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Jie Chen, François P. Brissette, and Zhi Li

Abstract

This study proposes a new statistical method for postprocessing ensemble weather forecasts using a stochastic weather generator. Key parameters of the weather generator were linked to the ensemble forecast means for both precipitation and temperature, allowing the generation of an infinite number of daily times series that are fully coherent with the ensemble weather forecast. This method was verified through postprocessing reforecast datasets derived from the Global Forecast System (GFS) for forecast leads ranging between 1 and 7 days over two Canadian watersheds in the Province of Quebec. The calibration of the ensemble weather forecasts was based on a cross-validation approach that leaves one year out for validation and uses the remaining years for training the model. The proposed method was compared with a simple bias correction method for ensemble precipitation and temperature forecasts using a set of deterministic and probabilistic metrics. The results show underdispersion and biases for the raw GFS ensemble weather forecasts, which indicated that they were poorly calibrated. The proposed method significantly increased the predictive power of ensemble weather forecasts for forecast leads ranging between 1 and 7 days, and was consistently better than the bias correction method. The ability to generate discrete, autocorrelated daily time series leads to ensemble weather forecasts’ straightforward use in forecasting models commonly used in the fields of hydrology or agriculture. This study further indicates that the calibration of ensemble forecasts for a period up to one week is reasonable for precipitation, and for temperature it could be reasonable for another week.

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Kuiping Li, Qin He, Yang Yang, Zhi Li, and Weidong Yu

Abstract

The atmospheric 10–20-day quasi-biweekly mode (QBWM) significantly modulates the active–break spells of the South Asian monsoon. Current knowledge, however, is limited concerning the diversity of the QBWM in the Indian Ocean (IO). Based on extended empirical orthogonal function analysis, two dominant summer modes are constructed in the IO. The first mode (QBWM1) generally depicts IO basin-dependent variability, while the second mode (QBWM2) exhibits a close relationship with the northwestern Pacific. QBWM1 initiates in the equatorial western IO and propagates toward the eastern IO along the equator. Two Rossby wave cells evolve in the off-equatorial eastern IO when convection encounters the Maritime Continent, and subsequently the northern cell develops and moves westward in the South Asian monsoon region. In contrast, QBWM2 originates in the northwestern Pacific and passes westward across the South Asian monsoon region in the form of convectively coupled Rossby waves. The maintenance mechanism of the peculiar IO basin-dependent QBWM1 is understood in terms of moisture dynamics. Significant moisture anomalies are found to precondition convection initiation in the western IO and subsequent eastward movement along the equator. Afterward, two off-equatorial moisture centers are generated in the double Rossby wave cells along with convection dissipation in the eastern IO, and the moisture anomalies are delivered from the southern cell toward the convection initiation area in the western IO via a moisture conveyor belt without coupling with convection. Moisture budget analysis indicates that the horizontal moisture advection associated with QBWM1 is regulated by the mean clockwise circulation in the tropical IO.

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Zhi Li, Weidong Yu, Kuiping Li, Huiwu Wang, and Yanliang Liu

Abstract

Globally, the highest formation rate of super tropical cyclones (TCs) occurs over the Bay of Bengal (BoB) during the premonsoon transition period (PMT), but TC genesis has a low frequency here. TCs have occurred over the BoB in only 20 of the past 36 years of PMTs (1981–2016). This study investigates which environmental conditions modulate TC formation during the PMT over the BoB by conducting a quantitative analysis based on the genesis potential parameter, vorticity tendency equation, and specific humidity budget equation. The results show that there is a cyclonic anomaly in the TC genesis group compared to the non-TC genesis group, which is mainly due to the divergence term. A significant difference in vorticity contributes to TC formation over the BoB during the PMT. Furthermore, anomalous cyclonic flow enhances ascending motion, transporting moisture to the midlevel atmosphere. A change in specific humidity (SH) causes an increase in relative humidity, which contributes positively to TC formation. The vertical wind shear also makes a small positive contribution. In contrast to the previous three terms, the contribution from the instability term associated with 500- and 850-hPa air temperatures is negative and almost negligible. In addition, the synoptic-scale disturbance energy is more powerful in the TC genesis group than in the non-TC genesis group, which is favorable for TC breeding. Together, these conditions determine whether TCs are generated over the BoB during the PMT.

