Methodological Development of the Conditional Sampling Method. Part II: Quality Control Criteria of Relaxed Eddy Accumulation Flux Measurements

Determination of biosphere–atmosphere exchanges requires accurate quantification of the turbulent fluxes of energy and of a wide variety of trace gases. Relaxed Eddy Accumulation (REA) is a method that has received increasing attention in recent years, because it does not require any rapid sensor for the scalar measurements as the Eddy Correlation method (EC) does. As in all micrometeorological studies, REA measurements in the atmospheric surface layer are valid under some restrictive conditions so as to be representative of a specific ecosystem. These conditions are the homogeneity of the underlying surface, stationary and horizontally homogeneous turbulence. For most experiments these conditions are not fully satisfied. Data uncertainties can also be related to not fulfilling the method principles or to the technical characteristics of the REA system itself. In order to assess REA measurement quality, a methodological approach of data analysis is developed in this study. This methodological analysis is based on the establishment of criteria for data quality control. A set of them, deduced from the mean and turbulent flow, are called ‘Dynamic criteria’ and are designated to control the stationarity and homogeneity of the w function and the validation of Taylor’s hypothesis. A second set (‘REA operational criteria’) is designed to check the sampling process and, more precisely, the homogeneity of the negative and positive selection process throughout the sampling period. A third set of criteria (‘Chemical scalar criteria’) concerns the scalar measurements. Results of the criteria application to data measured at two different experimental sites are also presented. Cut-off limits of criteria are defined based on their statistical distribution and shown to be specific for each site. Strictness of each criterion, defined by the percentage of flagged samples, is analysed in conjunction with the meteorological conditions and atmospheric stability. It is found that flagged samples mainly correspond to neutral and stable nocturnal conditions. During daytime, nearly free convection conditions can also yield poor quality data.     

A Re-Evaluation of Long-Term Flux Measurement Techniques Part II: Coordinate Systems

To convert measurements of windspeed, eddy flux and scalar concentration into estimates of surface scalar exchange, we implicitly or explicitly assimilate the measurements into mathematical statements of the mass balance in a control volume on a representative patch of the surface. The form of this statement depends on the coordinate system in which it is written and the coordinate system should be chosen so that measurements can be used optimally. This requirement imposes a set of conditions on the coordinates. Here we perform a comparative analysis of some candidate coordinate systems, concentrating on the Cartesian and physical streamline systems. We show that over gentle topography there are definite advantages in working in streamline coordinates. Transforming measurements of vector and tensor quantities measured in the reference frame, s _i, of the anemometer into the reference frame, e _i, of the chosen coordinate system involves using the measured statistics of the wind field to define three Euler rotation angles. We compare the method in most common use, which employs the components of the mean wind vector and the Reynolds stress tensor to define these angles, with the more recent planar-fit method that uses instead an ensemble of mean wind vectors to define the rotations. We find that, in real flows, the standard method has a previously unrecognized closure problem that ensures that the third rotation angle defined using the stress tensor or scalar flux vector will always be in error and often give unphysical results. An alternative procedure is recommended. Finally, the relationships between measurements and model outputs are discussed.     

One-Equation Turbulence Modelling for Atmospheric and Engineering Applications

A new algebraic turbulent length scale model is developed, based on previous one-equation turbulence modelling experience in atmospheric flow and dispersion calculations. The model is applied to the neutral Ekman layer, as well as to fully-developed pipe and channel flows. For the pipe and channel flows examined the present model results can be considered as nearly equivalent to the results obtained using the standard k– model. For the neutral Ekman layer, the model predicts satisfactorily the near-neutral Cabauw friction velocities and a dependence of the drag coefficient versus Rossby number very close to that derived from published (G. N. Coleman) direct numerical simulations. The model underestimates the Cabauw cross-isobaric angles, but to a less degree than the cross-isobar angle versus Rossby dependence derived from the Coleman simulation. Finally, for the Cabauw data, with a geostrophic wind magnitude of 10 ms^–1, the model predicts an eddy diffusivity distribution in good agreement with semi-empirical distributions used in current operational practice.     

