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- Author or Editor: Jun Yang x
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Abstract
Comparisons between anomaly and full-field methods have been carried out in weather analysis and forecasting over the last decade. Evidence from these studies has demonstrated the superiority of anomaly to full field in the following four aspects: depiction of weather systems, anomaly forecasts, diagnostic parameters, and model prediction. To promote the use and further discussion of the anomaly approach, this article summarizes those findings. After examining many types of weather events, anomaly weather maps show at least five advantages in weather system depiction: 1) less vagueness in visually connecting the location of an event with its associated meteorological conditions, 2) clearer and more complete depictions of vertical structures of a disturbance, 3) easier observation of time and spatial evolution of an event and its interaction or connection with other weather systems, 4) simplification of conceptual models by unifying different weather systems into one pattern, and 5) extension of model forecast length due to earlier detection of predictors. Anomaly verification is also mentioned. The anomaly forecast is useful for raising one’s awareness of potential societal impact. Combining the anomaly forecast with an ensemble is emphasized, where a societal impact index is discussed. For diagnostic parameters, two examples are given: an anomalous convective instability index for convection, and seven vorticity and divergence related parameters for heavy rain. Both showed positive contributions from the anomalous fields. For model prediction, the anomaly version of the beta-advection model consistently outperformed its full-field version in predicting typhoon tracks with clearer physical explanation. Application of anomaly global models to seasonal forecasts is also reviewed.
Abstract
Comparisons between anomaly and full-field methods have been carried out in weather analysis and forecasting over the last decade. Evidence from these studies has demonstrated the superiority of anomaly to full field in the following four aspects: depiction of weather systems, anomaly forecasts, diagnostic parameters, and model prediction. To promote the use and further discussion of the anomaly approach, this article summarizes those findings. After examining many types of weather events, anomaly weather maps show at least five advantages in weather system depiction: 1) less vagueness in visually connecting the location of an event with its associated meteorological conditions, 2) clearer and more complete depictions of vertical structures of a disturbance, 3) easier observation of time and spatial evolution of an event and its interaction or connection with other weather systems, 4) simplification of conceptual models by unifying different weather systems into one pattern, and 5) extension of model forecast length due to earlier detection of predictors. Anomaly verification is also mentioned. The anomaly forecast is useful for raising one’s awareness of potential societal impact. Combining the anomaly forecast with an ensemble is emphasized, where a societal impact index is discussed. For diagnostic parameters, two examples are given: an anomalous convective instability index for convection, and seven vorticity and divergence related parameters for heavy rain. Both showed positive contributions from the anomalous fields. For model prediction, the anomaly version of the beta-advection model consistently outperformed its full-field version in predicting typhoon tracks with clearer physical explanation. Application of anomaly global models to seasonal forecasts is also reviewed.
Abstract
Cloud detection is the precondition for deriving other information (e.g., cloud cover) in ground-based sky imager applications. This paper puts forward an effective cloud detection approach, the Hybrid Thresholding Algorithm (HYTA) that fully exploits the benefits of the combination of fixed and adaptive thresholding methods. First, HYTA transforms an input color cloud image into a normalized blue/red channel ratio image that can keep a distinct contrast, even with noise and outliers. Then, HYTA identifies the ratio image as either unimodal or bimodal according to its standard deviation, and the unimodal and bimodal images are handled by fixed and minimum cross entropy (MCE) thresholding algorithms, respectively. The experimental results demonstrate that HYTA shows an accuracy of 88.53%, which is far higher than those of either fixed or MCE thresholding alone. Moreover, HYTA is also verified to outperform other state-of-the-art cloud detection approaches.
Abstract
Cloud detection is the precondition for deriving other information (e.g., cloud cover) in ground-based sky imager applications. This paper puts forward an effective cloud detection approach, the Hybrid Thresholding Algorithm (HYTA) that fully exploits the benefits of the combination of fixed and adaptive thresholding methods. First, HYTA transforms an input color cloud image into a normalized blue/red channel ratio image that can keep a distinct contrast, even with noise and outliers. Then, HYTA identifies the ratio image as either unimodal or bimodal according to its standard deviation, and the unimodal and bimodal images are handled by fixed and minimum cross entropy (MCE) thresholding algorithms, respectively. The experimental results demonstrate that HYTA shows an accuracy of 88.53%, which is far higher than those of either fixed or MCE thresholding alone. Moreover, HYTA is also verified to outperform other state-of-the-art cloud detection approaches.
