Atmospheric Science Letters最新文献

Wind retrievals from high-frequency radars (HFRs) provide high-density, hourly wind estimates over the ocean near the coast. These wind retrievals are a promising new source of near-surface wind estimates in coastal regions where it is difficult to deploy large networks of buoys and where scatterometers cannot observe because of land contamination. In addition to improving ocean monitoring, these wind retrievals are also useful in numerical weather prediction. The wind estimates are retrieved from the HFR Doppler spectra in conjunction with the Simulating WAves Nearshore (SWAN) model, HFR forward model, and HFR adjoint model. In this work, wind retrievals were generated from three Coastal Ocean Dynamics Applications Radar (CODAR) SeaSonde HFR sites located near the mouth of the Chesapeake Bay in August 2017 and assimilated in the Coupled Ocean/Atmosphere Mesoscale Prediction System® (COAMPS) model. The impact of the assimilated HFR wind retrievals on near-surface weather forecasts is measured with adjoint-derived forecast sensitivity observation impact (FSOI) and a comparison to forecasts from a data-denial observing system experiment (OSE). The FSOI measurement indicates the HFR winds had neutral impact on the 12-h forecast while the OSE comparison suggests a small improvement in the 10-m u and v winds at lead times up to 36 h. Compared to a similar study in the Southern California Bight, the small impact on the forecast seen in this Chesapeake Bay study could be related to there being a smaller number of wind retrievals in a smaller region during a period where the HFR wind retrievals were already similar to the background wind field. We suggest that HFR wind retrievals used for numerical weather prediction may be more beneficial when generated for long swaths of coastlines rather than small, isolated areas.

Three windshear/microburst detection algorithms are used operationally at the Hong Kong International Airport. Their performance is studied in the present article by assuming the availability of the complete set of meteorological data from numerical weather prediction model output. Only selected tropical cyclone cases are considered. The performance is evaluated using pilot reports of windshear. It turns out that the glide path scan windshear detection algorithm has the best overall performance by considering the area under curve using the relative operating characteristic curves. This may be related to the better geometry in covering the glide paths of the airport for this particular algorithm.

This study quantitatively evaluated the typhoon-induced sea surface temperature (SST) cooling caused by typhoons Faxai (2019) and Hagibis (2019) using a high-resolution ocean model and the cooling parameter (Co). Faxai and Hagibis both passed over the ocean south of the eastern part of the Japanese main island, but the associated average SST decreases differed. Faxai produced a decrease of less than 2°C, whereas Hagibis produced a decrease of more than 2°C. The average Co value was 1.6 for Faxai and 3.6 for Hagibis, indicating that SST was more easily cooled by Hagibis than by Faxai, consistent with the observations. The impact of ocean conditions on the typhoon-induced SST cooling by Hagibis was 2.6 times larger than the impact by Faxai, indicating that the ocean before Hagibis passes is less hard to cool ocean than Faxai. In short, it is important for ocean cooling not only ocean conditions but also typhoon characteristics because in fact, Hagibis cooled the ocean more than Faxai. In addition, the impact of Hagibis's characteristics on the typhoon-induced SST cooling was 4.8 times larger than the impact of Faxai's characteristics. Thus, SST was more likely to cool by typhoon characteristics in the case of Hagibis. In particular, among Hagibis's characteristics, typhoon size in the horizontal direction had the most efficient effect on SST cooling. Although Co does not consider the effects of the advection of ocean water, we suggest that Co is a practical indicator for estimating SST cooling caused by a typhoon and comparing factors of typhoon-induced SST cooling in multiple cases.