500hPa环流变化与山东春季降水异常

应用SVD技术,诊断分析了北半球500hPa高度场与山东春季降水的关系。结果表明：山东春季降水与北半球500hPa高度场关系密切,山东春季降水与东亚上空500hPa高度场具有很好的同步联系。前期(冬季)日本东北部500hPa高度场是影响山东南部春季降水的关键区,具有预测意义。500hPa高度距平场东高西低型是造成山东春季降水的主要大气环流形势,西高东低型是造成山东少雨的主要大气环流形势。     

Atmospheric densities derived from CHAMP/STAR accelerometer observations

The satellite CHAMP carries the accelerometer STAR in its payload and thanks to the GPS and SLR tracking systems accurate orbit positions can be computed. Total atmospheric density values can be retrieved from the STAR measurements, with an absolute uncertainty of 10-15%, under the condition that an accurate radiative force model, satellite macro-model, and STAR instrumental calibration parameters are applied, and that the upper-atmosphere winds are less than . The STAR calibration parameters (i.e. a bias and a scale factor) of the tangential acceleration were accurately determined using an iterative method, which required the estimation of the gravity field coefficients in several iterations, the first result of which was the EIGEN-1S (Geophys. Res. Lett. 29 (14) (2002) 10.1029) gravity field solution. The procedure to derive atmospheric density values is as follows: (1) a reduced-dynamic CHAMP orbit is computed, the positions of which are used as pseudo-observations, for reference purposes; (2) a dynamic CHAMP orbit is fitted to the pseudo-observations using calibrated STAR measurements, which are saved in a data file containing all necessary information to derive density values; (3) the data file is used to compute density values at each orbit integration step, for which accurate terrestrial coordinates are available. This procedure was applied to 415 days of data over a total period of 21 months, yielding 1.2 million useful observations. The model predictions of DTM-2000 (EGS XXV General Assembly, Nice, France), DTM-94 (J. Geod. 72 (1998) 161) and MSIS-86 (J. Geophys. Res. 92 (1987) 4649) were evaluated by analysing the density ratios (i.e. “observed” to “computed” ratio) globally, and as functions of solar activity, geographical position and season. The global mean of the density ratios showed that the models underestimate density by 10-20%, with an rms of 16-20%. The binning as a function of local time revealed that the diurnal and semi-diurnal components are too strong in the DTM models, while all three models model the latitudinal gradient inaccurately. Using DTM-2000 as a priori, certain model coefficients were re-estimated using the STAR-derived densities, yielding the DTM-STAR test model. The mean and rms of the global density ratios of this preliminary model are 1.00 and 15%, respectively, while the tidal and latitudinal modelling errors become small. This test model is only representative of high solar activity conditions, while the seasonal effect is probably not estimated accurately due to correlation with the solar activity effect. At least one more year of data is required to separate the seasonal effect from the solar activity effect, and data taken under low solar activity conditions must also be assimilated to construct a model representative under all circumstances.     

Pyrolisis of phosphorylated molecules and survivability limits during the atmospheric passage in earth-like planets

There is evidence that space energy sources could give place to the appearance of phosphorylated nucleosides outside of Earth. These compounds may have been delivered mainly by interplanetary dust particles due to the lower temperatures experienced during atmospheric deceleration and impacts to the terrestrial surface. In this report, we communicate the results of pyrolytic studies to simulate atmospheric survivability of adenosine-5′-diphosphates (ADP) (and adenosine-5′-monophosphate, adenosine and adenine as degradation products) at temperatures <500°C and at various time intervals. Our results revealed that phosphorylated and non-phosphorylated nucleosides transported by IDPs having sizes of 10⁻⁶- could resist temperatures up to 500°C generated during atmospheric entry. However, atmospheric passage should not exceed a time due to the thermal lability of these molecules. Because of the high half-life showed by ADP in the presence of meteoritic powder, it is thought that extraterrestrial delivery of very complex biomolecules is more suitable under such protected conditions. These data indicate that the formation of a Fe²⁺- and/or Ca²⁺-complex could increase the stability of the molecules in the presence of meteoritic matter. Therefore, if the synthesis of nucleosides, nucleotides or oligonucleotides could take place in icy bodies, then micron-sized dust could have contributed significantly to the availability of phosphorylated nucleosides in the early Earth or in extrasolar early Earth-like planets, and thereby could have allowed the arising of an early biological activity.     

