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Yang P.,China Meteorological Administration Training Center | Ren G.,National Climate Center | Liu W.,Institute of Urban Meteorology
Journal of Applied Meteorology and Climatology | Year: 2013

An hourly dataset of automatic weather stations over Beijing Municipality in China is developed and is employed to analyze the spatial and temporal characteristics of urban heat island intensity (UHII) over the built-up areas.Atotal of 56 stations that are located in the built-up areas [inside the 6th Ring Road (RR)] are considered to be urban sites, and 8 stations in the suburban belts surrounding the built-up areas are taken as reference sites. The reference stations are selected by using a remote sensing method. The urban sites are further divided into three areas on the basis of the city RRs. It is found that the largest UHII generally takes place inside the 4th RR and that the smallest ones occur in the outer belts of the built-up areas, between the 5th RR and the 6th RR, with the areas near the northern and southern 6th RR experiencing the weakest UHI phenomena. On a seasonal basis, the strongest UHII generally occurs in winter and weak UHII is dominantly observed in summer and spring. The UHII diurnal variations for each of the urban areas are characterized by a steadily strong UHII stage from 2100 local solar time (LST) to 0600 LST and a steadily weak UHII stage from 1100 to 1600 LST, with the periods 0600-1100 LST and 1600-2100 LST experiencing a swift decline and rise, respectively. UHII diurnal variation is seen throughout the year, but the steadily strong UHII stage at night is longer (shorter) and the steadily weak UHII stage during the day is shorter (longer) during winter and autumn (summer and spring). © 2013 American Meteorological Society.

News Article | November 22, 2016
Site: www.eurekalert.org

The Beijing Metropolitan Region (BMR) often suffers from heavy rainfall events. The complex topography with the Yan Mountains to the north and the Taihang Mountains to the west, as well as the diverse underlying urban surfaces make it much difficult to predict convective initiations (CIs) and local heavy rainfall events over the BMR. Especially, the CIs and consequent heavy rainfall events occurring under weak synoptic forcing are extremely great challenges to today's operational NWP models and even experienced forecasters. Recently, scientists from the Institute of Atmospheric Physics/CAS, University of Maryland, Institute of Urban Meteorology, and Beijing Meteorological services performed observational and modeling studies on two local heavy rainfall events over the BMR occurring under weak synoptic forcing. The CI processes were specially investigated. Their results revealed that the cold pool outflows associated with precipitation systems around the BMR, the underlying urban surface and the local topography dominated where and when CIs and consequent heavy rainfall occurred. A sudden local heavy rainfall event produced by several scattered convective storms in 2008 interrupted the scheduled matches of the ongoing 2008 Beijing Olympic Games. The detailed observational analysis showed that small-scale topography and cold pool outflows were two key influencing factors in the development of the convective storms. As for another local heavy rainfall event over the BMR in 2011, an isolated convective storm was initiated suddenly over Haidian district of Beijing, far away from the outflow boundary associated with a precipitation system over the northwestern mountains. They revealed that the "northwestward-concaved valley" near Haidian and Changping districts and the urban surface accounted for the formation of a favorable convergence zone near the border of the above two districts, facilitating the confluence of high equivalent potential temperature air. The isolated convective outbreak would not be possible without the sustained low-level convergence of high equivalent potential temperature air between south- to southeasterly flows and a northerly flow. The latter occurred far ahead of the outflow boundary associated with a convectively generated cold pool by the northwestern precipitation system. Their studies revealed the mechanisms for the development of local storms and consequent heavy rain in the BMR, which have important implications for improving relative prediction skills. The results were recently published in Atmospheric Science Letters and Monthly Weather Review.

