Dynamics of Atmospheres and Oceans最新文献

Maximum potential intensity (MPI), which a TC may reach in certain environment conditions, can be affected by microphysical processes. Latent heat released in the process of TC development plays a significant role in it. However, the impacts of hail added both to single-moment and double-moment microphysics parameterization scheme on the MPI remain unclear. In this study, high-resolution sensitivity experiments are conducted in the Weather Research and Forecasting (WRF) model by using four bulk microphysics schemes belonging to a family, namely, WRF Single-Moment 6-Class (WSM6) scheme, WRF Double-Moment 6-Class (WDM6) scheme, WRF Single-Moment 7-Class (WSM7) scheme, WRF Double-Moment 7-Class (WDM7) scheme. Results show that SM schemes simulate the greater MPI than DM schemes. Adding hail in SM scheme increases the MPI while in DM scheme makes less difference. There is a close relationship between the MPI and the radial peak location and intensity of latent heat. The closer the latent heat peak is to the TC center and the greater the peak intensity is, the greater the MPI can be achieved. Though the presence of hail plays a cooling effect thermally, it may affect the TC structures due to the larger sedimentation speed. WSM7 scheme including hail microphysics simulates the TC with smaller size and eye wall inclination, and thus the latent heating efficiency in the eye wall is higher, which is more conducive to TC intensification. However, the larger content of hail resulting from the accretion of liquid water in WDM7 scheme brings a stronger cooling effect and probably offsets the dynamic advantage.

Chang, Wolfe, Stewart, and McWilliams commented on my recent work regarding the horizontal gravity disturbance vector in atmospheric and oceanic dynamics. Their comments are incorrect. They ignored the horizontal pressure gradient error, claimed the validity of the spheroidal geopotential approximation subjectively based only on small metric error, and decomposed gravity into gravitational and centrifugal accelerations, which should never have been done. Here, I explain further why the spheroidal geopotential approximation is invalid and why gravity cannot be decomposed into gravitational and centrifugal accelerations in atmospheric and oceanic dynamics. Physically, the horizontal gravity disturbance vector vanishes in the horizontal momentum equation using the true gravity g_t in the true geopotential coordinates but does occur in the horizontal momentum equation using the true gravity g_t in the spheroidal geopotential coordinates due to the horizontal pressure gradient error in the coordinate transformation. The error of horizontal pressure gradient force in transforming true geopotential to spheroidal geopotential coordinates equals to the horizontal gravity disturbance vector. The spheroidal geopotential approximation claimed by Chang, Wolfe, Stewart, and McWilliams is invalid.

This study aims to investigate the role of tropical-extratropical interactions in the formation of atmospheric rivers as an important source of moisture in extreme-widespread precipitation of Iran. Extreme precipitation events are extracted based on the 95th percentile index from 1989 to 2019 in Iran. Then, the threshold of widespread precipitation is determined. A day is defined as having extreme-widespread precipitation if one-third of the selected stations have mostly extreme precipitation. Finally, 9 days with the highest extreme precipitation and spatial continuity are selected. The upper air data of the 9 selected days are extracted and, accordingly, synoptic charts are plotted. The used data include ERA5, which are extracted from the lower (800 and 850 hPa) and middle (500, 600 and 700 hPa) levels. The results indicate an interaction with tropical circulation patterns by penetration of extratropical circulation patterns into tropical and subtropical regions. The interaction between patterns could lead to the formation of atmospheric rivers with tropical origin from ITCZ, their transport to subtropical and extratropical regions and their moisture supply along the path by different moisture sources in tropical, subtropical and extratropical regions. The formed atmospheric rivers are divided into two categories, namely continental and oceanic, based on their formation origin. The continental atmospheric river is formed at the lower level and, in some cases, at the middle level, while the oceanic atmospheric river is only formed at the middle level. With the emergence of atmospheric rivers in Iran, there have been extreme-widespread precipitation events due to unstable conditions and rising atmosphere.

On September 29, 2021, Cyclone Shaheen attained significant development in the Arabian Sea. It proceeded to cross the Gulf of Oman on October 3, causing substantial economic damage and casualties due to the heavy rain and high waves it generated. Using meteorological and satellite data, measuring the physical and chemical properties of the water column by CTD a few days before and after the cyclone, and coastal observations, we analyzed the impact of Cyclone Shaheen on the northern shelf of the Gulf of Oman, Iran. High sea surface temperature in the Arabian Sea favored to strengthen the cyclone toward the Gulf of Oman. Strong winds over 20 m s^-1 caused a dust storm on the Iranian Makran coast followed by heavy rains of 72 mm during a day with extensive flooding. Before the passage of cyclone Shaheen, the surface water temperature in the northern shelf of the Gulf of Oman was about 32°C and the dissolved oxygen concentration was 6 mg l^-1, which reached the hypoxia threshold at a depth of about 60 m. The cyclone intensified the vertical mixing in the upper layer, leading to a decrease in surface water temperature by approximately 2–5 °C. Additionally, it pushed the hypoxia boundary down to a depth of 110 m, thereby causing the oxygenated upper layer to become thicker. The current research demonstrates that the Gulf of Oman stratified waters above the oxygen minimum zone could benefit from the passage of the tropical cyclone during the warm season in terms of temperature, dissolved oxygen, and probably dissolved nutrients.

