Atmospheric Science Letters最新文献

By using hourly rain gauge records from 1961 to 2020, we find that the annual precipitation differences between highland stations and lowland stations in eastern China are reducing. Specifically, precipitation frequency has decreased significantly in the highlands while precipitation intensity has increased considerably in the lowlands, both contributing to a reduced elevation–dependency of precipitation. The decreasing precipitation frequency correlates with reduced convective available potential energy (CAPE), whereas the increasing precipitation intensity is linked to enhanced low-level convergence.

The study of tropical cyclones re-entering the ocean or making ‘seafall’ has been limited. Here, idealised simulations are used to study the re-intensification of seafalling tropical cyclones. They follow a two-stage fast-slow process driven predominately by a change in surface friction initially and then by heating. The previous land decay causes seafalling tropical cyclones to be larger and intensify more slowly with milder inner-core contraction than in ocean-only cases. Nonetheless, they reach the same intensity but with almost twice the integrated kinetic energy, so that the second landfall made by seafalling tropical cyclones can cause more damage due to their larger footprint of destructive wind.

Drop size distributions (DSDs) obtained over ocean in the tropical warm pool using a shipboard optical disdrometer are investigated. Quality control procedures, including the suppression of effects primarily related to the ship-relative wind, are applied to extract reliable data. The procedures suppressed the number of data points to 2% of the total rain duration. The characteristics of the obtained DSD parameters such as averaged size and intercept parameter are reasonably consistent with those of previous studies, with the larger size for a specific rain rate. The categorization of the data according to precipitation characteristics contrasts the DSD parameters, especially for the stratiform precipitation over the coastal ocean prior to the MJO active period. This precipitation is also an outlier when the obtained DSDs are applied to the scattering simulation for the radar-based quantitative precipitation estimation, while the contrast between the open ocean and the coastal ocean is not clearly separated.

Based on the extent and eccentricity characteristics, mesoscale convective systems (MCSs) formed in the middle reaches of the Yangtze River Basin were classified into six subtypes: large circular (LC), large elongated (LE), medium circular (MC), medium elongated (ME), small circular (SC), and small elongated (SE). The lifespans of all L-scale and most M-scale MCSs exceed 6 h, whereas the majority of S-scale MCSs last less than 6 h. The cold cloud coverage frequency for E-type MCSs exhibits relative uniformity, with high-frequency regions located south of the Yangtze River. In contrast, C-type MCSs display a more scattered high-frequency distribution with higher maxima. E-type MCSs predominantly retain an elongated shape throughout their life cycles. Additionally, as the area of LE and ME MCSs expands, their eccentricity progressively decreases, leading to a greater inclination towards the east–west direction. For C-type MCSs, they maintain a circular shape for less than half of their duration but tend to adopt an elongated shape during the development or dissipation stages. These findings provide a foundation for further investigation into the formation mechanisms and associated mesoscale systems of MCSs, which could enhance the prediction accuracy of the location and intensity of severe weather events linked to different types of MCSs.

Polar lows (PLs) are intense maritime mesoscale cyclones that often form during marine cold air outbreaks. The objective of this study is to determine the atmospheric model horizontal resolution needed to correctly represent PLs for climate modelling. Three simulations have been conducted with the Weather Research and Forecasting (WRF) model using grid spacings of 50, 25 and 12.5 km. PLs have been tracked using a combination of objective and subjective tracking methods. The number of PLs detected in each simulation increases, and their average equivalent radius decreases, as the model resolution increases. A comparison against three PL track climatologies shows that the hit rate increases with increasing resolution of the atmospheric model. The lifetime maxima of the area-maximum 10-m wind speed and area-average surface sensible heat fluxes associated with PLs are on average 12% and 20% larger, respectively, in the higher-resolution simulations than in the lower-resolution one. The lifetime maximum of the area-maximum 1-h accumulated precipitation is 67% and 133% larger in the 25- and 12.5-km simulations, respectively, than in the lower-resolution one. We conclude that a better representation of PLs can be obtained by increasing the resolution of atmospheric models from 50 to 25 km, but further increasing the resolution to 12.5 km will not result in a substantial improvement.

The Arctic climate system is experiencing large changes associated with global warming. Precipitation is a crucial factor linking the atmosphere with other climate compartments, for example, ocean and cryosphere. Using atmospheric reanalysis (ERA5) we assess the role of atmospheric weather systems, that is, atmospheric rivers, cyclones, and fronts. When: Averaged over the whole Arctic (> 70° N), a strong seasonal cycle exists with twice as much precipitation in summer than in winter when frozen precipitation is mainly brought by cyclones. In summer, the highest total precipitation amounts are rather equally contributed by all weather systems. Where: In winter, the Arctic North Atlantic region experiences by far the highest precipitation amounts, whereas in summer precipitation is more evenly distributed over the whole Arctic. How: Overall, cyclones are the most important contributor to precipitation. The highest precipitation intensity occurs when atmospheric rivers, cyclones, and fronts coincide, whereas the lowest precipitation rates occur when precipitation cannot be attributed to any of these weather systems. This residual makes up almost half of the annual snowfall, most of it in the central Arctic, and 25% of rainfall. Marine Cold Air Outbreaks can explain part of the residual. The amount and drivers for light “trace” precipitation requires further investigation.

