Modeling three-dimensional forest structures to drive canopy radiative transfer simulations of bidirectional reflectance factor

Three-dimensional(3-D)Monte Carlo-based radiative transfer(MCRT)models are usually used for benchmarking in intercomparisons of the canopy radiative transfer(RT)simulations.However,the 3-D MCRT models are rarely applied to develop remote sensing algorithms to estimate essential climate variables of forests,due mainly to the difficulties in obtaining realistic stand structures for different forest biomes over regional to global scales.Fortunately,some of important tree structure parameters such as canopy height and tree density distribution have been available globally.This enables to run the intermediate complexities of the 3-D MCRT models.We consequently developed a statistical approach to generate forest structures with intermediate complexities depending on the inputs of canopy height and tree density.It aims at facilitating applications of the 3-D MCRT models to develop remote sensing retrieval algorithms.The proposed approach was evaluated using field measurements of two boreal forest stands at Estonia and USA,respectively.Results demonstrated that the simulations of bidirectional reflectance factor(BRF)based on the measured forest structures agreed well with the BRF based on the generated structures from the proposed approach with the root mean square error(RMSE)and relative RMSE(rRMSE)ranging from 0.002 to 0.006 and from 0.7%to 19.8%,respectively.Comparison of the computed BRF with corresponding MODIS reflectance data yielded RMSE and rRMSE lower than 0.03 and 20%,respectively.Although the results from the current study are limited in two boreal forest stands,our approach has the potential to generate stand structures for different forest biomes.

The Characteristics of the Cold Front Cloud in Doppler Radar Reflectivity Factor

[Objective] The aim was to summarize the characteristics of refelectivity factors of Doppler radar of the cold front cloud system. [Method] Judging from the characteristics of reflectivity factors, by dint of the new generation weather radar in Harbin from 2002 to 2007, the features of the reflectivity factors of the cold front cloud system were summarized. [Result] The cloud formed by the cold front was in banded form in general. However, there was void in the cloud and its intensity was uneven. Most fast moving cold front was long and narrow banded echo and basically the radial velocity turned from northwest wind to southwest. With the changes of month, the feature of the reflective rate also changed. In winter, the cold front cloud was in layer form. The feature of the reflectivity factors was weak and in large area. However, the structure was loose and there was space in the echo. Among them, there were several strong echoes. Strong convection cell echo formed in the two sides of the cold front, and it moved with the entire cloud belt. When the dry cold front moved, regional strong convective current formed, mainly by convective cloud and small echo area. Generally, the changes of the wind direction can not be expounded from the radial velocity. However, the intensity of the convection cell was distinct, 'three-body scattering', 'side lobe echo', and 'weak echo', as well as features of super convection cell. [Conclusion] The study provided positive role for the application of Doppler radar in the surveillance of weather in Heilongjiang Province.

Application of wind profiler data to rainfall analyses in Tazhong Oilfield region,Xinjiang,China

To improve the level of meteorological service for the Oilfield region in the Taklimakan Desert, the Urumqi Institute of Desert Meteorology of the China Meteorological Administration （CMA） conducted a detection experiment by means of wind profiling radar （WPR） in Tazhong Oilfield region of Xinjiang, China in July 2010. By using the wind profiler data obtained during the rainfall process on 27 July, this paper analyzed the wind field fea- tures and some related scientific issues of this weather event. The results indicated that： （1） wind profiler data had high temporal resolution and vertical spatial resolution, and could be used to analyze detailed vertical structures of rainfall processes and the characteristics of meso-scale systems. Before and after the rain event on 27 July, the wind field showed multi-layer vertical structures, having an obvious meso-scale wind shear line and three airflows from different directions, speeding up the motion of updraft convergence in the lower atmosphere. Besides, the wind directions before and after the rainfall changed inversely with increasing height. Before the rain, the winds blew clockwise, but after the onset of the rain, the wind directions became counterclockwise mainly; （2） the temperature advection derived from wind profiler data can reproduce the characteristics of low-level thermodynamic evolution in the process of rainfall, which is capable to reflect the variation trend of hydrostatic stability in the atmosphere. In the early stage of the precipitation on 27 July, the lower atmosphere was mainly affected by warm advection which had accumulated unstable energy for the rainfall event and was beneficial for the occurrence of updraft motion and precipitation; （3） the ＂large-value zone＂ of the radar reflectivity factor Z was virtually consistent with the onset and end of the rainfall, the height for the formation of rain cloud particles, and precipitation intensity. The reflectivity factor Z during this event varied approximately in the range of 18-38 dBZ and the rain droplets formed mainly at the layer of 3,800-4,500 m.

