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Kaicun Wang and Shunlin Liang

Abstract

Changes in surface net radiation Rn control the earth’s climate, the hydrological cycle, and plant photosynthesis. However, Rn is not readily available. This study develops a method to estimate surface daytime Rn from solar shortwave radiation measurements as well as conventional meteorological observations (or satellite retrievals) including daily minimum temperature, daily temperature range, and relative humidity, and vegetation indices from satellite data. Measurements collected at 22 U.S. and 2 Tibetan Plateau, China, sites from 2000 to 2006 are used to develop and validate the method. Land cover types include desert, semidesert, croplands, grasslands, and forest. Site elevations range from 98 to 4700 m. The results show that the method estimates Rn under clear and cloudy conditions accurately over a range of land cover types, elevations, and climates without requiring local calibration. The results show that the method estimates Rn accurately. The bias varies from −7.8 to 9.7 W m−2 (±3% in relative value) for different sites, and the root-mean-square error ranges from 12.8 to 21 W m−2 (from +5% to +9% in relative value) for different sites, with an average of 16.9 W m−2 (+6% relative) for all sites. The correlation coefficient for all sites is about 0.99. The correlation coefficient between the measured and predicted annual anomaly (year average subtracted from multiyear average) in daytime Rn is as high as 0.91, demonstrating that the method accurately estimates long-term variation in Rn.

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Liang Wang and Dan Li

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In this study, we simulate the magnitude of urban heat islands (UHIs) during heat wave (HWs) in two cities with contrasting climates (Boston, Massachusetts, and Phoenix, Arizona) using the Weather Research and Forecasting (WRF) Model and quantify their drivers with a newly developed attribution method. During the daytime, a surface UHI (SUHI) is found in Boston, which is mainly caused by the higher urban surface resistance that reduces the latent heat flux and the higher urban aerodynamic resistance r a that inhibits convective heat transfer between the urban surface and the lower atmosphere. In contrast, a daytime surface urban cool island is found in Phoenix, which is mainly due to the lower urban r a that facilitates convective heat transfer. In terms of near-surface air UHI (AUHI), there is almost no daytime AUHI in either city. At night, an SUHI and an AUHI are identified in Boston that are due to the stronger release of heat storage in urban areas. In comparison, the lower urban r a in Phoenix enhances convective heat transfer from the atmosphere to the urban surface at night, leading to a positive SUHI but no AUHI. Our study highlights that the magnitude of UHIs or urban cool islands is strongly controlled by urban–rural differences in terms of aerodynamic features, vegetation and moisture conditions, and heat storage, which show contrasting characteristics in different regions.

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Kaicun Wang and Shunlin Liang

Abstract

A simple and accurate method to estimate regional or global latent heat of evapotranspiration (ET) from remote sensing data is essential. The authors proposed a method in an earlier study that utilized satellite-determined surface net radiation (Rn), a vegetation index, and daytime-averaged/daily maximum air temperature (Ta) or land surface temperature (Ts) data. However, the influence of soil moisture (SM) on ET was not considered and is addressed in this paper by incorporating the diurnal Ts range (DTsR). ET, measured by the energy balance Bowen ratio method at eight enhanced facility sites on the southern Great Plains in the United States and by the eddy covariance method at four AmeriFlux sites during 2001–06, is used to validate the improved method. Site land cover varies from grassland, native prairie, and cropland to deciduous forest and evergreen forest. The correlation coefficient between the measured and predicted 16-day daytime-averaged ET using a combination of Rn, enhanced vegetation index (EVI), daily maximum Ts, and DTsR is about 0.92 for all the sites, the bias is −1.9 W m−2, and the root-mean-square error (RMSE) is 28.6 W m−2. The sensitivity of the revised method to input data error is small. Implemented here is the revised method to estimate global ET using diurnal Ta range (DTaR) instead of DTsR because DTsR data are not available yet, although DTaR-estimated ET is less accurate than DTsR-estimated ET. Global monthly ET is calculated from 1986 to 1995 at a spatial resolution of 1° × 1° from the International Satellite Land Surface Climatology Project (ISLSCP) Initiative II global interdisciplinary monthly dataset and is compared with the 15 land surface model simulations of the Global Soil Wetness Project-2. The results of the comparison of 118 months of global ET show that the bias is 4.5 W m−2, the RMSE is 19.8 W m−2, and the correlation coefficient is 0.82. Incorporating DTaR distinctively improves the accuracy of the estimate of global ET.

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Xue-Liang Wang and Takio Murakami

Abstract

The longitude-time cross section of outgoing longwave radiation and zonal winds at the equator indicates a regular eastward propagation of interannual time-scale perturbations all the way from the Indian Ocean, across the maritime continent, to as far east as the eastern Pacific during the 5 years of 1979–83. These interannual perturbations also exhibit standing wave character. Associated with this are phase changes from the pre anti-ENSOF, in January 1982, via the mid-ENSO in January 1983, to the post anti-ENSO of December 1983. Both anti-ENSO and mid-ENSO are phase locked with the seasonal cycle. Anti-ENSO synoptics are the manifestation of anomalous enhancement of the normal winter circulation, whereas the mid-ENSO patterns reflect an anomalous weakening of the normal winter circulation. Thus, the mid-ENSO exhibits an approximate reversal of synoptics (anomaly) from the anti-ENSO phase.

