Contrasting the impact of dynamic data assimilation on the numerical simulations of cyclogenesis during GALEIOP 10 and IOP 1

Summary The nudging assimilation scheme described in the companion paper by Brill et al. is applied to study oceanic cyclogenesis during GALEIOP 10 on 27–28 February 1986. A 36-h control simulation statically initialized from 0000 UTC 27 February 1986 data moves the cyclone too far north and east in the 12-h period of most rapid deepening limiting the usefulness of the simulation for diagnostic study. The use of nudging to dynamically assimilate special 3-h and routine 12-h rawindsonde and dropsonde data into the model during the entire 36-h forecast period failed to deepen the cyclone as it moved northeast off the Atlantic seaboard beyond the area covered 3-h by sounding data. Subjectively analyzed mean sea-level pressures (MSLP) were included in the data base to allow the model to nudge toward 3-h surface pressure analyses extended to cover the region of cyclogenesis over the ocean. The assimilation of 3-h surface data over the ocean is insufficient to produce a realistic simulation of cyclogenesis. This result motivated the use of the nudging technique to assimilate surface pressure and upper air data over land during the 12-h pre-cyclogenetic period (i.e. dynamic initialization) and compare the subsequent 24-h simulation with one initialized statically at the same synoptic time.Dynamic initialization produced the best simulation of the occanic cyclone based upon the standard statistical scores and positions of the MSLP minima. This simulation is used to diagnose differences between cyclogenesis during GALEIOP 1 and IOP 10. Isentropic analyses and vertical cross sections are derived from the model simulations and are used to contrast the strength of the upper tropospheric forcing and the low-level static stability associated with each case. The results of the diagnostic analyses reveal that stronger surface response (based upon MSLP minima) to weaker upper-level forcing during GALEIOP 10 (compared with GALEIOP 1) was associated with differences in the lower tropospheric static stability and thermal advection patterns and their interaction with upper tropospheric features.With 19 Figures     

Variability of Surface Sensible Heat Flux over Northwest China

The present study documents the variability of surface sensible heat flux over Northwest China using station observations for the period 1961 2000.It is found that the afternoon and nighttime sensible heat flux variations are remarkably different.The variability of the instant flux in the afternoon is much larger than in the nighttime.The afternoon and nighttime flux anomalies tend to be opposite.The diurnal and seasonal dependence of sensible heat flux variations is closely related to the diurnal cycle of mean land-air temperature difference.The relationship of sensible heat flux with land-air temperature difference based on the instant value differs from that based on the daily mean.The present study indicates the importance for the models to properly simulate mean land-air temperature difference and its diurnal and seasonal variations in order to capture surface sensible heat flux variability over Northwest China and predicts its plausible impacts on climate.     

Observed Changes in One-in-20 Year Extremes of Canadian Surface Air Temperatures

Weather and climate extremes are often associated with substantial adverse impacts on society and the environment. Assessment of changes in extremes is of great and broad interest. This study first homogenizes daily minimum and maximum surface air temperatures recorded at 146 stations in Canada. In order to assess changes in one-in-20 year extremes (i.e., extremes with a 20-year return period) in temperature, annual maxima and minima of both daily minimum temperatures and daily maximum temperatures are derived from the homogenized daily temperature series and analyzed with a recently developed extreme value analysis approach based on a tree of generalized extreme value distributions (including stationary and non-stationary cases). The procedure is applied to estimate the changes over the period 1911 to 2010 at 115 stations, located mainly in southern Canada, and over the period 1961 to 2010 at 146 stations across Canada (including 37 stations in the North). The results show that warming is strongest for extreme low temperature and weakest for extreme high temperature and is much stronger in the Canadian Arctic than in southern Canada. Warming is stronger in winter than in summer and stronger during nighttime than daytime of the same season.     