This study identified a prominent temporal seesaw haze intensity case that occurred between the late winter months of 2010 in the North China Plain (NCP), featuring considerably suppressed haze intensity in January and enhanced haze intensity in the adjacent month of February in 2011. We suggest that dramatic alternations of atmospheric and oceanic anomalies played fundamental roles in forming this seesaw haze intensity case, rather than changes in manmade emission anomalies. The suppressed haze intensity in January 2011 was tied to an equivalent barotropic cyclonic anomaly that dominated the NCP and its surroundings, which generated in situ haze-suppressed meteorology characterized by strengthened lower-level northerly anomalies with cold and dry conditions, as well as elevated boundary layer height and destabilized atmospheric stratification. In stark contrast, the enhanced haze intensity in February 2011 was connected to an equivalent barotropic anticyclonic anomaly, linking a haze-favourable meteorology opposite to that in January 2011. The pronounced North Atlantic sea surface temperature (SST) tripole anomalies, with positive anomalies in the tropical and mid-latitudinal North Atlantic and negative anomalies in the subtropical North Atlantic, made a significant contribution to the above-mentioned seesaw haze intensity case. Diagnostic analyses suggested that the January North Atlantic SST tripole anomalies were linked to a significant negative North Atlantic Oscillation (NAO)-like pattern, which acted as the source of the Rossby wave train to generate concurrent haze-suppressed meteorology over the NCP. In February, although the NAO-like pattern was drastically dampened, the enhanced barotropic cyclonic anomaly centred southeast of the Yamal Peninsula played a critical role in relaying the impact of January tripole SST anomalies, thus inducing concurrent haze-favourable meteorology. Consequently, January North Atlantic SST tripole anomalies could exert an effective modulation effect on the generation of seesaw haze intensity. The proposed mechanism was further verified using the Community Earth System Model Large Ensemble Numerical Simulation (CESM-LENS) datasets.

Soil freeze–thaw alternation is a natural characteristic of the Tibetan Plateau (TP), and plays an important role in surface energy balance and eco-hydrological processes. The soil freeze–thaw process on the TP has changed significantly owing to global warming, affecting the alpine ecosystem structure and function. This study used high-resolution atmospheric forcing datasets to drive the Community Land Model version 5.0 (CLM5.0) to simulate the near-surface soil freeze–thaw status between 1979 and 2020. The simulated results were compared with in situ observations, and then the spatiotemporal distribution of the freeze start-date (FSD), freeze end-date (FED), freeze duration (FD), and thaw duration (TD) at a depth of 0.1 m were analyzed. The Nash–Sutcliffe efficiency coefficients (NSEs) of FSD, FED, FD, and TD between simulations and in situ observations were 0.77, 0.90, 0.98 and 0.92, and the correlation coefficients of FSD, FED, FD, TD were 0.97, 0.99, 0.99 and 0.98, respectively. The spatial distribution of FSD and TD was characterized by gradually increasing from northwest to southeast while FED and FD exhibited the opposite characteristics. FSD, FED, FD, and TD changed at an area-mean rate of 1.1, −1.4, −2.5, and 2.5 days decade⁻¹, respectively. This study provides an important reference for analyzing and predicting the changes in near surface soil freeze–thaw status on the TP under the warming climate.

This study investigated a rainfall event under a typhoon influence using a 2D video disdrometer and weather radar observations to characterize raindrop size distribution (DSD) in a mixed convective and stratiform precipitating system. During the time period when both convective and stratiform rainfalls existed, the DSDs generally indicated a monotonically decreasing shape with increasing particle size, with a relatively gradual decrease at intermediate particle size observed at certain times; this feature is attributed to the combined effect of convective and stratiform rainfalls. During the transitional period between convective and stratiform rainfalls, the DSDs exhibited a bimodal shape. The DSDs were well approximated by a newly proposed gamma raindrop distribution combined with exponential (GRACE) distribution function, which was defined as the sum of the exponential distribution and the gamma distribution. A comparison of the volume ratio of the exponential and gamma components of the GRACE distribution revealed that the exponential component of the DSD was larger than the gamma component in the bimodal DSD. These results suggest that the DSD became bimodal during the period when stratiform rainfall predominated because of the weakening of convective rainfall. The GRACE distribution is useful for understanding cloud-microphysical processes in mixed stratiform and convective precipitation conditions.

