In situ observation of a buoys/moorings array and a model simulation were used to study the modulation of upper ocean thermal structure by Typhoon Kalmaegi in September 2014. The inertial period signals were significant after forcing of Kalmaegi, but they did not account for the net heat change. Removing the inertial period signals showed that the net thermal response biased to the right of Kalmaegi's track. Vertical mixing caused surface cooling with an inverted-cone structure and subsurface warming with a double-wing structure. Net upwelling converted the left wing of the subsurface warming to cooling, while net downwelling warmed the upper ocean in front and on both sides of the net upwelling zone. Horizontal advection was not as important as vertical mixing and vertical advection in modulating the thermal structure but contributed to the net outward advection of thermal anomaly in the mixed layer during the forced stage and also in the net along-track recovery of subsurface anomaly during the relaxation stage. In general, horizontal and vertical advection modulated thermal anomalies in the upper ocean across a broader horizontal range and into the deeper ocean compared with the effect of vertical mixing. Our results indicate the need to consider both mixing and advection (rather than only mixing) when studying the effects of tropical cyclones on local ocean heat uptake and global ocean heat transport.Plain Language Summary Tropical cyclones are strong natural phenomena occurring on the ocean. Tropical cyclones intensify ocean mixing and deepen surface mixed layer (defined as a layer with uniform temperature). In so doing, it creates cold anomaly at the surface and warm anomaly in the subsurface, which can be considered as a downward pump of warm water (heat pump effect). The subsurface warming cannot be directly recovered by air-sea surface interaction; it may stay in the ocean and contribute to global ocean heat transport and then influence the climate system. This work studied the upper ocean thermal response to a tropical cyclone (typhoon Kalmaegi) in September 2014. The results show that besides the surface cooling and subsurface warming, typhoon Kalmaegi also cools the subsurface by an upwelling process. Upwelling brings up cold water, and part of subsurface warming is modulated outside of the main response area and into the deeper ocean (cold suction effect). This work indicates that the upper ocean thermal response to a tropical cyclone is more complicated than only heat pump effect. Cold suction effect needs to be taken into consideration when estimating the tropical cyclones' contribution to global ocean heat budget.
Flue gas desulfurization (FGD) gypsum mainly composed of calcium sulfate dihydrate (DH) was used as a raw material to obtain α‐calcium sulfate hemihydrate (α‐HH) through dehydration in a Ca–Mg–K–Cl‐solution medium at 95°C under atmospheric pressure. The effects of potassium sodium tartrate and sodium citrate on the preparation of α‐HH in the electrolyte solution were investigated. The results revealed that the addition of potassium sodium tartrate (1.0 × 10−2–2.5 × 10−2M) decreased the dehydration rate of FGD gypsum and increased the length/width (l/w) ratio of α‐HH crystals, which could yield unfavorable strength properties. Addition of sodium citrate (1.0 × 10−5– 2.0 × 10−5M) slightly increased the dehydration rate of FGD gypsum and decreased the l/w ratio of α‐HH crystals, which could be beneficial to increase strength. However, it also led to a partial formation of anhydrite (AH) crystals. AH was also the only dehydration product when the concentration of sodium citrate increased to 1.0 × 10−4M. Therefore, sodium citrate rather than potassium sodium tartrate could be used as an additive in Ca–Mg–K–Cl electrolyte solutions if α‐HH with a shorter l/w ratio is the desired product from FGD gypsum dehydration. The concentration of sodium citrate should be properly controlled to reduce the formation of AH.
pH is one of the most important parameters that determine the crystallization process, but it is always neglected in the preparation of a-calcium sulfate hemihydrate (a-HH) from calcium sulfate dihydrate (DH) with the hydrothermal method. Flue gas desulfurization (FGD) gypsum, which is mainly composed of DH, was used as raw material to obtain a-HH through dehydration in a Ca-Mg-K-Cl-solution medium at 951C under atmospheric pressure. The initial pH values of the suspensions were adjusted from 1.2 to 8.0 to explore the influence of pH on the dehydration process and the product characteristics. The results showed that a-HH crystal was the only dehydration product with the pH ranging from 1.2 to 8.0. With the increase of initial pH, the dehydration rate decreased and the formed a-HH crystal had a larger particle size. The length/width ratio decreased markedly from 4.8 to 2.9 as the initial pH increased from 1.2 to 7.3. pH had a profound influence on the dehydration of DH and the morphology of a-HH via its effect on the supersaturation and perhaps also the precipitation of Ca(OH) 2 in an alkaline environment.
The phase transitions of α-calcium sulfate hemihydrate (α-HH) in KCl solutions were studied at (80 to 105) °C under atmospheric pressure in an attempt to estimate the metastability of α-HH and to sketch out the routes for α-HH’s hydration, dehydration, or recrystallization. The crystal water content and morphology of the product during the reaction period were comprehensively examined. It was found that the metastability of α-HH heavily relied on KCl concentration and temperature. The phase transition of α-HH is successfully outlined as a function of KCl concentration and temperature. Calcium sulfate dihydrate (DH) is stable in region I and metastable in region III. Anhydrite (AH) is stable in regions III and IV and coexists with goergeyite. HH is metastable in only region II. The results show that α-HH tends to hydrate or dehydrate in KCl solution, but there is still a narrow metastable region for α-HH crystallization in which KCl concentration and temperature are strictly limited.
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