The von Kármán constant determined by large eddy simulation

The present study explores the extent to which the logarithmic region of the adiabatic atmospheric boundary layer can be modeled using a three-dimensional large eddy simulation. A value of the von Kármán constant (_LES) is obtained by determining the slope of a logarithmic portion of the velocity profile. Its numerical value is found to be dependent on the value of the Smagorinsky-Model Reynolds number, Re_SM: the value of _LES increases with Re_SM. Results indicate that _LES approaches a value of 0.35 as Re_SM reaches about 7.75 × 10⁵ for the largest domain. The sensitivity of _LES to the profile region over which it is evaluated has been tested. Results show that _LES is not sensitive to the depth of this evaluation region when we employ five grids above the sub-grid buffer layer where sub-grid-scale effects dominate. The maximum _LES is obtained when the lower boundary of the evaluation region is just above the top of the sub-grid-scale buffer layer. This result is consistent with modelled mean speed and resolved-scale shear stress profiles.     

Tropical convective responses to microphysical and radiative processes: a sensitivity study with a 2-D cloud resolving model

Summary Prognostic cloud schemes are increasingly used in weather and climate models in order to better treat cloud-radiation processes. Simplifications are often made in such schemes for computational efficiency, like the scheme being used in the National Centers for Environment Prediction models that excludes some microphysical processes and precipitation-radiation interaction. In this study, sensitivity tests with a 2-D cloud resolving model are carried out to examine effects of the excluded microphysical processes and precipitation-radiation interaction on tropical thermodynamics and cloud properties. The model is integrated for 10 days with the imposed vertical velocity derived from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The experiment excluding the depositional growth of snow from cloud ice shows anomalous growth of cloud ice and more than 20% increase of fractional cloud cover, indicating that the lack of the depositional snow growth causes unrealistically large mixing ratio of cloud ice. The experiment excluding the precipitation-radiation interaction displays a significant cooling and drying bias. The analysis of heat and moisture budgets shows that the simulation without the interaction produces more stable upper troposphere and more unstable mid and lower troposphere than does the simulation with the interaction. Thus, the suppressed growth of ice clouds in upper troposphere and stronger radiative cooling in mid and lower troposphere are responsible for the cooling bias, and less evaporation of rain associated with the large-scale subsidence induces the drying in mid and lower troposphere.     

Periodicities and irregularities of indices describing the daily pluviometric regime of the Fabra Observatory (NE Spain) for the years 1917–1999

Summary Several patterns of a daily pluviometric regime are obtained from an homogeneous set of daily rainfall recorded at the Fabra Observatory (NE Spain) for the period 1917–1999. Power spectral analyses of four annual pluviometric indices, determined from different daily rain amount percentiles, are performed. Periodicities of the quasi-biannual oscillation (QBO), North Atlantic Oscillation (NAO) and sunspot cycles are correlated in some cases with empirical spectral peaks of the indices exceeding Markov red-noise with a confidence level of 95%. Depending on the rainy day percentiles, a set of short periods (2.1–4.6 years), a second group of intermediate periods (5.5 and 9.2 years) and, finally, a third group of long periods of 11.8, 20.8 and 41.5 years can be distinguished. These periods are compared with those derived for other European and African emplacements. Additionally, the consecutive irregularity of every annual series is quantified by means of a concept similar to the entropy.     

The effects of bushfires on hydrological processes using a paired-catchment analysis