Recent state transition of the Beaufort Gyre has drawn great interest in the Arctic research community, but how the upper ocean hydrographic structure varies with this transition remains poorly understood. The upper ocean mixed layer plays an important role in climatic and ecological processes. Therefore, we analyze the Ice-Tethered Profiler (ITP) observations over the last two decades (2004–2022) to investigate the long-term trend of the mixed layer in the Arctic Ocean’s Beaufort Gyre (BG) from an observational perspective. Results show that the linear trend of the BG surface mixed layer depth (MLD) before and after 2015 has changed significantly, characterized by the vanishing or even reversal of the significant deepening trend. This transition is most pronounced in winter. The BG winter mixed layer is significantly cooler, saltier and denser in the mid-transition period (2013–2017) compared to the pre-transition period (2004–2012), but becomes significantly warmer, fresher and lighter in the post-transition period (2018–2022). The transition feature of the depth of maximum buoyancy frequency in the upper BG is similar to that of MLD, while this maximum decreases significantly in both the mid- and post-transition period when compared to their previous period. The deepening signal of MLD is propagated eastward, which coincides with the recent transition of BG position and freshwater distribution. Mechanism analysis further reveals that the reversal of winter MLD trend before and after 2015 may be due to changes in surface wind stirring and Ekman pumping. This study extends the investigation of the recent state transition of BG considering the upper hydrographic structure.

Precipitation is a crucial component of the water cycle and is essential for the livelihood of people and ecosystems; therefore, understanding precipitation parameters is vital. Stable isotopes in precipitation can provide important information on precipitation sources, atmospheric circulation patterns, and hydrological processes. In this study, stable isotopes in precipitation for four cities in India were analyzed, namely New Delhi, Hyderabad, Shillong, and Calicut, using data from the International Atomic Energy Agency (IAEA) Global Network for Isotopes in Precipitation (GNIP). The GNIP data were supplemented with in-situ measurements. Results showed the correlations between climate-related factors such as surface air temperature and precipitation levels with the stable isotope composition of precipitation. The relationships critically explore interannual variations in the isotope data over the last three decades. The Local Meteoric Water Line (LMWL) for New Delhi and Hyderabad had intercepts less than 10‰, implying a higher evaporation effect over precipitation, consistent with arid and semi-arid regions with increased altitude. The weighted average value of d-excess for southern and Himalayan points were 10.7 and 12.7, respectively, and the average value of δ¹⁸O were − 3.65 and − 5.84, and δ²H were − 16.8 and − 35.8. The d-excess value was significantly lower in the Northern part (New Delhi), with an average weighted value of 6.6. The key values include the isotopic composition of rainfall in different regions of India, the LMWL for different stations, the d-excess value, and the consistency of meteoric water lines with regional and global values. The results of this study provide valuable information on the variability of stable isotopes in precipitation in India. The study's outcomes can be compared with the isotopic composition of surface water and groundwater. This discovery offers more understanding of the isotopic differences that occur on a smaller scale during organized convection and the factors that affect them. As a result, it enhances our ability to decipher the paleoclimate data in arid, semi-arid, and subtropical monsoon regions.

Deforestation in tropical areas is broadly reported to change the climate both locally and regionally. Warmer and drier conditions, as well as changes in precipitation patterns, are linked to deforestation in the Amazon. In this study, we identified two areas of distinct land use in Amazon: a preserved native forest and an increasingly deforested region southward. Due to the scarcity of available ground data, we propose assessing the impacts of deforestation on surface climate using two different datasets: a station-based reference product and the ERA5 reanalysis. However, as ERA5 does not include a recent and dynamic land use map in its development, an additional goal is to evaluate the potential discrepancies in the reanalysis for not accounting for these changes. Despite some consistent and similar patterns in relative humidity and low intensity (9th decile) precipitation, our results show, indeed, different trends among the datasets, with reference trends always more accentuated than in the reanalysis. Despite being broadly used in numerous studies, reanalysis data under intensive land use change and ungauged areas need to be used with caution to avoid inconclusive or misleading findings.