A Simulation Study on the Extreme Temperature Events of the 20th Century by Using the BCC_AGCM

Extreme temperature events are simulated by using the Beijing Climate Center Atmospheric General Circulation Model (BCC AGCM) in this paper. The model has been run for 136 yr with the observed external forcing data including solar insolation, greenhouse gases, and monthly sea surface temperature (SST). The daily maximum and minimum temperatures are simulated by the model, and 16 indices representing various extreme temperature events are calculated based on these two variables. The results show that the maximum of daily maximum temperature (TXX), maximum of daily minimum (TNX), minimum of daily maximum (TXN), minimum of daily minimum (TNN), warm days (TX90p), warm nights (TN90p), summer days (SU25), tropical nights (TR20), and warm spell duration index (WSDI) have increasing trends during the 20th century in most regions of the world, while the cold days (TX10p), cold nights (TN10p), and cold spell duration index (CSDI) have decreasing trends. The probability density function (PDF) of warm/cold days/nights for three periods of 1881-1950, 1951- 1978, and 1979-2003 is examined. It is found that before 1950, the cold day/night has the largest probability, while for the period of 1979-2003, it has the smallest probability. In contrast to the decreasing trend of cold days/nights, the PDF of warm days/nights exhibits an opposite trend. In addition, the frost days (FD) and ice days (ID) have decreasing trends, the growing season has lengthened, and the diurnal temperature range is getting smaller during the 20th century. A comparison of the above extreme temperature indices between the model output and NCEP data (taken as observation) for 1948-2000 indicates that the mean values and the trends of the simulated indices are close to the observations, and overall there is a high correlation between the simulated indices and the observations. But the simulated trends of FD, ID, growing season length, and diurnal temperature range are not consistent with the observations and their correlations are low or even negative. This indicates that the model is incapable to simulate these four indices although it has captured most indices of the extreme temperature events.     

西藏地区雷暴与大气热源关系研究

利用1979～2008年NCEP/NCAR全球再分析月平均资料和雷暴记录资料, 运用相关分析和合成分析等方法分别对西藏地区大气热源和雷暴日数的关系以及热源强弱不同时期雷暴日数变化特征进行了分析。结果表明, 大气热源与雷暴日数存在着明显的正相关关系, 月相关系数高达0.86。在雷暴高发区的那曲东部、拉萨、日喀则中部、山南地区东北部和林芝地区西北部, 相关系数均超过了0.4。进一步分析表明, 当大气热源强值年时, 整个日喀则地区、拉萨地区、山南地区、那曲中西部和东部地区以及昌都地区中西部, 雷暴日数是偏多的。当大气热源偏弱时, 雷暴日数在日喀则中部地区减少最为明显, 其次在日喀则南木林县东北部和林芝西北部地区。     

An analysis on observed and simulated PNA associated atmospheric diabatic heating

This study examines the PNA associated atmospheric diabatic heating by linearly isolating the influence of ENSO. The analysis is based on the NCEP–NCAR and ERA-40 reanalyses and a 1,000-year-long integration of the CCCma coupled climate model. Both the vertically integrated and three-dimensional diabatic heating are examined. The Rossby wave sources in association with the PNA are also diagnosed. The PNA-related heating is confined outside the tropics and is dominated by anomalies in the eastern Pacific, with a north–south dipole structure in mid-latitudes and the northern subtropics. The heating anomalies change sign with height in mid-latitudes but have the same sign throughout the troposphere in the northern subtropics. Relatively weak heating anomalies also appear in mid-latitudes, downstream of the heating dipole over North America and the western North Atlantic. The heating anomalies are largely supported by the advections related to the mean state throughout the troposphere, and partially damped by the advections related to the eddy effect, particularly at the upper troposphere over the North Pacific. Broadly similar patterns are seen from the NCEP–NCAR and ERA-40 reanalyses. Yet anomalous heating centers are generally located at relatively lower troposphere for the ERA-40 with respect to the NCEP–NCAR. The tropical heating anomalies are rather weak, remarkably different from those related to ENSO variability. In addition, the Rossby wave source collocates with the atmospheric diabatic forcing in the mid-high latitudes over the PNA sector, and shows no forcing source in the tropics. The results demonstrate possible forcing in the mid-high latitudes, regardless of tropical heating for the PNA teleconnection. The modeled heating and wave forcing anomalies in association with the modeled PNA compare reasonably well with the reanalysis-based estimates, increasing confidence in the observational results. The analysis provides further evidence of the independence of the PNA on ENSO from the diabatic heating point of view.