展对比分析,研究结果表明：(1)相比于地面自动站降水数据,Parsivel与地面观测一致性较高,DSG5则有一定的偏小。(2)相对于DSG5,Parsivel观测的粒子数浓度N_w相对较多、质量加权平均直径D_m则相对较小。(3)对于分钟雨强RR≥20 mm·h^-1的降水,Parsivel粒子直径小于(大于)3 mm数浓度高于(低于) DSG5的观测。(4)对于直径大于4 mm的粒子,Parsivel的观测的下落速度略低于理论值,DSG5则明显大于理论值。另外,2次雨雪转换过程中降水现象观测对比研究表明DSG5对降水现象有着良好的观测,相比于Parsivel,其在降水相态转换过程中的天气现象与人工观测结果一致性更好。

     

On the scavenging of SO2 by cloud and raindrops: II. An experimental study of SO2 absorption and desorption for water drops in air

For the purpose of testing our previously described theory of SO₂ scavenging a laboratory investigation was carried out in the UCLA 33 m long rainshaft. Drops with radii between 250 and 2500 m were allowed to come to terminal velocity, after which they passed through a chamber of variable length filled with various SO₂ concentrations in air. After falling through a gas separating chamber consisting of a fluorocarbon gas the drops were collected and analyzed for their total S content in order to determine the rate of SO ₂ absorption.The SO₂ concentration in air studied ranged between 1 and 60% (v). Such relatively large concentrations were necessary due to the short times the drops were exposed to SO₂ in the present setup. The present experimental results were therefore not used to simulate atmospheric conditions but rather to test our previously derived theory which is applicable to any laboratory or atmospheric condition. Comparison of our studies with the results from our theory applied to our laboratory conditions led to predicted values for the S concentration in the drops which agreed well with those observed if the drops had radii smaller than 500 m. In order to obtain agreement between predicted and observed S concentrations in larger drops, an empirically derived eddy diffusivity for SO₂ in water had to be included in the theory to take into account the effect of turbulent mixing inside such large drops.In a subsequent set of experiments, drops initially saturated with S (IV) were allowed to fall through S-free air to determine the rate of SO ₂ desorption. The results of these studies also agreed well with the results of our theoretical model, thus justifying the reversibility assumption made in our theoretical models.In a final set of experiments, the effects of oxidation on SO₂ absorption was studied by means of drops containing various amounts of H₂O₂. For comparable exposure times to SO₂, the S concentration in drops with H₂O₂ was found to be up to 10 times higher than the concentration in drops in which no oxidation occurred.     

夏季青藏高原及周边上对流层水汽质量及其向平流层传输年际异常. II：向平流层的绝热和非绝热传输

夏季7～8月青藏高原及周边地区上对流层水汽质量的年际异常分布为整体异常型和东西偶极异常型所主导。本文基于ERA-Interim再分析资料并利用HYSPLIT（Hybrid Single Particle Lagrangian Integrated Trajectory）轨迹模式,分析了两个主导分布型对应的水汽质量向平流层绝热和非绝热传输的异常特征,结果表明:青藏高原上空水汽质量整体偏多（少）时,对应南亚高压和青藏高原地区垂直向上的水汽质量非绝热输送偏强（弱）,青藏高原及周边水汽质量向平流层的绝热和非绝热传输均偏强（弱）。水汽质量整体偏多与偏少年,水汽质量向平流层绝热和非绝热传输的主要区域和层次相近,只是水汽质量整体偏多年,水汽质量向平流层非绝热传输的层次略高。当青藏高原上空水汽质量呈西多/东少分布时,对应南亚高压偏西,青藏高原西北、东北侧水汽质量向中纬度平流层的绝热传输偏强,青藏高原南侧高层水汽质量向热带平流层的经向绝热传输也偏强,而青藏高原北侧水汽质量向中纬度平流层的经向绝热传输明显减弱。同时青藏高原主体上空水汽质量向平流层的非绝热传输偏强,而青藏高原南侧高层和北侧低层水汽质量向平流层的非绝热传输偏弱。水汽质量呈西少/东多分布时有相反的结果。轨迹模式模拟的结果证实了水汽质量整体偏多年,青藏高原及周边地区绝热进入平流层的轨迹频次偏多;也证实了水汽质量呈西多/东少分布时,青藏高原西北、东北和南侧绝热进入平流层的轨迹频次偏多,而青藏高原北侧绝热进入平流层的轨迹频次偏少。