Journal of Geophysical Research: Atmospheres最新文献

This study investigates the factors influencing runoff response to atmospheric rivers (ARs) over the U.S. West Coast. We focused on runoff time series variations impacted by AR characteristics (e.g., category and frequency) and land preconditions during Northern Hemisphere cool seasons in the period of 1940–2023. Results show that high-category ARs significantly increase local runoff with higher hourly precipitation rates leading to a greater incremental rate and peak runoff. Extreme runoff increases greatly with the AR category with an increase rate up to 12.5 times stronger than non-extreme runoff. Besides the AR category, land preconditions such as soil moisture and snowpack also play crucial roles in modulating runoff response. We found that runoff induced by weak-category ARs is more sensitive to land preconditions than high-category ARs, with high peak runoff occurring when soil is nearly saturated. Additionally, more than 50% of high-peak-runoff events in snow-covered grid cells are associated with rain-on-snow events particularly for the events associated with weaker ARs. Regression analysis reveals that AR precipitation and land preconditions jointly influence runoff, emphasizing the importance of including soil moisture and snowpack levels in AR impact assessments. The study also highlights the intensified runoff response to back-to-back ARs with short intervals, which may become more frequent with climate warming, posing increased flood risks via facilitating wet soil conditions. Our findings have significant implications for AR risk predictions and the development of prediction models for AR-induced runoff.

Heatwave preconditioned-heavy rainfall (HW_HR), a preconditioned compound event, can cause more damage than a single heatwave or rainstorm. Both heatwaves and rainstorms can be exacerbated by the presence of cities, but the response of their compounding to urbanization remains unclear especially at the hourly scale. Here, we investigate the spatial and temporal responses of hourly HW_HR to a typical urban agglomeration, the Pearl River Delta, using observations and scenario-based numerical simulations. Compared to rural areas, the observations show that HW_HR in urban areas has a higher probability of occurrence and mean intensity, and its diurnal cycle of frequency is narrower, peaking in the afternoon. The temporal and spatial response of HW_HR intensity to urbanization effects is the most significant, with the urbanization-induced increase in HW_HR being five times that in non-heatwave preconditioned-heavy rainfall (noHW_HR). Our simulations support the observations and suggest that urbanization-induced changes are intense and spatially heterogeneous in HW_HR but relatively weak and spatially continuous in noHW_HR. The simulations also suggest that heatwave preconditioning not only amplifies urbanization-induced changes in key variables that alter atmospheric conditions but also provides a pre-storm unstable environment for the urban-induced warm-dry surface to trigger and enhance convection. The sub-daily pre-storm environment suggests that the preconditioning-induced thermodynamic changes gradually decrease, whereas the dynamic changes gradually increase as the event approaches. Our study highlights the importance of understanding urbanization effects on preconditioned events, providing new insights into the role of preconditions in the urban water cycle.

Using long-term reanalysis data, extreme cold events (ECEs) over East Asia are categorized into long-duration ECEs (L-ECEs, lasting more than 7 days) and short-duration ECEs (S-ECEs, lasting 3–7 days) according to the duration of ECEs during boreal winters from 1959/1960 to 2019/2020. Our results show that L-ECEs over East Asia exhibit greater intensity, higher frequency, and longer persistence than S-ECEs. On annual average, L-ECEs are 1.8 K colder, contribute over 60% of total ECE days, and dominate in longer durations, persisting beyond day 5 and lasting up to 12 days. L-ECEs in East Asia are attributed to the extension of the stratospheric polar vortex (SPV) toward the Atlantic-Euro region. This SPV extension tends to enhance the Atlantic-Euro trough, which is coupled with a strengthened and persistent Eurasian teleconnection pattern and a strengthened Ural ridge and East Asia trough during the decay of ECEs (after day 3). The strengthened East Asia trough, accompanied with cyclonic circulation, facilitates the transport of cold air from higher to lower latitudes. This process makes the ECEs more persistent and longer in duration, potentially developing into L-ECEs. The findings of this study provide valuable insights into the linkages between the extension of the SPV and ECEs over East Asia from a long-term climate statistics perspective instead of relying solely on case-by-case analysis. This study may offer a valuable new predictor for forecasting long-lasting ECE events over East Asia.