Summary Several problems associated with capturing the effects of bushfires on hydrological processes in the Goulburn River catchment (6810km²), Hunter Valley, Australia are investigated using a paired-catchment analysis. It is suggested, first, that the within-year-missing data, as defined in this paper, need to be examined carefully when using the double-mass curve of annual total discharge in a paired-catchment analysis. Second, in order to provide an accurate precipitation background, which is one of the most important prerequisites for a paired-catchment analysis, the use of annual average precipitation is strongly recommended together with annual total precipitation when there are large amounts of within-year-missing data. Third, caution is needed in comparing multi-year average precipitation and streamflow data before and after the fire when the data series is not statistically long enough, since the average values for precipitation and streamflow over a different number of years may produce completely contrasting results. Fourth, the analysis of the flow duration curve, which is another useful technique in paired-catchment analysis, needs to be interpreted from the precipitation duration curve. This is because the change of flow duration curve can be caused either from the change of precipitation or the fire. Fifth, the change in streamflow, calculated by subtracting the average streamflow for the non-fire years from the observed streamflow for the years in which fires occurred, is not an efficient way of capturing the fire effects. The problem associated with this approach is not just that the streamflow is strongly dependent on rainfall, as reported elsewhere, but also, it can lead to misinterpretation using the hydrograph when the average streamflow in non-fire years is close to the average streamflow in fire years.By taking into account the above problems there was no effect of fires found on streamflow in the Goulburn River catchment. This result contrasts with the conclusions reported from other studies that have reported an immediate increase in streamflow by experimental analyses, paired-catchment analyses or modelling studies. Instead, it is shown that the spatial pattern of precipitation over the Goulburn River catchment is more important in shaping the hydrograph than the effects of bushfires. The ratio of fire extent to catchment area is approximately 4% in this study, which we suggest is a minimum area required to identify a hydrological response to the fires. The fact that other studies have focused more on capturing the generally expected effect of an immediate increase in streamflow after fire, than on why this effect occurs, makes it highly desirable to undertake micro-meteorological experiments to obtain observed evapotranspiration data before and after fire. Also, it is important to develop a coupled soil-vegetation-atmosphere-transfer dynamic mechanism and high resolution numerical weather prediction model with a distributed hydrological model in order to simulate more realistically the effect of fire on hydrological processes.     

Reproducibility of seasonal ensemble integrations with ECMWF GCM and its association with ENSO

Summary This study evaluates seasonal climate potential predictability with a dataset of nine-member ensemble seasonal integrations produced by the ECMWF GCM for the ERA-15 period (1979–1993). The methodology used here is the measure of the ensemble reproducibility for a particular season defined by Yang et al (1998). High reproducibility reflects the dominant role of the lower boundary forcing in seasonal climate anomaly, indicating good potential predictability. Spatial patterns of the reproducibility for selected variables are documented, which exhibit obvious regionality and seasonality. Such variables are always highly reproducible over most of the tropical regions. Over the northern extratropics, primary reproducible information, taking the 500hPa geopotential height for example, is found over the PNA region during winter while over most of Asia during summer. Winter has the largest reproducible area; autumn has the lowest, while summer and spring are in between. Association of the reproducibility with ENSO events was examined regionally. Internal variances due to ensemble spread were broken down for individual years, and the reproducibility was computed for four categories: El Niño, La Niña, ENSO, and non-ENSO years. The reproducibility during winter especially over the PNA region is insensitive to ENSO events, with exceptions over the tropical western Pacific, central Siberia and Western Europe. Contrarily, ENSO events have significant impacts on the reproducibility over the southwest USA and most of Asian monsoon region during summer. These results suggest that ENSO events may not be helpful to the seasonal climate predictability over the PNA region during winter, but they may increase predictable information over many regions of the northern extratropical continents during summer.     

The impact of soil moisture–atmosphere coupling on daily maximum surface temperatures in Southeastern South America

Based on a series of experiments conducted by two regional climate models (RCA4 and LMDZ) with and without soil moisture-atmosphere coupling, we investigate the role of soil moisture on the occurrence of surface air temperature extremes and its persistence in Southeastern South America. Our analysis reveals that both factors, soil moisture-atmosphere coupling and relatively drier soil conditions, enhance the temperature extremes. In addition, the existence of soil-atmosphere coupling and the associated soil moisture variability is crucial for the development of the extremes in SESA. The key role of soil-atmosphere coupling is also reflected in the intrinsic persistence of hot days, which is greater in simulations with interactive soil moisture than in those with prescribed soil conditions. In the absence of soil-atmosphere coupling, the imprint of the anomalous dry (and also wet) soil conditions on the intensity and persistence of hot days is weaker.