青藏高原探空大气水汽偏差及订正方法研究

水汽是大气的主要成分和降水的主要物质来源.青藏高原大气水汽分布对区域天气和气候有很大影响,为了探讨探空观测的大气水汽总量（R）资料的可靠性,本文以地基GPS遥感的大气水汽总量（G）为参照标准,对拉萨（1999~2010年）和那曲（2003年）的R进行对比分析和偏差（R-G）订正.结果表明:近10多年拉萨站R比G明显偏小,偏小程度随使用不同的探空仪而异.GZZ-2型机械探空仪和GTS-1型电子探空仪多年平均的PW偏差分别为-8.8%和-3.9%,随机误差分别为17.6%和13.6%.近10多年PW偏差变化呈减少趋势,这与探空仪性能改进有关.分析发现,青藏高原PW偏差具有明显季节变化和日变化特征,夏季比冬季明显,1200 UTC比0000 UTC明显.拉萨站GZZ-2型和GTS-1型探空仪在1200 UTC多年平均的PW偏差分别为-15.8%和-7.3%,在0000 UTC分别为-1.6%和-0.4%.那曲站GZZ-2型探空仪在1200 UTC和0000 UTC的PW偏差分别为-12.4%和-0.3%.分析还表明,太阳辐射加热与气温的日变化和季节变化是造成高原PW偏差日变化和季节变化的重要原因.据此,提出了高原PW偏差的订正方法,并以拉萨和那曲站为例进行PW偏差订正,订正后的PW系统偏差显著减少,随机误差也相应得到了改善.     

Dynamical controls on the diurnal cycle of temperature in complex topography

We examine the climatological diurnal cycle of surface air temperature in a 6 km resolution atmospheric simulation of Southern California from 1995 to the present. We find its amplitude and phase both have significant geographical structure. This is most likely due to diurnally-varying flows back and forth across the coastline and elevation isolines resulting from the large daily warming and cooling over land. Because the region’s atmosphere is generally stably stratified, these flow patterns result in air of lower (higher) potential temperature being advected upslope (downslope) during daytime (nighttime). This suppresses the temperature diurnal cycle amplitude at mountaintops where diurnal flows converge (diverge) during the day (night). The nighttime land breeze also advects air of higher potential temperature downslope toward the coast. This raises minimum temperatures in land areas adjacent to the coast in a manner analogous to the daytime suppression of maximum temperature by the cool sea breeze in these same areas. Because stratification is greater in the coastal zone than in the desert interior, these thermal effects of the diurnal winds are not uniform, generating spatial structures in the phase and shape of the temperature diurnal cycle as well as its amplitude. We confirm that the simulated characteristics of the temperature diurnal cycle as well as those of the associated diurnal winds are also found in a network of 30 observation stations in the region. This gives confidence in the simulation’s realism and our study’s findings. Diurnal flows are probably mainly responsible for the geographical structures in the temperature diurnal cycle in other regions of significant topography and surface heterogeneity, their importance depending partly on the degree of atmospheric stratification.     

Analysis of the surface temperature and wind forecast errors of the NCAR-AirDat operational CONUS 4-km WRF forecasting system

Investigating the characteristics of model-forecast errors using various statistical and object-oriented methods is necessary for providing useful guidance to end-users and model developers as well. To this end, the random and systematic errors (i.e., biases) of the 2-m temperature and 10-m wind predictions of the NCAR-AirDat weather research and forecasting (WRF)-based real-time four-dimensional data assimilation (RTFDDA) and forecasting system are analyzed. This system has been running operationally over a contiguous United States (CONUS) domain at a 4-km grid spacing with four forecast cycles daily from June 2009 to September 2010. In the result an exceptionally useful forecast dataset was generated and used for studying the error properties of the model forecasts, in terms of both a longer time period and a broader coverage of geographic regions than previously studied. Spatiotemporal characteristics of the errors are investigated based on the 24-h forecasts between June 2009 and April 2010, and the 72-h forecasts between May and September 2010. It was found that the biases of both wind and temperature forecasts vary greatly seasonally and diurnally, with dependency on the forecast length, station elevation, geographical location, and meteorological conditions. The temperature showed systematic cold biases during the daytime at all station elevations and warm biases during the nighttime above 1,000 m above sea level (ASL), while below 600 m ASL cold biases occurred during the nighttime. The forecasts of surface wind speed exhibited strong positive biases during the nighttime, while the negative biases were observed in the spring and summer afternoons. The surface wind speed was mostly over-predicted except for the stations located between 1,000 and 2,100 m ASL, for which negative biases were identified for most forecast cycles. The highest wind-speed errors were found over the high terrain and near sea-level stations. The wind-direction errors were relatively large at the high-terrain elevation in the Rocky and Appalachian mountain ranges and the western coastal areas and the error structure exhibited notable diurnal variability.     