Open access
Li-Zhi Shen, Chun-Chieh Wu, and Falko Judt

Abstract

This study attempts to understand how surface heat fluxes in different storm regions affect tropical cyclone (TC) size. The Advanced Research version of the Weather Research and Forecasting (ARW-WRF) Model (version 3.5.1) is used to simulate Typhoon Megi (2016). A series of numerical experiments are carried out, including a control simulation and several sensitivity experiments with surface heat fluxes suppressed in different TC regions [to mimic the reduction of the wind-induced surface heat exchange (WISHE) feedback in the inner and/or outer core]. The results show that with surface heat fluxes suppressed in the entire domain, the TC tends to be smaller. Meanwhile, the TC size is more sensitive to the surface heat flux change in the outer core than to that in the inner core. Suppressing surface heat fluxes can weaken the rainbands around the suppressed area, which in turn slows down the secondary circulation. When the surface heat flux is suppressed in the inner-core region, the weakening of the secondary circulation associated with the diminished inner rainbands is limited to the inner-core region, and only slightly affects the absolute angular momentum import from the outer region, thus having negligible impact on TC size. However, suppression of surface heat fluxes in the outer-core region leads to less active outer rainbands and a more substantial weakening of secondary circulation. This results in less absolute momentum import from the outer region and in turn a smaller TC.

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Zhi Li, Sjoerd Groeskamp, Ivana Cerovečki, and Matthew H. England

Abstract

Using observationally based hydrographic and eddy diffusivity datasets, a volume budget analysis is performed to identify the main mechanisms governing the spatial and seasonal variability of Antarctic Intermediate Water (AAIW) within the density range γn = [27.25−27.7] kg m−3 in the Southern Ocean. The subduction rates and water mass transformation rates by mesoscale and small-scale turbulent mixing are estimated. Firstly, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter variety AAIW and Subantarctic Mode Water (SAMW). The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. The circumpolar-integrated total subduction yields–5 – 19 Sv of AAIW volume loss. Secondly, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing (2–13 Sv) near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing (1–10 Sv). Lastly, 3–14 Sv of AAIW are exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the ACC and northward into the subtropical gyres. The volume budget analysis reveals strong seasonal variability in the rate of subduction, vertical mixing, and volume transport driving volume change within the AAIW layer. The non-zero volume budget residual suggests that more observations are needed to better constrain the estimate of geostrophic flow, mesoscale and small-scale mixing diffusivities.

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Chen Zhang, Hua Liao, Fang-Zhi Wang, and Ru Li

Abstract

Human behaviors are believed to be sensitive to environmental conditions. However, little is known about the role of temperature in individual daily behaviors. We examine the links between temperature and food intake using nearly one million purchasing records from China. The results show that a 1°C increase in temperature would cause a 0.11% decrease in food intake, which amounts to USD 4.2 million of daily food expenditures nationwide. Moreover, females appear to be more sensitive to the temperature in their food intake than males. In addition, we observe a U-shaped relationship between the temperature and the willingness to order a takeout online, and this observation is robust under multiple alternative estimations. Our results indicate that a higher temperature would reduce energy demand for body thermoregulation, resulting in less food intake. Both extreme high and low temperatures can cause disutility. Therefore, the consumers who still want to satisfy their needs for food intake feel compelled to alter their willingness to pay under the extreme temperature events. The quantitative analysis can provide helpful references for modeling the climate–consumer relationship in integrated assessment models. Thus, it is an interesting avenue for future research to bridge the climate and consumers to identify welfare loss and inequality due to climate change.