     

Permo-Triassic magnetostratigraphy in China: northern Tarim

The upper boundary of the Permo-Carboniferous Reversed Polarity Superchron has been identified in a palaeomagnetic study of the Permo-Triassic of the northern part of the Tarim Basin, China. This boundary serves as an important marker horizon for correlation with other Permo-Triassic sequences both in China and world-wide. A Permo-Triassic palaeomagnetic pole for the Tarim Block is estimated to be at 71.8°N, 187.6°E.Comparison with similar age poles from the adjacent blocks of China and Asia suggests that the Tarim was widely separated from the Sino-Korean Block in Permo-Triassic times but was not yet sutured to Kazakhstan.     

气候模式CAS-ESM-C对1月热带太平洋流场模态的模拟评估

利用气候模式CAS-ESM-C从1922年起84年的模拟资料,对1月份热带太平洋上层流场作复EOF分解及小波分析,并与实况以及理论解析解作对比讨论,以考察模式对赤道大洋上层流场的模拟能力,得到主要结论：（1）复EOF分解前3个模态的方差贡献为53.5%、12.9%、9.5%,累积方差贡献为75.9%,累积方差贡献比实况更高。（2）第一、二模态空间场与实况相比总体相像,流场都为赤道所俘获,在俘获区内的流场均以偏纬向流为主;差异在于模拟资料分析的赤道俘获区范围较实况要大,流场的经向流分量及越赤道流也较实况明显。（3）第一、二模态实时间系数序列无线性变化趋势,而实况则有。复EOF模态年际及年代际变化与实况相同或相近;第一、二模态中3～7年的年际变化是厄尔尼诺与南方涛动（ENSO）的反映;第一模态22～23年的年代际变化受北太平洋主要气候模态北太平洋年代际振荡（PDO）对热带太平洋的影响,而第二模态13年的年代际变化是受北太平洋次要气候模态北太平洋环流振荡（NPGO）对热带太平洋的影响;第一、二模态还都有峰值16年的年代际变化,这可能与印尼穿越流有关。（4）模拟资料分析的结果具有理论解析解中流...     

Monthly precipitation mapping of the Iberian Peninsula using spatial interpolation tools implemented in a Geographic Information System

Summary In this study, spatial interpolation techniques have been applied to develop an objective climatic cartography of precipitation in the Iberian Peninsula (583,551 km²). The resulting maps have a 200 m spatial resolution and a monthly temporal resolution. Multiple regression, combined with a residual correction method, has been used to interpolate the observed data collected from the meteorological stations. This method is attractive as it takes into account geographic information (independent variables) to interpolate the climatic data (dependent variable). Several models have been developed using different independent variables, applying several interpolation techniques and grouping the observed data into different subsets (drainage basin models) or into a single set (global model). Each map is provided with its associated accuracy, which is obtained through a simple regression between independent observed data and predicted values. This validation has shown that the most accurate results are obtained when using the global model with multiple regression mixed with the splines interpolation of the residuals. In this optimum case, the average R ² (mean of all the months) is 0.85. The entire process has been implemented in a GIS (Geographic Information System) which has greatly facilitated the filtering, querying, mapping and distributing of the final cartography.     

Characteristics and simulation of lightning current waveforms during one artificially triggered lightning

In the summer of 2005, one negative lightning flash was artificially triggered in Shandong Province (117°48′ E, 37°42′N), middle latitude region of eastern China. The flash included 10 return strokes, and the geometric mean value of the current peak was 11.9 kA (the average value was 12.6 kA) with a maximum of 21.0 kA and a minimum of 6.6 kA, similar to the subsequent return strokes in natural lightning. The geometric mean value of half peak width was 39 μs (the average value was 40 μs), which was much larger than the usual result. Based on the Diendorfer and Uman (DU) model, the return-stroke current waveforms and charge distribution along the lightning channel are discussed. The simulated current waveforms, being divided into breakdown and corona current components, are in agreement with the optical measurements when the two different discharge time constants are properly chosen.