Abstract
The total mass of the atmosphere [or equivalently, the background surface pressure (SP)] may have varied significantly over the evolutionary histories of Earth and other planets. Atmospheric mass can affect climate by modifying physical processes, including shortwave scattering, the emissivity of greenhouse gases, the atmospheric heat capacity, and surface fluxes. We apply a three-dimensional global climate model to explore the dependence of climate on SP over the range of 0.5–2.5 bar. Our simulations show an intriguing, nonmonotonic dependence of climate on SP. Over the SP range of 0.5–0.9 and 1.5–2.5 bar, the surface temperature increases with SP; however, over the SP range of 0.9–1.5 bar, the surface temperature decreases with SP. The negative correlation is due to a convection–circulation–cloud coupled feedback. As SP increases, the moist adiabatic lapse rate increases, leading to upper-troposphere cold anomalies in the tropics and middle latitudes that increase the midlatitude baroclinicity and eddy activity. In association with these changes, the eddy-driven jet is strengthened and shifts equatorward, and two separate westerly jets merge into a single jet. These abrupt circulation changes result in an equatorward shift of the midlatitude cloud belt and reduction of polar clouds, which induce strong negative cloud radiative forcing that cools the climate. Our results demonstrate that the regime transition of flow state (e.g., the merge of jets here) may induce large anomalies in clouds and radiative forcing, resulting in nonlinear climate responses.
Abstract
The total mass of the atmosphere [or equivalently, the background surface pressure (SP)] may have varied significantly over the evolutionary histories of Earth and other planets. Atmospheric mass can affect climate by modifying physical processes, including shortwave scattering, the emissivity of greenhouse gases, the atmospheric heat capacity, and surface fluxes. We apply a three-dimensional global climate model to explore the dependence of climate on SP over the range of 0.5–2.5 bar. Our simulations show an intriguing, nonmonotonic dependence of climate on SP. Over the SP range of 0.5–0.9 and 1.5–2.5 bar, the surface temperature increases with SP; however, over the SP range of 0.9–1.5 bar, the surface temperature decreases with SP. The negative correlation is due to a convection–circulation–cloud coupled feedback. As SP increases, the moist adiabatic lapse rate increases, leading to upper-troposphere cold anomalies in the tropics and middle latitudes that increase the midlatitude baroclinicity and eddy activity. In association with these changes, the eddy-driven jet is strengthened and shifts equatorward, and two separate westerly jets merge into a single jet. These abrupt circulation changes result in an equatorward shift of the midlatitude cloud belt and reduction of polar clouds, which induce strong negative cloud radiative forcing that cools the climate. Our results demonstrate that the regime transition of flow state (e.g., the merge of jets here) may induce large anomalies in clouds and radiative forcing, resulting in nonlinear climate responses.
Abstract
The Dual-Frequency Precipitation Radar (DPR) on board the Global Precipitation Measurement (GPM) mission core satellite provides the new-generation global observation of rain since 2014. The main objective of this paper is to evaluate the suitability and limitation of GPM-DPR level-2 products over China. The DPR rain rate products are compared with rain gauge data during the summers of 5 years (2014–18). The ground observation network is composed of more than 50 000 rain gauges. The DPR precipitation products for all scans (DPR_NS, DPR_MS, and DPR_HS) generally underestimate rain rates. However, DPR_MS agrees better with gauge estimates than DPR_NS and DPR_HS, yielding the lowest mean error, systematic deviation, and highest Pearson correlation coefficient. In addition, all three swath types show obvious overestimation over gauge estimates between 0.5 and 1 mm h−1 and underestimation when gauge estimates are larger than 1 mm h−1. The DPR_HS and DPR_MS agree better with gauge estimates below and above 2.5 mm h−1, respectively. A deeper investigation was carried out to analyze the variation of DPR_MS’s performance with respect to terrains over China. An obvious underestimation, relative to gauge estimates, occurs in Tibetan Plateau while a slight overestimation occurs in the North China Plain. Furthermore, our comprehensive analysis suggests that in Sichuan Basin, the DPR_MS exhibit the best agreement with gauge estimates.
Abstract
The Dual-Frequency Precipitation Radar (DPR) on board the Global Precipitation Measurement (GPM) mission core satellite provides the new-generation global observation of rain since 2014. The main objective of this paper is to evaluate the suitability and limitation of GPM-DPR level-2 products over China. The DPR rain rate products are compared with rain gauge data during the summers of 5 years (2014–18). The ground observation network is composed of more than 50 000 rain gauges. The DPR precipitation products for all scans (DPR_NS, DPR_MS, and DPR_HS) generally underestimate rain rates. However, DPR_MS agrees better with gauge estimates than DPR_NS and DPR_HS, yielding the lowest mean error, systematic deviation, and highest Pearson correlation coefficient. In addition, all three swath types show obvious overestimation over gauge estimates between 0.5 and 1 mm h−1 and underestimation when gauge estimates are larger than 1 mm h−1. The DPR_HS and DPR_MS agree better with gauge estimates below and above 2.5 mm h−1, respectively. A deeper investigation was carried out to analyze the variation of DPR_MS’s performance with respect to terrains over China. An obvious underestimation, relative to gauge estimates, occurs in Tibetan Plateau while a slight overestimation occurs in the North China Plain. Furthermore, our comprehensive analysis suggests that in Sichuan Basin, the DPR_MS exhibit the best agreement with gauge estimates.