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Kuiping Li, Lin Feng, Yanliang Liu, Yang Yang, Zhi Li, and Weidong Yu

Abstract

The intraseasonal oscillations (ISOs) activate in the tropical Indian Ocean (IO), exhibiting distinct seasonal contrasts in active regions and propagating features. The seasonal northward migration of the ISO activity initiates in spring–early summer, composed of two stages. Strong ISO activity first penetrates into the northern Bay of Bengal (BoB) around mid-April, and then extends to the northern Arabian Sea (AS) by mid-May. The northward-propagating ISOs (NPISOs) during their initiation periods, which are referred to as the primary northward-propagating (PNP) events, are analyzed with regard to the BoB and the AS, respectively. In terms of the BoB PNP event, the northward branch could be observed only in the BoB, and the eastward movement is still clear as the winter ISOs. For the AS PNP event, a strong northward branch spreads across the wider northern IO, as obvious as the summer ISOs. The relative roles of the seasonal environmental fields in modulating the PNP events are diagnosed based on a 2.5-layer atmospheric model. The results indicate that the seasonal variations of the surface moisture dominantly regulate the BoB PNP event, while both the surface moisture and the vertical wind shear are necessary for the AS PNP event. Additionally, the leading BoB PNP event is hypothesized to potentially act as a precondition of the following AS PNP event in terms of their internal ISO reinitiation processes and in terms of creating a favorable easterly shear environment in the northern IO.

Open access
Zhi Li, Weidong Yu, Tim Li, V. S. N. Murty, and Fredolin Tangang

Abstract

The annual cycle of tropical cyclone (TC) frequency over the Bay of Bengal (BoB) exhibits a notable bimodal character, different from a single peak in other basins. The causes of this peculiar feature were investigated through the diagnosis of a genesis potential index (GPI) with the use of the NCEP Reanalysis I dataset during the period 1981–2009. A methodology was developed to quantitatively assess the relative contributions of four environmental parameters. Different from a conventional view that the seasonal change of vertical shear causes the bimodal feature, it was found that the strengthened vertical shear alone from boreal spring to summer cannot overcome the relative humidity effect. It is the combined effect of vertical shear, vorticity, and SST that leads to the GPI minimum in boreal summer. It is noted that TC frequency in October–November is higher than that in April–May, which is primarily attributed to the difference of mean relative humidity between the two periods. In contrast, more supercyclones (category 4 or above) occur in April–May than in October–November. It is argued that greater ocean heat content, the first branch of northward-propagating intraseasonal oscillations (ISOs) associated with the monsoon onset over the BoB, and stronger ISO intensity in April–May are favorable environmental conditions for cyclone intensification.

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Xi-Bin Ji, Wen-Zhi Zhao, Er-Si Kang, Zhi-Hui Zhang, Bo-Wen Jin, and Li-Wen Zhao

Abstract

Continuous eddy covariance measurements of CO2, water vapor, and heat fluxes were obtained from a maize field within an oasis in northwest China from 1 May 2008 to 30 April 2009. The experimental setup used was shown to provide reliable flux estimates on the basis of cross-checks made using various quality tests of the flux data. Results show that the highest half-hourly CO2 fluxes (Fc) were −55.7 and 6.9 μmol m−2 s−1 during the growing and nongrowing seasons, respectively. The daily net ecosystem exchange of carbon (NEE) ranged from −14.7 to 2.2 g C m−2 day−1 during the growing season; however, the daily NEE fell to between 0.2 and 2.1 g C m−2 day−1 during the nongrowing season. The annual NEE calculated by integrating flux measurements and filling in missing and spurious data was about −487.9 g C m−2. The total NEE during the growing season (−692.9 g C m−2) and the annual NEE were in the middle of the range, when compared with results obtained for maize fields in different studies and regions, whereas the differences between the off-season NEE from this study (205.0 g C m−2) and those defined in previous studies were very small. In addition, the seasonal variations in energy balance and evapotranspiration over the maize field were also addressed.

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You-xi Gao, Mao-cang Tang, Si-wei Luo, Zhi-bao Shen, and Ci Li

The Qinghai-Xizang (Tibet) Plateau has a profound influence on atmospheric circulation patterns on all time and space scales. This report constitutes a short summary of work being performed at the Lanzhou Institute of Plateau Atmospheric Physics of the Academia Sinica. A short discussion of the climatic characteristics of the plateau is followed by a description of the main features of annual and diurnal cycles in pressure and circulation patterns.

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