Impact of urbanization on boundary layer structure in Beijing

The Beijing meteorological tower is located in an area of Beijing, China, which has developed from a suburban to an inner city setting over the past 30 years. The impacts of this urbanization process on both the vertical profile and diurnal cycles of air temperature are investigated using hourly data collected from a series of monitoring levels (up to 325 m high) on the Beijing meteorological tower since 1984. We find that the inter-decadal temperature has increased gradually, and that a more significant increase occurred during the 1980s and 1990s due to the effects of urbanization. A well-defined change in temperature stratification was also observed over this period. The height of the temperature inversion layer decreased from the 1980s to the 2000s. A well-defined nighttime temperature inversion developed below 50 m during the summer in the 1980s, but this near-surface inversion is not seen in data from the 1990s and 2000s. This change can be related to an increase in turbulent mixing caused by urban roughness and surface heat storage that disturbs the near-surface temperature inversion layer. In addition, the diurnal change in temperature in the city in summer shows a maximum increase from sunrise to the early afternoon, which is mainly caused by the nature variability and global warming in both the summer and winter. The urbanization mainly contributes to the temperature increase in the afternoon and nighttime. Moreover the urbanization dominates the increase in daily mean near-surface temperature.     

On the relation between the temperature anomaly duration and the value of its extrema

A statistical structure of the time series of air temperature with a daily resolution is studied from the data of nine meteorological stations of Russia for the period from 1961 to 2006. A quantitative relation is obtained between the temperature anomaly duration and its extrema for different seasons of the year. The analysis also enabled to reveal the increase in the duration of long-term air temperature anomalies from the west to the east.     

Climatology of summer midtropospheric perturbations in the US northern plains. Part II: large-scale effects of the Rocky Mountains on genesis

Propagating convective storms across the US northern plains are often coupled with preexisting midtropospheric perturbations (MPs) initiated over the Rocky Mountains. A companion study (Part I) notes that such MPs occur most commonly at 12 UTC (early morning) and 00 UTC (late afternoon). Using a regional reanalysis and a general circulation model (GCM), this study investigates how such a bimodal distribution of the MP frequency is formed. The results point to two possible mechanisms working together while each has a different timing in terms of maximum effect. The diurnal evolutions between the midtropospheric flows over the Rockies and over the Great Plains are nearly out-of-phase due to inertial oscillation. During the nighttime, the westerly flows at 700?C500?mb over the Rockies intensify while flows at the same level over the Great Plains turn easterly. These two flows converge over the eastern Rockies and induce cyclonic vorticity through vortex stretching. After sunrise, the convergence dissipates and the cyclonic vorticity is redistributed by horizontal vorticity advection, moving it downstream. This process creates a climatological zonally propagating vorticity signal which, in turn, facilitates the early-morning MP genesis at 12 UTC. The analysis also reveals marked dynamic instability conducive to subsynoptic-scale disturbances in the midtroposphere over the Rockies. Strong meridional temperature gradients appear over the north-facing slopes of the Rockies due to terrain heating to the south and the presence of cooler air to the north. This feature, along with persistent vertical shear, creates a Charney?CStern type of instability (i.e. sign changes of the meridional potential vorticity gradient). Meanwhile, the development of terrain boundary layer reduces the Rossby deformation radius which, subsequently, enhances the likelihood for baroclinic short waves. Such effects are most pronounced in the late afternoon and therefore are supportive to the MP genesis around 00 UTC. Examination of GCM experiments with and without orography further supports the critical role of the Rocky Mountains and its associated boundary layer impacts on the formation of MPs.     

