Browse

You are looking at 11 - 20 of 9,663 items for :

  • Journal of Applied Meteorology and Climatology x
  • Refine by Access: All Content x
Clear All
Warwick Grace and Graeme Tepper

Abstract

Pesticide applications during surface inversions can lead to spray drift causing severe damage up to several kilometers off-target. Current regulations in Australia prohibit spray application of certain agricultural chemicals when hazardous surface inversions exist. This severely limits spray opportunities. Surface inversions can be classified as weakly or strongly stable. In the weakly stable case, moderate to strong turbulent mixing is not supportive of long-distance concentrated drift. In the very stable case, weak turbulent mixing can support the transport of high concentrations of fine material over long distances. A review of the literature and our analyses indicate that if the turbulence, as measured by the standard deviation of the vertical wind speed σ w, is greater than about 0.2 m s−1 then turbulence-driven mixing and dispersion is moderate to strong and conversely if σ w is less than about 0.2 m s−1 then turbulence-driven mixing and dispersion is weaker (an order of magnitude). The concept of maximum downward heat flux as a natural division between the regimes is applied within Monin–Obukhov stability theory, and it is shown that the observed mean σ w of 0.2 m s−1 aligns with the ridge line of maximum heat flux in stable conditions. The level of turbulence in the weakly stable regime is comparable to the turbulence typically observed in near-neutral conditions that are recommended under current guidelines as suitable for spraying and is therefore seen as an acceptable prerequisite to avoid nondispersive spraying conditions.

Restricted access
Thomas O. Mazzetti, Bart Geerts, Lulin Xue, Sarah Tessendorf, Courtney Weeks, and Yonggang Wang

Abstract

Glaciogenic cloud seeding has long been practiced as a way to increase water availability in arid regions, such as the interior western United States. Many seeding programs in this region target cold-season orographic clouds with ground-based silver iodide generators. Here, the “seedability” (defined as the fraction of time that conditions are suitable for ground-based seeding) is evaluated in this region from 10 years of hourly output from a regional climate model with a horizontal resolution of 4 km. Seedability criteria are based on temperature, presence of supercooled liquid water, and Froude number, which is computed here as a continuous field relative to the local terrain. The model’s supercooled liquid water compares reasonably well to microwave radiometer observations. Seedability peaks at 20%–30% for many mountain ranges in the cold season, with the best locations just upwind of crests, over the highest terrain in Colorado and Wyoming, as well as over ranges in the northwest interior. Mountains farther south are less frequently seedable, because of warmer conditions, but when they are, cloud supercooled liquid water content tends to be relatively high. This analysis is extended into a future climate, anticipated for later this century, with a mean temperature 2.0 K warmer than the historical climate. Seedability generally will be lower in this future warmer climate, especially in the most seedable areas, but, when seedable, clouds tend to contain slightly more supercooled liquid water.

Restricted access
Xiaoxiong Lu, Qinglan Li, Wei Zhao, Aiguo Xiao, Guangxin Li, and Zifeng Yu

Abstract

Based on daily meteorological observation data in South China (SC) from 1967 to 2018, the spatiotemporal characteristics of the precipitation in SC over the past 52 years were studied. Only 8% of the stations showed a significant increase in annual rainfall, and there was no significant negative trend at any weather stations at a confidence level of 90%. Monthly rainfall showed the most significant decreasing and increasing trends in April and November, respectively. During the entire flooding season from April to September, the monthly rainfall at the weather stations in the coastal areas showed almost no significant change. The annual rainfall gradually decreased toward the inland area with the central and coastal areas of Guangdong Province as the high-value rainfall center. By using the empirical orthogonal function decomposition method, it was found that the two main monthly rainfall modes had strong annual signals. The first modal spatial distribution was basically consistent with the average annual rainfall distribution. Based on the environmental background analysis, it was found that during the flooding season the main water vapor to SC was transported by the East Asian summer monsoon and the Indian summer monsoon. In late autumn and winter, the prevailing wind from northeastern China could not bring much water vapor to SC and led to little precipitation in these two seasons. The spatial distribution of precipitation in SC during summer was more consistent with the moisture flux divergence distribution of the bottom layer from 925 to 1000 hPa rather than that of the layer from 700 to 1000 hPa.

Open access
Dehe Xu, Qi Zhang, Yan Ding, and De Zhang

Abstract

Drought is a common natural disaster that greatly affects the crop yield and water supply in China. However, the spatiotemporal characteristics of drought in China are not well understood. This paper explores the spatial and temporal distributions of droughts in China over the past 40 years using multiscale standardized precipitation evapotranspiration index (SPEI) values calculated by monthly precipitation and temperature data from 612 meteorological stations in China from 1980 to 2019 and combines the space–time cube (STC), Mann–Kendall test, emerging spatiotemporal hot-spot analysis, spatiotemporal clustering, and local outliers for the analysis. The results were as follows: 1) the drought frequency and STC show that there is a significant difference in the spatiotemporal distribution of drought in China, with the most severe drought in Northwest China, followed by the western part of Southwest China and the northern part of North China. 2) The emerging spatiotemporal hot-spot analysis of SPEI6 over the past 40 years reveals two cold spots in subregion 4, indicating that future droughts in the region will be more severe. 3) A local outlier analysis of the multiscale SPEI yields a low–low outlier in western North China, indicating relatively more severe year-round drought in this area than in other areas. The low–high outlier in central China indicates that this region was not dry in the past and that drought will become more severe in this region in the future.