Wang H.,U.S. National Center for Atmospheric Research | Sun J.,U.S. National Center for Atmospheric Research | Fan S.,Institute of Urban Meteorology | Huang X.-Y.,U.S. National Center for Atmospheric Research
Journal of Applied Meteorology and Climatology | Year: 2013

An indirect radar reflectivity assimilation scheme has been developed within the Weather Research and Forecasting model three-dimensional data assimilation system (WRF 3D-Var). This scheme, instead of assimilating radar reflectivity directly, assimilates retrieved rainwater and estimated in-cloud water vapor. An analysis is provided to show that the assimilation of the retrieved rainwater avoids the linearization error of the Z-qr (reflectivity-rainwater) equation. A new observation operator is introduced to assimilate the estimated in-cloud water vapor. The performance of the scheme is demonstrated by assimilating reflectivity observations into the Rapid Update Cycle data assimilation and forecast system operating at Beijing Meteorology Bureau. Four heavy-rain-producing convective cases that occurred during summer 2009 in Beijing, China, are studied using the newly developed system. Results show that on average the assimilation of reflectivity significantly improves the short-term precipitation forecast skill up to 7 h. Adiagnosis of the analysis fields of one case shows that the assimilation of reflectivity increases humidity, rainwater, and convective available potential energy in the convective region. As a result, the analysis successfully promotes the developments of the convective system and thus improves the subsequent prediction of the location and intensity of precipitation for this case. © 2013 American Meteorological Society.

Hu H.,Institute of Urban Meteorology
Journal of Applied Meteorology and Climatology | Year: 2015

The rainfall data of 20 rain gauge stations are used for analysis on the spatiotemporal characteristics of rainstorm-induced hazards in Beijing. A local model used to calculate rainstorm hazards index values (RHIVs) has been developed to reflect the degree of rainstorm-induced hazards. The Mann-Kendall test on the RHIVs series recognizes 1984 as the changepoint in the series, which is shortly after the beginning of a rapid urban expansion period in 1981. The RHIV trend analysis reveals that the trends indices of all stations are negative before 1984 but mostly positive after 1984. Although the climate in north China shows drought conditions, and the annual rainfalls have decreased in recent years, no relationship is implied to a reduction in rainstorm-induced hazards. By using the lognormal distribution model, the probability analysis on the RHIVs of 20 stations indicates that very extreme precipitation occurred in increasing frequency after 1984. Moreover, the estimated spatial distributions of 100-, 150-, and 200-mm daily rainfall exceedance probabilities (EPs) indicate that these EPs have increased mainly in the urban areas and northward, which are the downwind of the summer monsoon, whereas the EPs to the south of the urban areas have decreased since 1984. Such spatiotemporal characteristics of the RHIVs can be attributed to modification of precipitation by the changed land use and land cover in urban areas. Moreover, the urban-induced rainfall downwind of the urban areas enhanced the rain intensity and rain rate, which led to an increase in RHIVs and contributed to the frequent occurrences of flash floods in Beijing metropolis. © 2015 American Meteorological Society.

Sun J.,U.S. National Center for Atmospheric Research | Chen M.,Institute of Urban Meteorology | Wang Y.,Institute of Urban Meteorology
Weather and Forecasting | Year: 2010

The Variational Doppler Radar Analysis System (VDRAS) was implemented in Beijing, China, and contributed to the Beijing 2008 Forecast Demonstration Project (B08FDP) in support of the Beijing Summer Olympics. VDRAS is a four-dimensional variational data assimilation system that produces frequently updated analyses using Doppler radar radial velocities and reflectivities, surface observations, and mesoscale model data. The system was tested in real time during the B08FDP pretrials in the summers of 2006 and 2007 and run during the Olympics to assist the 0-6-h convective weather nowcasting. This paper provides a description of the upgraded system and its Beijing implementation, an evaluation of the system performance using data collected during the pretrials, and its utility on convective weather nowcasting through two case studies. Verification of VDRAS wind against a wind profiler shows that the analyzed wind is reasonably accurate with a smaller RMS difference for 2006 than for 2007 due to better radar data coverage in 2006. The analyzed cold pools in three convective episodes are compared with surface observations at selected stations. The result shows good agreement between the analysis and the observations. The two case studies demonstrate the role that VDRAS could play in nowcasting convective initiation. © 2010 American Meteorological Society.