How Accurate Are Satellite-Derived Surface Solar Radiation Products over Tropical Oceans?Abstract
Surface solar radiation (SSR) over the ocean is essential for studies of ocean–atmosphere interactions and marine ecology, and satellite remote sensing is a major way to obtain the SSR over ocean. A new high-resolution (10 km; 3 h) SSR product has recently been developed, mainly based on the newly released cloud product of the International Satellite Cloud Climatology Project H series (ISCCP-HXG), and is available for the period from July 1983 to December 2018. In this study, we compared this SSR product with in situ observations from 70 buoy sites in the Global Tropical Moored Buoy Array (GTMBA) and also compared it with another well-known satellite-derived SSR product from the Clouds and the Earth’s Radiant Energy System (CERES; edition 4.1), which has a spatial resolution of approximately 100 km. The results show that the ISCCP-HXG SSR product is generally more accurate than the CERES SSR product for both ocean and land surfaces. We also found that the accuracy of both satellite-derived SSR products (ISCCP-HXG and CRERS) was higher over ocean than over land and that the accuracy of ISCCP-HXG SSR improves greatly when the spatial resolution of the product is coarsened to ≥ 30 km.
Abstract
Surface solar radiation (SSR) over the ocean is essential for studies of ocean–atmosphere interactions and marine ecology, and satellite remote sensing is a major way to obtain the SSR over ocean. A new high-resolution (10 km; 3 h) SSR product has recently been developed, mainly based on the newly released cloud product of the International Satellite Cloud Climatology Project H series (ISCCP-HXG), and is available for the period from July 1983 to December 2018. In this study, we compared this SSR product with in situ observations from 70 buoy sites in the Global Tropical Moored Buoy Array (GTMBA) and also compared it with another well-known satellite-derived SSR product from the Clouds and the Earth’s Radiant Energy System (CERES; edition 4.1), which has a spatial resolution of approximately 100 km. The results show that the ISCCP-HXG SSR product is generally more accurate than the CERES SSR product for both ocean and land surfaces. We also found that the accuracy of both satellite-derived SSR products (ISCCP-HXG and CRERS) was higher over ocean than over land and that the accuracy of ISCCP-HXG SSR improves greatly when the spatial resolution of the product is coarsened to ≥ 30 km.
Collection of Gas-Tight Water Samples from the Bottom of the Challenger DeepAbstract
A new gas-tight pair sampler was designed for the collection of gas-tight fluid samples from the hadal zone. The sampler uses two titanium bottles and one sampling valve to collect two samples at once. The sampler can be deployed in the deepest trenches in the ocean as a result of its ability to resist ultrahigh pressure and its good bidirectional sealing performance. It can be used on manned submersibles, remotely operated vehicles, and deep-sea landers. Three sets of this new sampler were constructed and field tested in the Mariana Trench during the cruise TS-03 from 15 January to 23 March 2017, during which 3 L of water samples were successfully obtained from the bottom of the Challenger Deep.
Abstract
A new gas-tight pair sampler was designed for the collection of gas-tight fluid samples from the hadal zone. The sampler uses two titanium bottles and one sampling valve to collect two samples at once. The sampler can be deployed in the deepest trenches in the ocean as a result of its ability to resist ultrahigh pressure and its good bidirectional sealing performance. It can be used on manned submersibles, remotely operated vehicles, and deep-sea landers. Three sets of this new sampler were constructed and field tested in the Mariana Trench during the cruise TS-03 from 15 January to 23 March 2017, during which 3 L of water samples were successfully obtained from the bottom of the Challenger Deep.