Microwave absorption properties regulation and bandwidth formula of oriented Y_(2)Fe_(17)N_(3-δ)@SiO_(2)/PU composite synthesized by reduction-diffusion method

As concepts closely related to microwave absorption properties,impedance matching and phase matching were rarely combined with material parameters to regulate properties and explore related mechanisms.In this work,reduction–diffusion method was innovatively applied to synthesize rare earth alloy Y_(2)Fe_(17).In order to regulate the electromagnetic parameters of absorbers,the Y_(2)Fe_(17)N_(3-δ)particles were coated with silica(Y_(2)Fe_(17)N_(3-δ)@SiO_(2))and absorbers with different volume fractions were prepared.The relationship between impedance matching,matching thickness,and the strongest reflection loss peak(RLmin)was presented obviously.Compared to the microwave absorption properties of Y_(2)Fe_(17)N_(3-δ)/PU absorber,Y_(2)Fe_(17)N_(3-δ)@SiO_(2)/PU absorbers are more conducive to the realization of microwave absorption material standards which are thin thickness,light weight,strong absorbing intensity,and broad bandwidth.Based on microwave frequency bands,the microwave absorption properties of the absorbers were analyzed and the related parameters were listed.As an important parameter related to perfect matching,reflection factor(√ε_(r)/μ_(r))was discussed combined with microwave amplitude attenuation.According to the origin and mathematical model of bandwidth,the formula of EAB(RL<-10 dB)was derived and simplified.The calculated bandwidths agreed well with experimental results.

Understanding the Variability of Z-R Relationships Caused by Natural Variations in Raindrop Size Distributions(DSD):Implication of Drop Size and Number

In the issue of rainfall estimation by radar through the necessary relationship between radar reflectivity Z and rain rate R (Z-R), the main limitation is attributed to the variability of this relationship. Indeed, several pre-vious studies have shown the great variability of this relationship in space and time, from a rainfall event to another and even within a single rainfall event. Recent studies have shown that the variability of raindrop size distributions and thereby Z-R relationships is therefore, more the result of complex dynamic, thermody-namic and microphysical processes within rainfall systems than a convective/stratiform classification of the ground rainfall signature. The raindrop number and size at ground being the resultant of various processes mentioned above, a suitable approach would be to analyze their variability in relation to that of Z-R relation-ship. In this study, we investigated the total raindrop concentration number NT and the median volume di-ameter D0 used in numerous studies, and have shown that the combination of these two ‘observed’ parame-ters appears to be an interesting approach to better understand the variability of the Z-R relationships in the rainfall events, without assuming a certain analytical raindrop size distribution model (exponential, gamma, or log-normal). The present study is based on the analysis of disdrometer data collected at different seasons and places in Africa, and aims to show the degree of the raindrop size and number implication in regard to the Z-R relationships variability.

ANALYSIS AND CASE STUDY ON THE GENERATION OF RADAR ECHO BRIGHT BAND

In this study, the vertical profiles of radar refractive factor(Z) observed with an X-band Doppler radar in Jurong on July 13, 2012 in different periods of a stratiform cloud precipitation process were simulated using the Sim RAD software, and the contributions of each impact resulting in the bright band were analyzed quantitatively. In the simulation, the parameters inputted into Sim RAD were updated until the output Z profile was nearly consistent with the observation. The input parameters were then deemed to reflect real conditions of the cloud and precipitation. The results showed that a wider(narrower) and brighter(darker) bright band corresponded to a larger(smaller) amount, wider(narrower) vertical distribution, and larger(smaller) mean diameter of melting particles in the melting layer. Besides this,radar reflectivity factors under the wider(narrower) melting layer were larger(smaller). This may be contributed to the adequate growth of larger rain drops in the upper melting layer. Sensitivity experiments of the generation of the radar bright band showed that a drastic increasing of the complex refractive index due to melting led to the largest impact,making the radar reflectivity factor increase by about 15 d BZ. Fragmentation of large particles was the second most important influence, making the value decrease by 10 d BZ. The collision-coalescence between melting particles, volumetric shrinking due to melting, and the falling speed of raindrops made the radar reflectivity factor change by about 3-7d BZ. Shape transformation from spheres to oblate ellipsoids resulted in only a slight increase in the radar reflectivity factors(about 0.2 d BZ), which might be due to the fact that there are few large particles in stratiform cloud.

Character of Convective Systems Producing Short-Term Heavy Precipitation in Central China Revealed by Kilometer and Minute Interval Observations

Accurate forecasting of heavy precipitation in central China is still a challenge,within which a key issue is our still incomplete understanding of the convective systems(CSs)responsible for such events.In this study,through use of an iterative rain-cell tracking algorithm,the macroscale characteristics(scale,intensity,duration,etc.)of the CSs that produced 595 short-term heavy precipitation events in Hunan Province,central China,are quantitatively analyzed,based on radar reflectivity,echo top,and rainfall observations at 1-km and 6-min intervals in April-September of 2016-2018.The results show that CSs present significant seasonal and diurnal features.Spring CSs usually cover a larger echo area with stronger convective cores and thus generate more precipitation than summer CSs,though summer CSs develop more vigorously and frequently.CSs initiated at 1400-1600 local time are characterized by the strongest convection and a smaller spatiotemporal scale,causing violent and transient showers with typical areal precipitation of 0.5-1 mm km^(−2),but less total precipitation.Further analyses of the relationships among the scale,intensity,duration,and total precipitation of CSs reveal that the convective intensity is linearly correlated to the spatiotemporal scale of CSs,with the duration increasing on average by 0.0372 h dBZ^(−1);the echo area is significantly correlated to the total precipitation,and the duration and rainfall amount are connected with the area expansion rate(AER)of CSs:when the AER exceeds 50%,CSs expand rapidly with increasing total precipitation,but the duration is shorter.These findings provide a helpful reference for the forecasting of short-term heavy precipitation induced by CSs in central China.