Zheng Z.-F.,Institute of Urban Meteorology
Advanced Materials Research | Year: 2011

In order to analyze the variations of climate warming and human body comfort index on the basis of homogenized correction, the daily meteorological data from 1960 to 2009 in Beijing are examined. According the investigations, some conclusions could be drawn. Firstly, in the last 50 years, the warming rate is 0.39/10a in Beijing, but the warming is unsymmetrical. The warming rate of annual average lowest temperature is obviously higher than that of annual average highest temperature. Secondly, the main discomfort climatic factor is cold discomfort, while thermal discomfort is rare, so is extremely cold discomfort. Thirdly, climate warming and body comfort index are closely correlated in Beijing. The overall trend resulted in by climate warming is the rising in the number of comfortable days, declining in the number of uncomfortable days, and the rate of change is increasing. What's more, investigations also show that, the heating (cooling) degree days in Beijing dropped (rose) obviously, suggesting that the energy consumption for air conditioners will increase in summer and decrease in winter in Beijing in the future. © (2011) Trans Tech Publications.

Hu H.,Institute of Urban Meteorology
International Journal of Geographical Information Science | Year: 2014

An algorithm was developed for converting radar data from matrix format into polygons that can be easily visualized, processed, and analyzed using Geographic Information Systems. Spatial operators can be used to overlap radar polygons with land surface features represented by points, lines, and polygons to meet the demands of severe weather identification and tracking and risk recognition. Application and testing of the algorithm demonstrate that the converted radar polygons are suitable for use in weather modification, risk assessments of flash floods in urban areas, and the identification of lightning activity for lightning risk recognition, all of which are essential in real-time severe weather monitoring and warning. © 2014 Taylor & Francis.

Meng C.,Institute of Urban Meteorology
Meteorology and Atmospheric Physics | Year: 2016

Seasonal snow plays an important part in Earth’s climate system. Snow cover regulates the land surface energy balance through altering the albedo of the land surface. To utilize the satellite-retrieved snow cover fraction (SCF) and snow depth (SD) data sufficiently and avoid inconsistency, this paper developed a very simple but robust quality control method to assimilate Fengyun satellite-retrieved SCF and SD simultaneously. The results show that the assimilation method which this paper implemented can not only utilize the satellite-retrieved SCF and SD data sufficiently but also avoid the inconsistency of them. Two experiments were designed and performed to quantify the impacts of snow on land surface energy balance using the integrated urban land model. With the increase of the SCF and SD, the net radiation decreased significantly during the day and increased a little at night; the sensible heat flux decreased significantly during the day; the evapotranspiration and ground heat flux decreased during the day too. © 2016 Springer-Verlag Wien

Meng C.,Institute of Urban Meteorology
Journal of Advances in Modeling Earth Systems | Year: 2015

An integrated urban land model (IUM) was developed based on the Common Land Model (CoLM). A whole layer soil evaporation parameterization scheme was developed to improve soil evaporation simulation especially in arid areas. For the urban underlying surface, the energy and water balance model were modified; urban land parameters such as the anthropogenic heat (AH), albedo, surface roughness length, imperious surface evaporation etc. were also reparameterized. IUM was validated and compared with CoLM and the urbanized high-resolution land data assimilation system (u-HRLDAS) in single and regional scale. The validation results indicate that IUM can improve the simulation of land surface parameters and land-atmosphere interaction fluxes. © 2015. The Authors.

In this study, a genetic programming (GP) algorithm is introduced to solve the symbolic regression problem for flash flood risk zoning in Beijing. GP operates in simulation of biological revolution and can avoid arbitrariness in risk estimates. Herein, this revolutionary computing searched for an appropriate model to best fit the training samples which comprise the data fields of the predictand of Ripley’s K-values to be the posterior risk, and the predictors of the rainstorm hazard index value (RHIV), physical vulnerability, terrain factor, impervious surface area, and population density. After generations of revolution, the optimal fit regressions for estimating the risk value were determined in the form of function (parse) trees. Also, the grid risk values were calculated using the deduced regression. The risk zoning map indicates that the risk values are higher in urban areas, which is reasonable in comparison with the distribution of historical flash flood events. With an explicit model structure, this symbolic regression manifests that the risk value is mainly determined by the RHIV and impervious land surface and is weakly correlated with the other risk factors, e.g., the physical vulnerability, the terrain factor, and population density. Our research demonstrates that GP in an artificial intelligence manner meets the needs of risk assessment in determining the optimal fit regressions and is a promising technique for future applications. Meanwhile, approaches are still available for improving the GP application in the risk assessment, e.g., considering the historical losses in posterior risk estimations and improvement in the sampling training data. © 2016 Springer Science+Business Media Dordrecht

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