Diurnal variation of simulated 2007 summertime precipitation over South Korea in a real-time forecast model system

A real-time forecast (RTF) system using Weather Research and Forecast (WRF) model version 2.2 is used to evaluate the diurnal variation of precipitation over South Korea in the summer (June to August) of 2007. The characteristics of the observed precipitation are also analyzed. The analysis and simulation period is divided into two sub-periods following the end of the changma, or East Asian monsoon, in 2007: Period_1 is from 1 June to 21 July, and Period_2 is from 22 July to 31 August. A 24-h precipitation cycle is observed over the entire period. The diurnal variation of precipitation over the South Korea shows that the nighttime maximum precipitation in Period_1 is affected by a largescale system; in contrast, the daytime maximum precipitation in Period_2 resulted from mesoscale convections is induced by thermal instability and moisture advection. The phases of the diurnal variation of simulated precipitation are consistent with those of the observed precipitation. The daytime rainfall amount of simulated precipitation in Period_2 is overestimated, and the convective rain process significantly affects the simulated total precipitation. The daytime overestimated precipitation is associated with overestimations of low-level temperature and moisture during the daytime in the model simulations as compared with the observations.     

青藏高原大气水汽探测误差及其成因

青藏高原大气水汽分布对区域天气气候有很大影响,其探测资料的可靠性备受关注。以地基全球定位系统(GPS)遥感的大气可降水量为对比参照,分析了1999—2008年拉萨和2003年那曲探空观测大气可降水量的误差及其原因。结果表明,近10年拉萨站探空观测的可降水量比GPS遥感的可降水量明显偏小,偏小程度随使用不同的探空仪而异。GZZ-2型机械探空仪和GTS-1型电子探空仪多年平均的大气可降水量相对偏差分别为8.8%和4.4%,随机误差分别为19.8%和13.3%。近10年大气可降水量探测偏差具有减少的趋势,从12.7%减少至2.4%,主要是由探空仪性能改进所致。分析发现青藏高原大气可降水量探测偏差具有明显的日变化,12时(世界时)比00时大。拉萨站GZZ-2型和GTS-1型探空仪在12时多年平均的大气可降水量探测偏差分别为15.8%和8.3%,00时分别为1.6%和0.5%。那曲站GZZ-2型探空仪在12和00时的大气可降水量探测偏差分别为12.4%和0.3%。大气可降水量探测偏差还具有季节变化,夏季大,冬季小。对大气可降水量探测偏差日变化和季节变化的成因分析表明,12时气温比00时气温高以及夏季比冬季气温高是造成大气可降水量探测偏差日变化和季节变化的重要原因。     

Diurnal variations of winds over an upwelling region off Oregon

The diurnal variation of surface winds off the coast of Oregon is described and compared with a recent analysis of winds off the coast of Peru. The Oregon wind speeds have a distinct 24-h periodicity, while the Peru wind speeds were reported to have an irregular 12-h variation. The long-and trans-shore components of both winds exhibit 24-h periodicities; the ratio of the long-shore to trans-shore diurnal amplitudes off Oregon is 2.8, twice the ratio found off Peru. Although meteorological conditions off Oregon were quasi-stationary during the period investigated, there were considerable day-to-day variations in diurnal amplitudes and phases. Diurnal amplitudes were found to be correlated with the daily mean long-shore winds.     

Variability of the Mixed-Layer Height Over Mexico City

The diurnal and seasonal variability of the mixed-layer height in urban areas has implications for ground-level air pollution and the meteorological conditions. Measurements of the backscatter of light pulses with a commercial lidar system were performed for a continuous period of almost six years between 2011 and 2016 in the southern part of Mexico City. The profiles were temporally and vertically smoothed, clouds were filtered out, and the mixed-layer height was determined with an ad hoc treatment of both the filtered and unfiltered profiles. The results are in agreement when compared with values of mixed-layer height reconstructed from, (i) radiosonde data, and (ii) surface and vertical column densities of a trace gas. The daily maxima of the mean mixed-layer height reach values \(> 3\hbox { km}\) above ground level in the months of March–April, and are clearly lower (\(< 2.7\hbox { km}\)) during the colder months from September–December. Mean daily minima are typically observed at 0700 local time (UTC ? 6h), and are lowest during the winter months with values on average below 500 m. The data presented here show an anti-correlation between high-pollution episodes and the height of the mixed layer. The growth rate of the convective mixed-layer height has a seasonal behaviour, which is characterized together with the mixed-layer-height anomalies. A clear residual layer is evident from the backscattered signals recorded in days with specific atmospheric conditions, but also from the cloud-filtered mean diurnal profiles. The occasional presence of a residual layer results in an overestimation of the reported mixed-layer height during the night and early morning hours.     