Abstract
This study describes a new handheld sampler, specially designed to be deployed by scuba divers, to collect fluid samples from shallow hydrothermal vents. The new sampler utilizes a syringe-like titanium sampling bottle with a regulated filling rate to collect samples. The filling rate regulation mechanism of the new sampler was studied. Through theoretical analysis and simulation, it is found that the filling rate can be regulated by either an orifice or an annular gap on the sampler. Further study indicates that the orifice is superior to the annular gap, since the former has a much lower requirement of machining accuracy. Moreover, the filling rate regulated by the orifice is independent of temperature and ambient pressure. The new sampler also features a compact structure, simple operation, and gas-tight performance. Efforts were made to minimize the organic carbon blank of the sampler by careful selection of the materials that may come into contact with the fluid samples. The sampler has been tested at the shallow hydrothermal vents off northeastern Taiwan. High-purity organic samples were successfully collected.
Abstract
This study describes a new handheld sampler, specially designed to be deployed by scuba divers, to collect fluid samples from shallow hydrothermal vents. The new sampler utilizes a syringe-like titanium sampling bottle with a regulated filling rate to collect samples. The filling rate regulation mechanism of the new sampler was studied. Through theoretical analysis and simulation, it is found that the filling rate can be regulated by either an orifice or an annular gap on the sampler. Further study indicates that the orifice is superior to the annular gap, since the former has a much lower requirement of machining accuracy. Moreover, the filling rate regulated by the orifice is independent of temperature and ambient pressure. The new sampler also features a compact structure, simple operation, and gas-tight performance. Efforts were made to minimize the organic carbon blank of the sampler by careful selection of the materials that may come into contact with the fluid samples. The sampler has been tested at the shallow hydrothermal vents off northeastern Taiwan. High-purity organic samples were successfully collected.
Abstract
Photosynthetically active radiation (PAR) is absorbed by plants to carry out photosynthesis. Its estimation is important for many applications such as ecological modeling. In this study, a broadband transmittance scheme for solar radiation at the PAR band is developed to estimate clear-sky PAR values. The influence of clouds is subsequently taken into account through sunshine-duration data. This scheme is examined without local calibration against the observed PAR values under both clear- and cloudy-sky conditions at seven widely distributed Surface Radiation Budget Network (SURFRAD) stations. The results indicate that the scheme can estimate the daily mean PAR at these seven stations under all-sky conditions with root-mean-square error and mean bias error values ranging from 6.03 to 6.83 W m−2 and from −2.86 to 1.03 W m−2, respectively. Further analyses indicate that the scheme can estimate PAR values well with globally available aerosol and ozone datasets. This suggests that the scheme can be applied to regions for which observed aerosol and ozone data are not available.
Abstract
Photosynthetically active radiation (PAR) is absorbed by plants to carry out photosynthesis. Its estimation is important for many applications such as ecological modeling. In this study, a broadband transmittance scheme for solar radiation at the PAR band is developed to estimate clear-sky PAR values. The influence of clouds is subsequently taken into account through sunshine-duration data. This scheme is examined without local calibration against the observed PAR values under both clear- and cloudy-sky conditions at seven widely distributed Surface Radiation Budget Network (SURFRAD) stations. The results indicate that the scheme can estimate the daily mean PAR at these seven stations under all-sky conditions with root-mean-square error and mean bias error values ranging from 6.03 to 6.83 W m−2 and from −2.86 to 1.03 W m−2, respectively. Further analyses indicate that the scheme can estimate PAR values well with globally available aerosol and ozone datasets. This suggests that the scheme can be applied to regions for which observed aerosol and ozone data are not available.
Abstract
The “Snowball Earth” hypothesis, proposed to explain the Neoproterozoic glacial episodes in the period 750–580 million years ago, suggested that the earth was globally covered by ice/snow during these events. This study addresses the problem of the forcings required for the earth to enter such a state of complete glaciation using the Community Climate System Model, version 3 (CCSM3). All of the simulations performed to address this issue employ the geography and topography of the present-day earth and are employed to explore the combination of factors consisting of total solar luminosity, CO2 concentration, and sea ice/snow albedo parameterization that would be required for such an event to occur. The analyses demonstrate that the critical conditions beyond which runaway ice–albedo feedback will lead to global freezing include 1) a 10%–10.5% reduction in solar radiation with preindustrial greenhouse gas concentrations; 2) a 6% reduction in solar radiation with 17.5 ppmv CO2; or 3) 6% less solar radiation and 286 ppmv CO2 if sea ice albedo is equal to or greater than 0.60 with a snow albedo of 0.78, or if sea ice albedo is 0.58 with a snow albedo equal to or greater than 0.80. These bifurcation points are very sensitive to the sea ice and snow albedo parameterizations. Moreover, “soft Snowball” solutions are found in which tropical open water oceans stably coexist with year-round snow-covered low-latitude continents, implying that tropical continental ice sheets would actually be present. The authors conclude that a “soft Snowball” is entirely plausible, in which the global sea ice fraction may reach as high as 76% and sea ice margins may extend to 10°S(N) latitudes.