The summer precipitation in Northeast China (NEC) exerts vital effects on the local crop yield and food security of our country, however, the prediction skill of it is still limited as yet. This study investigated the impact of snow cover anomaly over the Russian Far East during spring (March–April) on the succeeding early summer (May–June) precipitation variability in NEC as well as the underlying physical mechanisms during the years 1961–2020 based on diagnostic analyses and model simulation. The results showed that, on the interannual timescale, the decreased (increased) spring snow cover over the Russian Far East is conducive to the increased (decreased) NEC early summer precipitation. Further analyses indicated that the negative anomaly of spring snow cover can persist into early summer, which then enhances the diabatic heating and thereby warms the overlying atmosphere via the snow-albedo effect. The warming atmosphere favors the intensified Okhotsk High and an associated prominent anticyclonic circulation over northeast Asia. The anomalous southeast winds on the southwest flank of the anticyclone could bring abundant water vapor to the NEC, induce moisture convergence and ascending motion, and eventually contribute to the above-than-normal precipitation. These physical processes operate in the opposite manner in the case of the positive anomaly of spring snow cover. Our findings are beneficial to further improve the understanding and prediction ability of the summer precipitation variability over NEC.

Inorganic nitrate (NO₃⁻), a crucial component of fine particulate matter (PM_2.5), has not shown a consistent decrease, despite an obvious decrease of nitrogen oxide (NOx) and PM_2.5. The atmospheric oxidation process for nitrate formation has been deemed a key factor in pollution; however, the changes of sources and oxidation pathways during a particular haze episode require further investigation. Here, daily dual isotopes (δ¹⁵N and δ¹⁸O) were used to quantify the sources and oxidation pathways of nitrate formation in Qingdao, a port city in Northern China, from September 2017 to February 2018. This study also includes a detailed introduction to two haze episodes. δ¹⁵N and δ¹⁸O results show that both fractions of nocturnal oxidation for nitrate formation and NOx from coal combustion were lower in warmer season and higher in colder season. The fractions increased with increasing PM_2.5 under low PM_2.5 concentration while the fractions were not significantly changed under higher PM_2.5 concentration, dominated by nocturnal oxidation (70.6% ± 9.7%) and coal combustion (66.1% ± 18.2%), respectively. The haze episode 1 was attributed to smoke transported over long distances, which provided a large amount of aerosol particles to absorb more locally formed gaseous HNO₃ or N₂O₅. In this episode, meteorological and air quality factors, nitrate sources, and formation mechanism did not obviously change. Haze episode 2 was caused by unfavorable meteorological factors that enhanced local nitrate accumulation. As pollution worsened, the oxidation pathway shifted from OH oxidation to N₂O₅ hydrolysis, and the primary source changed from coal combustion to more vehicle exhaust.

In this study, we used satellite observations to identify 10 typical dust-loading events over the Indian Himalayas. Next, the aerosol microphysical and optical properties during these identified dust storms are characterized using cotemporal in situ measurements over Mukteshwar, a representative site in Indian Himalayas. Relative to the background values, the mass of coarse particles (size range between 2.5 and 10 μm) and the extinction coefficient were found to be enhanced by 400% (from 24 ± 15 to 98 ± 40 μg/m³) and 175% (from 89 ± 57 Mm⁻¹ to 156 ± 79 Mm⁻¹), respectively, during these premonsoonal dust-loading events. Moreover, based on the air mass trajectory, these dust storms can be categorized into two categories: (a) mineral dust events (MDEs), which involve long-range transported dust plumes traversing through the lower troposphere to reach the Himalayas and (b) polluted dust events (PDEs), which involve short-range transported dust plumes originating from the arid western regions of the Indian subcontinent and traveling within the heavily polluted boundary layer of the Gangetic plains before reaching the Himalayas. Interestingly, compared to the background, the SSA and AAE decrease during PDEs but increase during MDEs. More importantly, we observe a twofold increase in black carbon concentrations and the aerosol absorption coefficient (relative to the background values) during the PDEs with negligible changes during MDEs. Consequently, the aerosol-induced snow albedo reduction (SAR) also doubles during MDEs and PDEs relative to background conditions. Thus, our findings provide robust observational evidence of substantial dust-induced snow and glacier melting over the Himalayas.