Diurnal variations of temperature as a cause of periodic variations of high-frequency geoacoustic emission

Investigated is the periodic component of geoacoustic emission within the frequency band from 2.0 to 6.5 kHz registered by the piezoceramic hydrophone installed near the bottom of Lake Mikizha in the Kamchatka Peninsula. It is revealed that the variation period is 24 hours and the maximum variation is observed in summer, when the average daily air temperature is above 10°C. In that period the close connection is revealed between the series of air temperature and geoacoustic emission. Taking into account the similar results of measurements of the ground surface slope at the observation site, the most probable reason for the diurnal variations of high-frequency geoacoustic emission is the deformation of near-surface sedimentary rocks caused by diurnal variations of temperature.     

Seasonal and diurnal variations of stability indices and environmental parameters using NCEP FNL data over East Asia

Seasonal and diurnal variations of commonly used stability indices (SIs) and environmental parameters (EPs) over East Asia were examined by using eight years (2000～2007) 6-hr National Centers for Environmental Prediction (NCEP) final (FNL) global reanalysis data. The quality of the FNL data was also evaluated by using the routine rawinsonde data over South Korea. Most of the SIs and EPs shows significant seasonal variation (SV) and diurnal variation (DV), but their magnitudes are dependent on the geographic locations and seasons. In general, the SV and DV of SIs and EPs are greater over the inland area than that over the waters around the Korean peninsula, and the magnitude of DV is significantly larger during summer than during other seasons. The mid-to-upper atmosphere of the East Sea also exhibits relatively stable conditions during the summer due to the extended Okhotsk sea air mass. As a result, the SIs and EPs of the summer show a “C” shape with a minimum instability over the East Sea. The spatial distribution of the SIs and EPs also shows that the eastern and south-eastern region of China is the most favorable for deep convection during the summer. The DV of SIs and EPs are more significant in the inland area than on the coast with maximum (minimum) at 0600 UTC (1800 UTC). Although the spatial distribution of DV pattern is well suited to that of rawinsonde data, the time of the maximum and minimum instability over South Korea does not correspond to that of the rawinsonde data, which occurred at 1200 UTC and 0000 UTC (Eom et al., 2008). Compared to the rawinsonde data, the FNL data have a large RMSE in the temperature, wind, and mixing ratio especially at the lower troposphere. Therefore, the FNL data should be used with caution, especially for the perfect boundary condition experiments with various numerical simulation models.     

The rapid growth and decay of severe cyclone JAL (2010) over the Bay of Bengal

This study investigates the life cycle of Bay of Bengal cyclone JAL, characterized by a rapid fluctuation in its intensity during 60-h interval. The cyclone JAL underwent a period of rapid intensification during 24-h from 0000 UTC 05 November to 0000 UTC 06 November 2010. It was quasi-static during subsequent 24 h followed by a 12-h period of unusually rapid decay. During the rapid cyclogenesis phase, the system intensified (by 25 kt) from deep depression (DD) to severe cyclonic storm (SCS) and weakened (by 30 kt) from SCS to DD during the 12-h period of rapid cyclolysis. European Centre for Medium Range Weather Forecasts (ECMWF) model analysis field is used to analyze the Q vectors, K index and potential vorticity (PV) to diagnose the life cycle of this unusual cyclone. The analysis reveals that the 500–700 hPa column-averaged Q-vector convergence above the surface cyclone had strengthened and very high values of the K index produced a burst of heavy precipitation during the development stage of the cyclone. The associated latent heat release produced a substantial diabatic positive PV anomaly in the lower and middle troposphere that caused rapid cyclogenesis. The rapid cyclolysis is coincident with the weakening of the upper and lower PV anomalies and the westward shearing of the upper PV from the cyclone centre. Thus, the very latent heat release that assisted the rapid development of the cyclone also played an important role in its subsequent rapid decay. ECMWF model forecast for track and intensity is also verified.     