Over the eastern North and South Pacific between about the dateline and 120°W, a key area for the 1982–83 ENSO, the main characteristics of the anti-ENSO and mid-ENSO phases are summarized as follows: Anti-ENSO is characterized by 1) an anomalously intensified indirect N–S vertical overturning with below normal equatorial convection in contrast to above normal rainfall over the extratropics, 2) an unusual intensification of upper oceanic troughs over both the North and South Pacific with prominent equatorial westerlies between them, and 3) substantial intraseasonal time-scale 200 mb disturbance activity. Mid-ENSO is characterized by 1) a direct N–S vertical overturning (anomaly) accompanied by equatorial convection and extratropical dry spells, 2) an unusual weakening of upper oceanic troughs (or anomalous upper anticyclones) with equatorial easterlies inhibiting intraseasonal disturbance activity, and 3) enhanced midlatitude westerlies poleward of twin anomalous anticyclones, facilitating above normal baroclinic disturbance activity.

The Japan–maritime continent–Australia region (100°–160°E) is another key area for a well-defined anomalous circulation reversal. Here, circulation changes are out of phase with those over the eastern North and South Pacific region. Less organized circulation reversals also take place over both the Afghanistan–Indian Ocean (40°–80°E) and Central America–South America (80°–40°W) regions.

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Liang Zhao and Jing-Song Wang

Abstract

This study provides evidence of the robust response of the East Asian monsoon rainband to the 11-yr solar cycle and first identify the exact time period within the summer half-year (1958–2012) with the strongest correlation between the mean latitude of the rainband (MLRB) over China and the sunspot number (SSN). This period just corresponds to the climatological-mean East Asian mei-yu season, characterized by a large-scale quasi-zonal monsoon rainband (i.e., 22 May–13 July). Both the statistically significant correlation and the temporal coincidence indicate a robust response of the mei-yu rainband to solar variability during the last five solar cycles. During the high SSN years, the mei-yu MLRB lies 1.2° farther north, and the amplitude of its interannual variations increases when compared with low SSN years. The robust response of monsoon rainband to solar forcing is related to an anomalous general atmospheric pattern with an up–down seesaw and a north–south seesaw over East Asia.

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Tao Zheng, Shunlin Liang, and Kaicun Wang

Abstract

Incident photosynthetically active radiation (PAR) is an important parameter for terrestrial ecosystem models. Because of its high temporal resolution, the Geostationary Operational Environmental Satellite (GOES) observations are very suited to catch the diurnal variation of PAR. In this paper, a new method is developed to derive PAR using GOES data. What makes this new method distinct from the existing method is that it does not need external knowledge of atmospheric conditions. The new method retrieves both atmospheric and surface conditions using only at-sensor radiance through interpolation of time series of observations. Validations against ground measurement are carried out at four “FLUXNET” sites. The values of RMSE of estimated and ground-measured instantaneous PAR at the four sites are 130.71, 131.44, 141.16, and 190.22 μmol m−2 s−1, respectively. At the four validation sites, the RMSE as the percentage of estimated mean PAR value are 9.52%, 13.01%, 13.92%, and 24.09%, respectively; the biases are −101.54, 16.56, 11.09, and 53.64 μmol m−2 s−1, respectively. The independence of external atmospheric information enables this method to be applicable to many situations in which external atmospheric information is not available. In addition, topographic impacts on surface PAR are examined at the 1-km resolution at which PAR is retrieved using the GOES visible band data.

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Kaicun Wang, Robert E. Dickinson, and Shunlin Liang

Abstract

Pan evaporation (EP), an index of atmospheric evaporative demand, has been widely reported to have weakened in the past decades. However, its interpretation remains controversial because EP observations are not globally available and observations of one of its key controls, surface incident solar radiation Rs, are even less available. Using global-distributed Rs from both direct measurements (available through the Global Energy Balance Archive) and derived from sunshine duration, the authors calculated the potential evaporation from 1982 to 2008 from approximately 1300 stations. The findings herein show that the contribution of water vapor pressure deficit (VPD) to monthly variability of EP is much larger than that of other controlling factors, of Rs, wind speed (WS), and air temperature Ta. The trend of the aerodynamic component of EP, which includes contributions of VPD, WS, and Ta, accounted for 86% of the long-term trend of EP. The aerodynamic component was then calculated from 4250 globally distributed stations and showed a negligible averaged trend from 1973 to 2008 because the reduction in WS canceled out the impact of the elevated VPD. The long-term trend of WS dominates the long-term trend of the aerodynamic component of EP at the 4250 stations. Atmospheric evaporative demand increased in most arid and semiarid areas, indicating a decrease in water availability in those areas.