The concurrent occurrence of temperature and precipitation extremes, known as compound temperature-precipitation extreme events (CTPEEs), leads to more pronounced consequences for human society and ecosystems than when these extremes occur separately. However, such compound extremes have not been sufficiently studied, especially during boreal spring. Spring is an important transition season, during which the CTPEEs plays a pivotal role in plant growth and revival of terrestrial ecosystems. This study investigates the spatio-temporal variation characteristics of spring CTPEEs in China, including warm-dry, warm-wet, cold-dry, and cold-wet combinations. The compound cold-wet extreme events occur most frequently, followed by warm-dry, warm-wet, and cold-dry events. The frequency of CTPEEs associated with warm (cold) extremes shows a marked interdecadal increase (decrease) around the mid-to-late 1990s. It is found that the interdecadal change in CTPEEs is primarily determined by the variation in temperature extremes. This interdecadal shift coincides with the phase transitions of the Atlantic Multidecadal Oscillation (AMO) and the Interdecadal Pacific Oscillation (IPO). After the mid-to-late 1990s, the configuration of a positive AMO and a negative IPO excited atmospheric wave trains over mid-high latitudes, causing high-pressure and anticyclonic anomalies over East Asia. This leads to less cloudiness, allowing an increase in downward solar radiation, which enhances surface warming and contributes to an increase (decrease) in warm-dry and warm-wet extremes. The above observations are confirmed by the Pacemaker experiments. The results of this study highlight a significant contribution of internal climate variability to interdecadal changes in CTPEEs at the regional scale.

This study presents a detailed analysis of the CORDEX-FPS multi-model ensemble of convection-permitting climate simulations over the greater Alpine region. These simulations cover 10-year time slices and were obtained by downscaling global climate model (GCM) projections, using regional climate models (RCMs) and kilometer-scale convection-permitting models (CPMs). Our analysis over the Alpine area agrees with previous studies in terms of projected summer precipitation changes for the end of the century, in particular regarding a decrease in mean precipitation and increases in hourly precipitation intensities. In addition, we assess projected changes over different subregions, provide analyses at monthly and seasonal basis for temporal aggregations ranging from 1 hr to 5 days, address different extreme precipitation indices, and present validation against an Alpine-scale daily precipitation data set and an hourly precipitation product based on 3 Doppler radars. The evaluation reveals that CPMs show a refinement of spatial patterns, reduce the overestimation of precipitation frequency, and better capture intense precipitation characteristics. The improvements are especially apparent on the sub-daily scale and in the summer season. Convection-Permitting Model climate projections show an increase in precipitation intensity for all seasons and across all temporal aggregations in all regions, except for the Mediterranean in summer. The projections from different CPMs qualitatively agree, despite significant differences in the GCMs circulation changes, suggesting that the increase in heavy events is primarily due to thermodynamic effects. We also present a hypothesis explaining why projections of relative changes in hourly precipitation percentiles are similar between CPMs and RCMs, despite large biases in RCMs.

A detailed analysis of temperature and relative humidity biases in ERA5 reanalysis under tropical cyclone (TC) conditions is conducted using a composite analysis method based on more than 17,000 dropsonde observations. The statistical results indicate that ERA5 underestimates the warm-core intensity of TCs. It shows a moist bias at the tropopause and a cold-moist bias in the boundary layer beyond four times the radius of maximum winds (RMW). In contrast, within 4 RMW, a warm-dry bias in the boundary layer was observed. The temperature bias is more pronounced at the TC center, and relative humidity exhibits a similar pattern, primarily in the mid and upper troposphere. The bias distribution is asymmetric, most prominent in the right front quadrant of the TC's motion direction. Besides, bias correction is performed using an eXtreme Gradient Boosting (XGBoost) model and the biases can be reduced by approximately 60%. The dependence relationships between relative humidity and the influencing factors are investigated through SHapley Additive exPlanations (SHAP) values. The results indicate that low-level relative humidity increases and instability intensifies in TCs at night. Additionally, the findings suggest that ERA5 may inadequately capture the diurnal variation of relative humidity in TCs. The SHAP analysis also shows that relative humidity tends to be stronger in TC's motion direction.