2015 Vol. 29 No. 5
Cover Note
Abstract：In model-based climate sensitivity studies, model errors may grow during continuous long-term integrations in both the reference and perturbed states and hence the climate sensitivity (defined as the difference between the two states). To reduce the errors, we propose a piecewise modeling approach that splits the continuous long-term simulation into subintervals of sequential short-term simulations, and updates the modeled states through re-initialization at the end of each subinterval. In the re-initialization processes, this approach updates the reference state with analysis data and updates the perturbed states with the sum of analysis data and the difference between the perturbed and the reference states, thereby improving the credibility of the modeled climate sensitivity. We conducted a series of experiments with a shallow-water model to evaluate the advantages of the piecewise approach over the conventional continuous modeling approach. We then investigated the impacts of analysis data error and subinterval length used in the piecewise approach on the simulations of the reference and perturbed states as well as the resulting climate sensitivity. The experiments show that the piecewise approach reduces the errors produced by the conventional continuous modeling approach, more effectively when the analysis data error becomes smaller and the subinterval length is shorter. In addition, we employed a nudging assimilation technique to solve possible spin-up problems caused by re-initializations by using analysis data that contain inconsistent errors between mass and velocity. The nudging technique can effectively diminish the spin-up problem, resulting in a higher modeling skill.More+
Abstract：Cumulus convection is a key linkage between hydrological cycle and large-scale atmospheric circulation. Cumulus parameterization scheme is an important component in numerical weather and climate modeling studies. In the Global/Regional Assimilation and Prediction Enhanced System (GRAPES), turbulent mixing and diffusion approach is applied in its shallow convection scheme. This method overestimates the vertical transport of heat and moisture fluxes but underestimates cloud water mixing ratio over the region of stratocumulus clouds. As a result, the simulated low stratocumulus clouds are less than observations. To overcome this problem, a mass flux method is employed in the shallow convection scheme to replace the original one. Meanwhile, the deep convection scheme is adjusted correspondingly. This modification is similar to that in the US NCEP Global Forecast System (GFS), which uses the simplified Arakawa Schubert Scheme (SAS). The planetary boundary layer scheme (PBL) is also revised by considering the coupling between the PBL and stratocumulus clouds. With the modification of both the cumulus and PBL schemes, the GRAPES simulation of shallow convective heating rate becomes more reasonable; total amounts of stratocumulus clouds simulated over the eastern Pacific and their vertical structure are more consistent with observations; the underestimation of stratocumulus clouds simulated by original schemes is less severe with the revised schemes. Precipitation distribution in the tropics becomes more reasonable and spurious precipitation is effectively suppressed. The westward extension and northward movement of the western Pa-cific subtropical high simulated with the revised schemes are more consistent with Final Operational Global Analysis (FNL) than that simulated with the original schemes. The statistical scores for the global GRAPES forecast are generally improved with the revised schemes, especially for the simulation of geopotential height in the Northern Hemisphere and winds in the tropics. Root mean square errors (RMSEs) decrease in the lower and upper troposphere with the revised schemes. The above results indicate that with the revised cumulus and PBL schemes, model biases in the tropics decrease and the global GRAPES performance is greatly improved.More+
Abstract：Based on the ECMWF Interim reanalysis (ERA-Interim) 500-hPa height data for the period 1979-2013, the Tibetan Plateau vortex (TPV) activities are investigated with an objective identification and tracking algorithm. The vortex tracks over the Tibetan Plateau (TP) and its vicinity are identified, and the cold/warm nature of the TPV is revealed from the 500-hPa temperature fields. A TPV activity dataset is thus derived. Examination of the TPV frequency, intensity, persistence, geographical location, and migration from the TP suggests an average of 53 TPVs annually during the study period, with 6.7 of them shifting out of the TP. For these vortexes, the longer the lifetime, the lower the frequency, and 81% of the TPVs are initially warm in nature. The particularly high-intensity vortexes occur dominantly in the rainy period (May-September), with their origin mainly in western Nagqu and northern Ali. They disappear largely in the Tanggula maintain area on the east side of the vortex high-frequency center and the Dangqu River valley in western Qinghai Province, possibly due to the topography. It is also found that the frequency of TPVs is decreasing, with 2 fewer TPV occurrences per 10 yr. Meanwhile, it is statistically significant that 1.4 fewer TPVs move out of the TP and the percentage of TPVs moving out of the TP versus the total number of TPVs has reduced by 2.3% every 10 yr.More+