The Sichuan Basin (SCB) and its surrounding mountainous areas have complicated topography, and the "Nocturnal Rainfall in the Basin (NRB)" and "Nocturnal Rainfall in the Mountainous Areas (NRMA)" are frequent. To further clarify the relationship and the interaction between the two types of "nocturnal rainfall" synoptic systems, the characteristics of the synoptic meteorology, thermodynamics, dynamics, and water vapor fields of the nocturnal precipitation process generated by two Mesoscale Convective Systems (MCSs) that originated from the mountainous area on the western side of the SCB on June 4, 2019 were analyzed and diagnosed in this paper. In addition, the WRF-LES model was used to simulate and analyze the macro and micro physical characteristics of two precipitation centers formed by the main system MCS1 in the center of the SCB and the mountainous areas around it. The results showed as follows. (1) Two MCSs originated from the mountainous area on the western side of the SCB were generated by the eastward movement of the low-pressure trough over the Tibetan Plateau (TP) coupling the higher Convective Available Potential Energy (CAPE) value with the unstable circulation of the upper level. They matured in the SCB and matured and split in the western end of Mt. Daba, respectively. (2) After splitting, the southern part of the sub-system MCS2 sank and moved south along the southern foot of Mt. Daba and uplifted the main system MCS1. After its explosive development, two precipitation centers with the characteristics of the NRB and NRMA were formed. (3) The vertical velocity, divergence, thermal helicity, and potential vorticity could be used as the thermodynamics and dynamics diagnostic quantities to indicate the occurrence and development of the two MCSs. The potential vorticity was an obvious precursory parameter compared with the other three. The water vapor flux divergence and moisture helicity could better indicate the vertical transport of water vapor in the systems. (4) The precipitation simulation result of the WRF-LES model on the main system MCS1 in the SCB was better than that on the sub-system MCS2 in the mountainous area on the northern margin of the SCB. In each MCS1 stage, the precipitation of the NRB was mainly induced by the cold cloud process (supplemented by the warm cloud process), while the precipitation of the NRMA was mainly induced by the warm cloud process. The combination of diagnostic analysis and numerical simulation could effectively promote an understanding of the relationship and interaction between the NRB and NRMA.

In the present study, we focused on the heat waves (HW) associated with the quasi-biweekly (QBW, 10–20-day period) variability (QBW-HW) over Southern China (SC, 102º–120ºE, 21º–30ºN) in the Flexible Global Ocean–Atmosphere–Land System Model, GridPoint version 3 (FGOALS-g3), and the HW-associated structures and surface air temperature budget investigated by using model outputs from historical experiment of Coupled Model Intercomparison Project Phase 6 (CMIP6). We found that the anomalous circulations related to the QBW-HW events over SC are closely linked to the southeastward propagation of the wave train from mid-high latitudes and the northwestward-propagating disturbances from the tropics. The results also showed that adiabatic and diabatic heating play a key role in the QBW-HW over SC. These results are in good agreement with observations from previous studies. In addition, QBW-HWs are dry in the FGOALS-g3 model, while observed humid HWs occur over SC. The difference is mainly due to the new boundary layer scheme incorporated in the FGOALS-g3 model, which overestimates the entrainment process at the top of the boundary layer during the QBW-HW over SC, resulting in more and drier air into the boundary layer, and thus less moisture. It implies that the entrainment equation at the top of the boundary layer in the FGOALS-g3 model does need to be improved to be suitable for humid HW processes, although the boundary layer scheme can improve the model precipitation and radiative forcing.

The absence of robust quantitative evaluation methods has led to insufficient knowledge of models capability on the rapid intensification (RI) prediction of tropical cyclones (TCs). In this study, we propose a method and define some indicators aiming to evaluate model capability on predicting RI in a more accurate manner. An assessment of model predictive capability on RI of TCs based on 10 years of operational forecasts has been conducted using different RI criteria. The Tropical Regional Atmosphere Model for the South China Sea of China Meteorological Administration (CMA-TRAMS) and the European Centre for Medium-Range Weather Forecasts (ECMWF) high resolution (HRES) operational forecasts were used. Analysis results revealed that the criterion of 6-hour sea level pressure (SLP) change is more appropriate to be used in RI operational forecast. The maximum lead time (MLT) of CMA-TRAMS and HRES was 72 and 78 h, and the maximum deviation of RI occurrence time of CMA-TRAMS and HRES was 48 h delay and 24 h ahead, respectively. Overall results suggest that the model predictive capability of RI is currently limited, and both models have inadequate capability in providing sufficient heat and energy to support RI in the long run. A tendency of CMA-TRAMS to have a lag in RI occurrence time was also demonstrated due to an air-sea interaction lag resulting from the fixed skin sea surface temperature used. Results of the present study provide insights and could be the basis for future efforts on improving parametrization schemes for properly describing RI process of TCs.