The summer monsoon onset in the northern Indian Ocean is crucial for the densely populated South Asia, as it ends the pre-monsoon heatwave and kicks off the rainy agricultural season. Climatologically, this onset occurs in two distinct phases: first in the Bay of Bengal (BoB), followed by the Arabian Sea (AS). However, the possible mechanistic linkage between these phases remains unclear. Based on observational analysis, this study investigates how the BoB monsoon onset preconditions the subsequent AS monsoon onset, with particular focus on the pivotal trigger of the AS monsoon—the intraseasonal oscillation (ISO-AS). We demonstrate that the BoB monsoon onset establishes an easterly vertical wind shear across the northern Indian Ocean. The shear environment interacts with the ascending motion of the ISO-AS, which develops approximately 1 month later, enhancing cyclonic vorticity north of the convection center of the ISO-AS. This process, in turn, promotes moisture convergence in the boundary layer and facilitates the northward propagation of the ISO-AS, ultimately triggering the summer monsoon onset in the AS. By elucidating the stepwise nature of monsoon onset in the northern Indian Ocean, this work offers valuable insights for improving predictions of the Asian summer monsoon.

Wildfires have extensively burned areas worldwide, with significant impacts in various aspects of life. Among these, wildfires affect land-surface properties, such as vegetation nature and soil characteristics, from active burning to years and decades afterward. Despite this, the qualitative effects of post-wildfire conditions on short-term weather forecasting remain obscure. In this study, we investigated the impact of post-wildfire conditions on weather forecasting by considering post-wildfire land-surface conditions using the Weather Research and Forecasting (WRF) model in two burned forest areas. The changes in land-surface properties caused by wildfires were considered, including vegetation fraction, leaf area index, roughness length, emissivity, and soil hydraulic conductivity. The results show that post-wildfire land-surface properties have noticeable impacts on near-surface variables and atmospheric profiles. Over the study areas, the simulated near-surface air temperature could be approximately 1 K cooler and 0.75 g/kg moister if post-wildfire conditions are ignored, with impacts extending more than 3 km high in the vertical direction. This study also demonstrates that the effects of changes in land-surface properties over burned areas could extend to surrounding unburned areas.

The water vapour continuum needs to be accurately accounted for in atmospheric radiative transfer calculations. The offline ECMWF radiation scheme was used to assess the impact of the current disagreements in shortwave continuum absorption on the accuracy of clear-sky calculations of solar fluxes and heating rates from 2000 to 20,000 cm⁻¹ for three standard atmospheres: tropical, mid-latitude summer, and sub-arctic winter. These calculations were carried out at a solar zenith angle of 45°, surface albedo of 0.2, and total solar irradiance of 1361.0 W m⁻². The MT_CKD 2.5, MT_CKD 4.2, and CAVIAR continuum models were compared. These continuum models were each used to train the gas-optics tables required by the radiation scheme. Differences in the shortwave continuum have a modest impact on shortwave absorption and heating rates. The largest differences are found in the tropical atmosphere due to its higher water vapour content. Absorbed fluxes computed with MT_CKD 2.5 and MT_CKD 4.2 differ by up to ~1.3 W m⁻² (0.6%) while those with CAVIAR and MT_CKD 4.2 differ by up to ~1.7 W m⁻² (0.8%). The heating rate calculated with MT_CKD 2.5 is up to ~0.018 K d⁻¹ (0.8%) less than that obtained with MT_CKD 4.2. Compared to the heating rate computed with MT_CKD 4.2, the heating rate with CAVIAR is up to ~0.035 K d⁻¹ (1.5%) higher.

We assess near-surface temperature and sea surface temperature trends in 8 seasonal forecast systems in the Copernicus Climate Change Service archive, over the common hindcast period (1993–2016). All but one of the systems show a faster warming of the global-mean, relative to observations in both boreal summer and winter seasons. On average, systems warm at 0.21 K/decade and 0.22 K/decade for winter and summer, respectively, compared to 0.17 K/decade and 0.19 K/decade for ERA5. In summer, forecast systems tend to show an excessive warming of the tropical Pacific, tropical Atlantic and southern mid-latitudes, which contributes to the difference in global warming rates compared to observations. In contrast, greater warming in the northern mid-latitudes contributes most to trend differences for winter. The faster warming of models over this period has important implications for seasonal forecasts of future global and regional temperature and suggests further work is required to understand this bias.