Abstract：The high ridge in the northern branch of the subtropical westerly (hereafter referred to as northern branch ridge) extends from the north of the Tibetan Plateau to the north of Barr Kashmir Lake (32.5-47.5 N, 70-95 E) during wintertime. The intensity index and zonal position index of the wintertime northern branch ridge (WNBR) are defined in this study by using the NCEP-NCAR reanalysis data and precipitation and temperature observations at 160 stations in China. Interannual variation features of the WNBR and its relation with precipitation and surface air temperature in China are discussed based on wavelet analysis, correlation analysis, and composite analysis. The results indicate that the intensity and zonal position of the WNBR exhibit significant interannual variability. The intensity of the WNBR gradually weakens and varies in periodic cycles of 4-6, quasi-2, and quasi-16 yr. Its zonal position shifts westward slightly and varies in periodic cycles of quasi-8 and quasi-16 yr. The WNBR is highly correlated with precipitation and surface air temperature in China. When the WNBR is strong (weak) and its zonal position shifts eastward (westward), winter precipitation in Northeast China and the northern region of Northwest China increases (decreases), whereas precipitation decreases in central China, South China, and eastern regions of Northwest and Southwest China. Meanwhile, surface air temperatures in most areas of China are higher (lower) than normal. Further investigation has revealed that the two indexes are closely related to variations in general atmospheric circulation, which explains why there exists a close linkage between the variation of the WNBR and climate in China. It is believed that the WNBR is also tied to the Rossby wave, the North Atlantic Oscillation, the East Asian trough, and the North Pacific jet stream.More+
2015, 29(5): 793-805.
DOI:
10.1007/s13351-015-4116-9
Abstract：Dust aerosol effects on the properties of cirrus and altocumulus cloud in Northwest China were studied for the period March-May 2007 by using the satellite data of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), Aqua, and CloudSat. Dusty clouds were defined as those mixed with dust aerosols or existing in dust aerosol conditions, while pure clouds were those in a dust-free environment. For dusty altocumulus clouds, the mean values of cloud optical depth (OPD), cloud liquid water path (LWP), cloud ice water path (IWP), cloud effective particle radius (Re), and cloud effective particle diameter (De) were 6.40, 40.23 g m-2, 100.70 g m-2, 8.76 m, and 40.72 m, respectively. For pure altocumulus clouds, the corresponding mean values were 9.28, 76.70 g m-2, 128.75 g m-2, 14.03 m, and 48.92 m, respectively. These results show a significant decrease of OPD, LWP, IWP, Re, and De of approximately 31%, 48%, 22%, 38%, and 17% because of the effects of dust aerosols. Moreover, the effects of dust aerosols on liquid-phase altocumulus clouds were greater than on ice-phase altocumulus clouds. Regarding dusty cirrus clouds, the mean values of OPD, IWP, and De were 5.11, 137.53 g m-2, and 60.44 m, respectively. In contrast, the mean values were 6.69, 156.17 g m-2, and 66.63 m, respectively, for pure cirrus clouds, with a 24% decrease in OPD, a 12% decrease in IWP, and a 9% decrease in De. These results indicate that dust aerosols can significantly change cloud properties, leading to a reduction of OPD, LWP, and effective particle size for both altocumulus and cirrus clouds in Northwest China.More+
Abstract：A quantitative analysis of cloud fraction, cloud radiative forcing, and cloud radiative heating rate (CRH) of the single-layered cloud (SLC) and the multi-layered cloud (MLC), and their differences is presented, based on the 2B-CLDCLASS-LIDAR and 2B-FLXHR-LIDAR products on the global scale. The CRH at a given atmospheric level is defined as the cloudy minus clear-sky radiative heating rate. The statistical results show that the globally averaged cloud fraction of the MLC (24.9%), which is primarily prevalent in equatorial regions, is smaller than that of the SLC (46.6%). The globally averaged net radiative forcings (NET CRFs) induced by the SLC (MLC) at the top and bottom of the atmosphere (TOA and BOA) and in the atmosphere (ATM) are -60.8 (-40.9), -67.5 (-49.6), and 6.6 (8.7) W m-2, respectively, where the MLC contributes approximately 40.2%, 42.4%, and 57% to the NET CRF at the TOA, BOA, and in the ATM, respectively. The MLC exhibits distinct differences to the SLC in terms of CRH. The shortwave CRH of the SLC (MLC) reaches a heating peak at 9.75 (7.5) km, with a value of 0.35 (0.60) K day-1, and the differences between SLC and MLC transform from positive to negative with increasing altitude. However, the longwave CRH of the SLC (MLC) reaches a cooling peak at 2 (8) km, with a value of -0.45 (-0.42) Kday-1, and the differences transform from negative to positive with increasing altitude. In general, the NETCRH differences between SLC and MLC are negative below 7.5 km. These results provide an observational basis for the assessment and improvement of the cloud parameterization schemes in global models.More+