Abstract
The “Snowball Earth” hypothesis, proposed to explain the Neoproterozoic glacial episodes in the period 750–580 million years ago, suggested that the earth was globally covered by ice/snow during these events. This study addresses the problem of the forcings required for the earth to enter such a state of complete glaciation using the Community Climate System Model, version 3 (CCSM3). All of the simulations performed to address this issue employ the geography and topography of the present-day earth and are employed to explore the combination of factors consisting of total solar luminosity, CO2 concentration, and sea ice/snow albedo parameterization that would be required for such an event to occur. The analyses demonstrate that the critical conditions beyond which runaway ice–albedo feedback will lead to global freezing include 1) a 10%–10.5% reduction in solar radiation with preindustrial greenhouse gas concentrations; 2) a 6% reduction in solar radiation with 17.5 ppmv CO2; or 3) 6% less solar radiation and 286 ppmv CO2 if sea ice albedo is equal to or greater than 0.60 with a snow albedo of 0.78, or if sea ice albedo is 0.58 with a snow albedo equal to or greater than 0.80. These bifurcation points are very sensitive to the sea ice and snow albedo parameterizations. Moreover, “soft Snowball” solutions are found in which tropical open water oceans stably coexist with year-round snow-covered low-latitude continents, implying that tropical continental ice sheets would actually be present. The authors conclude that a “soft Snowball” is entirely plausible, in which the global sea ice fraction may reach as high as 76% and sea ice margins may extend to 10°S(N) latitudes.
Abstract
This study investigates the climate dynamic feedbacks during a transition from the present climate to the extremely cold climate of a “Snowball Earth” using the Community Climate System Model, version 3 (CCSM3). With the land–sea distribution fixed to modern, it is found that by reducing solar luminosity and/or carbon dioxide concentration: 1) the amount of atmospheric water vapor and its attendant greenhouse effect decrease with the logarithm of sea ice cover, thereby promoting the expansion of sea ice; 2) over the sea ice, the cloud radiative feedback is positive, thus enhancing sea ice advance; over the ocean, the cloud radiative feedback is first negative and then becomes positive as sea ice enters the tropics; and 3) the strength of the atmospheric Hadley cell and the wind-driven ocean circulation increases significantly in the Southern Hemisphere, inhibiting the expansion of sea ice into the tropics. Meanwhile, the North Atlantic Deep Water cell disappears and the Antarctic Bottom Water cell strengthens and expands to occupy almost the entire Atlantic basin. In the experiment with 6% less solar radiation and 70 ppmv CO2 compared to the control experiment with 100% solar radiation and 355 ppmv CO2 near the ice edge (28°S latitude), the changes of solar radiation, CO2 forcing, water vapor greenhouse effect, longwave cloud forcing at the top of the model, and atmospheric and oceanic energy transport are −22.4, −6.2, −54.4, +6.2, and +16.3 W m−2, respectively. Therefore, the major controlling factors in producing global ice cover are ice albedo feedback (Yang et al., Part I) and water vapor feedback.
Abstract
This study investigates the climate dynamic feedbacks during a transition from the present climate to the extremely cold climate of a “Snowball Earth” using the Community Climate System Model, version 3 (CCSM3). With the land–sea distribution fixed to modern, it is found that by reducing solar luminosity and/or carbon dioxide concentration: 1) the amount of atmospheric water vapor and its attendant greenhouse effect decrease with the logarithm of sea ice cover, thereby promoting the expansion of sea ice; 2) over the sea ice, the cloud radiative feedback is positive, thus enhancing sea ice advance; over the ocean, the cloud radiative feedback is first negative and then becomes positive as sea ice enters the tropics; and 3) the strength of the atmospheric Hadley cell and the wind-driven ocean circulation increases significantly in the Southern Hemisphere, inhibiting the expansion of sea ice into the tropics. Meanwhile, the North Atlantic Deep Water cell disappears and the Antarctic Bottom Water cell strengthens and expands to occupy almost the entire Atlantic basin. In the experiment with 6% less solar radiation and 70 ppmv CO2 compared to the control experiment with 100% solar radiation and 355 ppmv CO2 near the ice edge (28°S latitude), the changes of solar radiation, CO2 forcing, water vapor greenhouse effect, longwave cloud forcing at the top of the model, and atmospheric and oceanic energy transport are −22.4, −6.2, −54.4, +6.2, and +16.3 W m−2, respectively. Therefore, the major controlling factors in producing global ice cover are ice albedo feedback (Yang et al., Part I) and water vapor feedback.