Reflections on Economic and Cultural Driving Factors for the Development of Creative Industry:A Book Review on Creative Cities in Practice:European and Asian Perspectives

Creative Cities in Practice:European and Asian Perspectives Authors:Yan TANG,Klaus R.Kunzmann Year:2013Publisher:Tsinghua University Press ISBN:9787302325192(281 pages,in Chinese)The book,titled Creative Cities in Practice:European and Asian Perspectives,by Dr.Yan TANG and Prof.Klaus R.Kunzmann,is an outstanding piece of work introducing creative industry and creative cities in a fairly comprehensive and

TRMM-Observed Summer Warm Rain over the Tropical and Subtropical Pacific Ocean:Characteristics and Regional Differences

Based on the merged measurements from the TRMM Precipitation Radar and Visible and Infrared Scanner,refined characteristics（intensity,frequency,vertical structure,and diurnal variation） and regional differences of the warm rain over the tropical and subtropical Pacific Ocean（40°S-40°N,120°E-70°W）in boreal summer are investigated for the period 1998-2012.The results reveal that three warm rain types（phased,pure,and mixed） exist over these regions.The phased warm rain,which occurs during the developing or declining stage of precipitation weather systems,is located over the central to western Intertropical Convergence Zone,South Pacific Convergence Zone,and Northwest Pacific.Its occurrence frequency peaks at midnight and minimizes during daytime with a 5.5-km maximum echo top.The frequency of this warm rain type is about 2.2%,and it contributes to 40%of the regional total rainfall.The pure warm rain is characterized by typical stable precipitation with an echo top lower than 4 km,and mostly occurs in Southeast Pacific.Although its frequency is less than 1.3%,this type of warm rain accounts for 95%of the regional total rainfall.Its occurrence peaks before dawn and it usually disappears in the afternoon.For the mixed warm rain,some may develop into deep convective precipitation,while most are similar to those of the pure type.The mixed warm rain is mainly located over the ocean east of Hawaii.Its frequency is 1.2%,but this type of warm rain could contribute to 80%of the regional total rainfall.The results also uncover that the mixed and pure types occur over the regions where SST ranges from 295 to 299 K,accompanied by relatively strong downdrafts at 500 hPa.Both the mixed and pure warm rains happen in a more unstable atmosphere,compared with the phased warm rain.

Study on the Clouds Detected by a Millimeter-Wave Cloud Radar over the Hinterland of the Taklimakan Desert in April–June 2018

This study was the first to conduct high-resolution consecutive detection of clouds over the hinterland of the Taklimakan Desert(TD)from April to June 2018 based on a ground-based Ka-band millimeter-wave cloud radar(MMCR),with focus on the structure and evolution of the desert clouds.We calculated reflectivity factor(Z),cloud boundary,and liquid water content(LWC)by use of the MMCR power spectrum data,which were verified against the observations from cloud profile radar(CPR)on board Cloud Sat.The results show that the TD clouds were mostly medium and high clouds,with thickness generally less than 2 km;moreover,the mean LWCs of these clouds were less than 0.01 gm^(-3),implying that cirrus and stratiform clouds were predominant.For the observed low clouds,however,the average thickness was 3166 m and accompanying drizzles were concentrated within 2.5-4.5 km,indicating that precipitation was more likely to occur in the low clouds.The mean LWC in the TD clouds was 0.0196 gm^(-3),less than that of clean clouds.Compared to other periods,the average durations and LWCs in the TD clouds increased significantly around noon owing to obvious surface sensible heating.The average time for evolution of high clouds into low clouds was approximately 2 h,and the average maximum LWC increased from 0.008 to 0.139 gm;.The results obtained herein provide a key reference for further studies of the structure and evolution characteristics of the desert clouds.

PROPAGATION EFFECTS AND APPLICATION OF PHASE INFORMATION FOR C-BAND DUAL-POLARIZATION RADAR

As a study on exploiting and popularizing the advanced dual-polarization weather radar technique in China,the physical mechanism for the propagation effect of the radar wave is discussed by using the widely adapted extended boundary condition method,and some theoretic results are provided for improving rain measurement accuracy.Furthermore,phase information, another important characteristic quantity of microwave,is considered for tapping the potentialities of the new meteorological radar system.