MULTIMODEL CONSENSUS FORECASTING OF LOW TEMPERATURE AND ICY WEATHER OVER CENTRAL AND SOUTHERN CHINA IN EARLY 2008

Based on the daily mean temperature and 24-h accumulated total precipitation over central and southern China, the features and the possible causes of the extreme weather events with low temperature and icing conditions,which occurred in the southern part of China during early 2008, are investigated in this study. In addition, multimodel consensus forecasting experiments are conducted by using the ensemble forecasts of ECMWF, JMA, NCEP and CMA taken from the TIGGE archives. Results show that more than a third of the stations in the southern part of China were covered by the extremely abundant precipitation with a 50-a return period, and extremely low temperature with a 50-a return period occurred in the Guizhou and western Hunan province as well. For the 24- to 216-h surface temperature forecasts, the bias-removed multimodel ensemble mean with running training period(R-BREM) has the highest forecast skill of all individual models and multimodel consensus techniques. Taking the RMSEs of the ECMWF 96-h forecasts as the criterion, the forecast time of the surface temperature may be prolonged to 192 h over the southeastern coast of China by using the R-BREM technique. For the sprinkle forecasts over central and southern China, the R-BREM technique has the best performance in terms of threat scores(TS) for the 24- to 192-h forecasts except for the 72-h forecasts among all individual models and multimodel consensus techniques. For the moderate rain, the forecast skill of the R-BREM technique is superior to those of individual models and multimodel ensemble mean.     

DRP-4DVar方法同化AIRS反演资料在一次江淮流域暴雨中的应用

利用经济省时的降维投影四维变分同化方法(DRP-4DVar),在2009年7月22～23日江淮流域的一次大暴雨过程中同化晴空条件下高光谱大气红外探测仪(AIRS)反演温度、湿度廓线,改进此次强降水过程的模拟。试验结果分析显示,同化AIRS反演的温度及湿度场后,基于四维变分同化系统的模式约束,能够改进湿度场、高度场、高低层散度场。从累积降水量偏差图及同化试验增量图可以看到,正降水量偏差对应于正湿度增量、负位势高度增量及低层负散度高层正散度增量,负降水量偏差则与之相反。同化试验较参照试验可更好地模拟出暴雨的天气形势、对暴雨的落区及强度有更好的反映。此外,从单次同化与连续同化的试验对比结果看出,连续同化试验结果较单次同化结果有进一步的改进,说明不断加入新的观测资料可以更好地模拟强降水过程。     

华南区域模式在湖南省的2 m温度预报检验与订正研究

基于2019年8月至2020年7月华南区域模式（CMA-GD）预报和湖南97个国家站2m温度实况,开展了模式温度预报检验和逐步回归订正技术研究。结果表明,华南区域模式2m温度预报与实况变化趋势基本一致,预报偏差具有明显日变化,白天准确率下降、夜间升高,随着预报时效的延长,偏差增大;夏半年预报偏差大于冬半年;湘西预报效果优于湘东;00时起报的2m温度预报优于12时起报。基于华南区域模式预报产品,区分起报时次和季节的2m温度预报逐步回归订正预报效果较好,订正后预报相对于模式预报误差下降、准确率提高,有明显正技巧,对12时起报的模式预报效果改善更大,不同站点订正效果略有差异,对预报误差较大站点,订正效果明显。     

厦门能见度的准周期性变化

利用厦门机场2000～2002三年白天（08～20时）逐时能见度及相应的相对湿度、风资料，通过统计方法和小波分析，发现厦门能见度有显著的日变化、季节变化、季节内和准两周等不同尺度的变化，并且分析了这几种变化的基本特征。在每天有记录的时刻08～20时，能见度日变化明显，在清晨、傍晚最低，午后最高；能见度存在季节变化，在一年中3～5月能见度最低，这正对应着厦门的雾季，厦门地区的能见度与当地的雾关系密切；能见度还存在显著的季节内（30～60d）变化和准双周（10～20d）变化，这与东亚季风区内的大气振荡有关。