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Dongliang Wang, Xudong Liang, Ying Zhao, and Bin Wang

Abstract

The impact of two bogussing schemes on tropical cyclone (TC) forecasts is compared. One scheme for bogussing TCs into the initial conditions of the nonhydrostatic version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is proposed by NCAR and the Air Force Weather Agency (AFWA), and four-dimensional variational data assimilation technology is employed for the other bogus data assimilation (BDA) scheme. The initial vortex structure adjusted by the NCAR–AFWA (N–A) scheme is more physically realistic, while the BDA scheme produces an initial vortex structure that is more consistent with the model. The results from 41 forecasts of TCs occurring over the western North Pacific (WNP) in 2002 suggest that the adjustment of the initial structure in the BDA scheme produces a greater benefit to the subsequent track and intensity forecasts, and the improvements in the track and intensity forecasts are significant using the BDA scheme. It seems that when using a model with 45-km grid length, the N–A scheme has a negative impact on the track forecasts for the recurving TCs and on the intensity predictions after 24 h.

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Qiang Zhang, Wenyu Wang, Sheng Wang, and Liang Zhang

Abstract

In most parts of the world, pan evaporation decreases with increased air temperature rather than increases, which is known as the “evaporation paradox.” The semiarid Loess Plateau, which is sensitive to global climate change and ecological variations, has a unique warming and drying climate. The authors of this study consider whether pan evaporation shows the same decreasing trend in this unique environment. Meteorological observations of the typical semiarid Dingxi in the Loess Plateau from 1960 to 2010 were used to analyze the variation in pan evaporation and its responses to climatic factors. It was found that the pan evaporation has increased considerably over the past 50 yr, which does not support the evaporation paradox proposed in previous studies. A multifactor model developed to simulate the independent impacts of climate factors on pan evaporation indicated that the temperature, humidity, wind speed, and low cloud cover variations contributed to pan evaporation by 46.18%, 25.90%, 2.48%, and 25.44%, respectively. The increased temperature, decreased relative humidity, and decreased low cloud cover all caused an increase in pan evaporation, unlike many parts of the world where increased low cloud cover offsets the effects of increased temperature and decreased relative humidity on pan evaporation. This may explain why the evaporation paradox occurs. If all relevant factors affecting pan evaporation are considered, it is possible the paradox will not occur. Thus in warm and drying regions, the increased pan evaporation will lead to increasingly arid conditions, which may exacerbate drought and flood disaster occurrences worldwide.

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Haibo Bi, Yunhe Wang, Yu Liang, Weifu Sun, Xi Liang, Qinglong Yu, Zehua Zhang, and Xiuli Xu

Abstract

Atmospheric circulation associated with the Arctic dipole (AD) pattern plays a crucial role in modulating the variations of summertime sea ice concentration (SIC) within the Pacific Arctic sector (PAS). Based on reanalysis data and satellite observations, we found that the impacts of atmospheric circulation associated with a positive AD (AD+) on SIC change over different regions of the PAS [including the East Siberian Sea (ESS), Beaufort and Chukchi Seas (BCS), and Canadian Arctic Archipelago (CAA)] are dependent on the phase shifts of Pacific decadal oscillation (PDO). Satellite observations reveal that SIC anomalies, influenced by AD+ during PDO− relative to that during PDO+, varies significantly in summer by 4.9%, −7.3%, and −6.4% over ESS, BCS, and CAA, respectively. Overall, the atmospheric anomalies over CAA and BCS in terms of specific humidity, air temperature, and thereby downward longwave radiation (DLR), are enhanced (weakened) in the atmospheric conditions associated with AD+ during PDO− (PDO+). In these two regions, the larger (smaller) increases in specific humidity and air temperature, associated with AD+ during PDO− (PDO+), are connected to the increased (decreased) poleward moisture flux, strengthened (weakened) convergence of moisture and heat flux, and in part to adiabatic heating. As a consequence, the DLR and surface net energy flux anomalies over the two regions are reinforced in the atmospheric scenarios associated with AD+ during PDO− compared with that during PDO+. Therefore, smaller SIC anomalies are identified over CAA and BCS in the cases related to AD+ during PDO− than during PDO+. Essentially, the changes of the DLR anomaly in CAA and BCS are in alignment with geopotential height anomalies, which are modulated by the anticyclonic circulation pattern in association with AD+ during varying PDO phases. In contrast, the SIC changes over ESS is primarily attributed to the variations in mechanical wind forcing and sea surface temperature (SST) anomalies. The cloud fraction anomalies associated with AD+ during different PDO phases are found not to be a significant contributor to the variations of sea ice anomaly in the studied regions. Given the oscillatory nature of PDO, we speculate that the recent shift to the PDO+ phase may temporarily slow the observed significant decline trend of the summertime SIC within PAS of the Arctic.

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