Soil moisture (SM) affects weather through its impact on surface flux partitioning, influencing vertical atmospheric profiles and circulations driven by differential surface heating. In West Africa, observational studies point to a dominant negative SM-precipitation feedback, where dry soils help to initiate and maintain convection. In this context, serious concerns exist about the ability of models with parameterised convection to simulate this observed sensitivity of daytime convection to SM. Here, we evaluate the effect of initial SM perturbations in a short-range ensemble forecast over West Africa, comparing the UK Met Office Global and Regional Ensemble Prediction System (MOGREPS) with parameterised convection (GLOB-ENS) to its regional convection-permitting counterpart (CP-ENS). Results from both models suggest SM perturbations introduce considerable spread into daytime evaporative fraction (EF) and near-surface temperatures. This spread is still evident on Day 3 of the forecast. Both models also show a tendency to increased afternoon rainfall frequency over negative EF anomalies, reproducing the observed feedback. However, this effect is more pronounced in CP-ENS than GLOB-ENS, which illustrates the potential for process-based forecast improvements at convection-permitting scales. Finally, we identify persistent biases in rainfall caused by land cover mapping issues in the operational GLOB-ENS setup, emphasising the need for careful evaluation of different mapping strategies for land cover.

The Once-in-a-Century extreme rainstorm event caused severe floods over Henan province during July 18–21, 2021, which resulted in large casualty and property losses. Although the rainstorm event occurred in Henan after July 18, the excessive rainfall had occurred to the east of Henan before July 18, with the 4-day accumulated rainfall exceeding +130 mm during July 14–17, 2021. How the rainfall evolving westward and intensifying after July 18 remained a puzzle, which is the focus of this study. The prerainstorm stage (July 14–17) was related to the South Asian High (SAH) extending eastward and the western Pacific subtropical high (WPSH) extending northwestward, and a low vortex between the SAH and WPSH caused above-normal rainfall to the east of Henan. The rainstorm stage (July 18–21) was associated with an inverted trough and excessive southerly and southeasterly water vapor transportation above Henan, which resulted from the combined effects of a deep trough in the upper troposphere and typhoon activities. Additionally, three subseasonal forecasting systems predicted this rainstorm event 3 days in advance, with the European Center for Medium Range Weather Forecasts (ECMWF) performing the best, which was related to a better prediction of the inverted trough and the water vapor transportation in the middle-lower troposphere. These results advance our understanding of the extreme rainstorm event in July 2021 in Henan.

The United Kingdom is committed to substantially increasing offshore wind capacity in its drive to decarbonise electricity production and achieve net zero. If low wind episodes—or ‘wind drought’ events—occur during high energy demand periods, energy security may be threatened without alternative supply. The challenge of managing the variability of wind power will increase into the future as its share in the energy mix increases. This study focuses attention on the North Sea as a centre of current and planned offshore wind resource, and on the winter season, given the occurrence of weather patterns that risk security of supply. We use a large climate model ensemble, providing a dataset an order of magnitude larger than the reanalysis-based observations, to better sample wind drought events. This leads to a more robust estimate of their frequency and persistence and their dynamical teleconnections compared with the observational record. We define week-long wind drought events, based on a 20th percentile threshold in 10 m wind speed, during which wind power is estimated to be around half that in a typical week of winter. Wind drought events of up to two consecutive weeks have been observed, but the model indicates a 1-in-40 chance of three or more continuous weeks of wind drought each winter, with the single most prolonged simulated event lasting 5 weeks. There is a doubling of the likelihood of these prolonged wind drought events during El Niño, indicating that monitoring and predicting the state of the tropical Pacific may be useful in assessing the risk of wind drought events in an upcoming winter.

The onset date of the South China Sea (SCS) summer monsoon (SCSSM) and the generation time of the first tropical cyclone (TC) over the Bay of Bengal (BoB) during late April and early June were significantly correlated with a correlation coefficient of 0.58 during 1979–2020. The composite analysis found that under the impact of BoB TCs, an enhanced southwesterly low-level flow transported abundant moisture from the BoB to the northern SCS. Besides, the diabatic heating related to TC convection stimulated an anticyclonic anomaly in the upper troposphere over the southern Tibetan Plateau, which was conducive to the enhancement and expansion of the South Asian high (SAH) over the Indo-China Peninsula. The stronger easterly outflows from the eastern periphery of the SAH overlapped with low-level water vapor convergence over the northern SCS, enhancing the development of monsoon convection. Thus, more condensation heating warmed the tropospheric atmosphere and reversed the meridional temperature gradient over the SCS, implying the SCSSM onset.