Abstract：Drought is one of the complex meteorological disasters affecting water resources, agriculture, livestock, and socioeconomic patterns of a region. Although drought prediction is difficult, it can be monitored based on climatological information. In this study, we provide high spatiotemporal resolution drought climatology, using observational, gridded precipitation data (0.50.5) from the Global Precipitation ClimatologicalCenter and soil moisture data from the Climate Prediction Center for the 60-yr period 1951-2010. The standardized precipitation index (SPI) based on a fitted Gamma distribution and Run method has been calculated from the regional drought identification model (ReDIM) for 3, 6, 9, 12, and 24 months. The results show strong temporal correlations among anomalies of precipitation, soil moisture, and SPI. Analysis of long-term precipitation data reveals that the drought vulnerability concentrates on monsoon season (July-September), which contributes 72.4% and 82.1% of the annual precipitation in northern and southern Sindh, respectively. Annual and seasonal analyses show no significant changes in the observed precipitation. The category classification criteria are defined to monitor/forecast drought in the selected area. Further analysis identifies two longest episodes of drought, i.e., 1972-1974 and 2000-2002, while 1969, 1974, 1987, and 2002 are found to be the most severe historical drought years. A drought hazard map of Sindh was developed, in which 10 districts are recognized as highly vulnerable to drought. This study helps to explain the time, duration, intensity, and frequency of meteorological droughts over Sindh as well as its neighboring regions, and provides useful information to disaster management agencies and forecasters for assessing both the regional vulnerability of drought and its seasonal predictability in Pakistan.More+
Abstract：To further verify the modified Becker's split-window approach for retrieving land surface temperature (LST) from long-term Advanced Very High Resolution Radiometer (AVHRR) data, a cross-validation and a radiance-based (R-based) validation are performed and examined in this paper. In the cross-validation, 3481 LST data pairs are extracted from the AVHRR LST product retrieved with the modified Becker's approach and compared with the Moderate Resolution Imaging Spectroradiometer (MODIS) LST product (MYD11A1) for the period 2002-2008, relative to the positions of 548 weather stations in China. The results show that in most cases, the AVHRR LST values are higher than the MYD11A1. When the AVHRR LSTs are adjusted with a linear regression, the values are close to the MYD11A1, showing a good linear relationship between the two datasets (R2=0.91). In the R-based validation, comparison is made between AVHRR LST retrieved from the modified Becker's approach and the inversed LST from the Moderate Resolution Transmittance Model (MODTRAN) consolidated with observed temperature and humidity profiles at four radiosonde stations. The results show that the retrieved AVHRR LST deviates from the MODTRAN inversed LST by -1.3 (-2.5) K when the total water vapor amount is less (larger) than 20 mm. This provides useful hints for further improvement of the LST retrieval algorithms' accuracy and consistency.More+
Abstract：Although sudden nocturnal warming events near the earth's surface in Australia and the United States have been examined in previous studies, similar events observed occasionally over the Loess Plateau of Northwest China have not yet been investigated. The factors that lead to these warming events in such areas with their unique topography and climate remain not clear. To understand the formation mechanisms and associated thermal and dynamical features, a nocturnal warming event recorded in Gansu Province (northwest of the Loess Plateau) in June 2007 was investigated by using observations and model simulations with the Weather Research and Forecasting (WRF) model. Observations showed that this near-surface warming event lasted for 4 h and the temperature increased by 2.5℃. During this event, a decrease in humidity occurred simultaneously with the increase of temperature. The model simulation showed that the nocturnal warming was caused mainly by the transport of warmer and drier air aloft downward to the surface through enhanced vertical mixing. Wind shear played an important role in inducing the elevated vertical mixing, and it was enhanced by the continuous development of the atmospheric baroclinicity, which converted more potential energy to kinetic energy.More+
2015, 29(5): 764-778.
DOI:
10.1007/s13351-015-4127-6
Abstract：The climatology of evaporation ducts is important for shipborne electromagnetic system design and application. The evaporation duct climatology that is currently used for such applications was developed in the mid 1980s; this study presents efforts to improve it over the South China Sea (SCS) by using a stateof-the-art evaporation duct model and an improved meteorology dataset. This new climatology provides better evaporation duct height (EDH) data over the SCS, at a higher resolution of 0.3120.313. A comparison between the new climatology and the old one is performed. The monthly average EDH in the new climatology is between 10 and 12 m over the SCS, higher than that in the old climatology. The spatiotemporal characteristics of the evaporation duct over the SCS in different months are analyzed in detail, based on the new climatology.More+
Vol. 29, No. 5, Nov 2015
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