Restricted access
Huanyan Gao, Yali Luo, Xiaoling Jiang, Da-Lin Zhang, Yang Chen, Yongqing Wang, and Xinyong Shen

Abstract

In this study, the total days, mean duration, and intensity of extreme hot events over southern China during the 1971–2020 warm seasons are analyzed on the basis of daily maximum and minimum temperatures, by comparing the newly proposed independent hot day (IHD), independent warm night (IWN), and compound extreme (CMPD; i.e., the continuous occurrences of hot days and hot nights) with the traditionally defined hot day and warm night. Relationships between the hot extremes and urbanization are explored with 1-km-resolution population density data. Results show obvious differences in the spatial distributions among IHD, IWN, and CMPD over southern China. Positive correlations of 0.43, 0.41, and 0.37 are found between the population density and the total days, mean duration, and mean intensity of CMPD, respectively, which are qualitatively similar to those using the traditional hot days and warm nights. In contrast, negative correlations between the IHD and IWN indices and the population density are found, because those indices are more apparent over rural areas. Moreover, total days, mean duration, and mean intensity of CMPD increase significantly, with trends of approximately 103%, 21%, and 38% decade−1, respectively, during the rapid urbanization period from the mid-1990s to 2020, which are about 4.9, 2.1, and 2.4 times their counterparts from 1970 to the mid-1990s, and less significant and smaller differences between the two eras are found in IHD and IWN. These results will provide a new scientific basis for evaluating climate models of hot extremes in southern China and have important implications for the other urbanized regions as well.

Restricted access
Robert Fritzen, Victoria Lang, and Vittorio A. Gensini

Abstract

Extratropical cyclones are the primary driver of sensible weather conditions across the midlatitudes of North America, often generating various types of precipitation, gusty nonconvective winds, and severe convective storms throughout portions of the annual cycle. Given ongoing modifications of the zonal atmospheric thermal gradient resulting from anthropogenic forcing, analyzing the historical characteristics of these systems presents an important research question. Using the North American Regional Reanalysis, boreal cool-season (October–April) extratropical cyclones for the period 1979–2019 were identified, tracked, and classified on the basis of their genesis location. In addition, bomb cyclones—extratropical cyclones that recorded a latitude-normalized pressure fall of 24 hPa in 24 h—were identified and stratified for additional analysis. Cyclone life span across the domain exhibits a log-linear relationship, with 99% of all cyclones tracked lasting less than 8 days. On average, ≈270 cyclones were tracked across the analysis domain per year, with an average of ≈18 yr−1 being classified as bomb cyclones. The average number of cyclones in the analysis domain has decreased in the last 20 years from 290 per year during 1979–99 to 250 per year during 2000–19. Decreasing trends in the frequency of cyclone track counts were noted across a majority of the analysis domain, with the most significant decreases found in Canada’s Northwest Territories, Colorado, and east of the Graah Mountain Range. No significant interannual or spatial trends were noted in the frequency of bomb cyclones.

Restricted access
XIANTONG LIU, HUIQI LI, SHENG Hu, QILIN WAN, HUI XIAO, TENGFEI ZHENG, MINGHUA LI, LANGMING YE, ZHEYONG GUO, YAO WANG, and ZHAOCHAO YAN

Abstract

According to the high accuracy linear shape-slope (μ-Λ) relationship observed by several 2-Dimensional-Video-Distrometers (2DVD) in South China, a high-precision and fast solution method of gamma (Γ) raindrop size distribution (RSD) function based on the zeroth order moment (M0) and the third order moment (M3) of RSD has been proposed. The 0-moment (M0) and 3-moment (M3) of RSD can be easily calculated from rain mass mixing ratio (Qr) and total number concentration (Ntr) simulated by the two-moment (2M) microphysical scheme, respectively. Three typical heavy rainfall processes and all RSD samples observed during 2019 in South China were selected to verify the accuracy of the method. Compared to the current widely used exponential RSD with a fixed shape parameter of zero in 2M microphysical scheme, the Γ RSD function using the linear constrained gamma (C-G) method agreed better with the Γ fit RSD from 2DVD observations. The characteristic precipitation parameters (e.g., rain rate, M2, M6 and M9) obtained by the proposed method are generally consistent with the parameters calculated by Γ fit RSD from 2DVD observations. The proposed method has effectively solved the problem that the shape parameter in the 2M microphysical scheme set to a constant, so that the Γ RSD functions are closer to the observations and have obviously smaller errors. This method has a good potential to be applied to the 2M microphysical schemes to improve the simulation of heavy precipitation in South China, but also paves the way for in-depth applications of radar data in numerical weather prediction models.

Restricted access
Hans Van de Vyver, Bert Van Schaeybroeck, Rozemien De Troch, Lesley De Cruz, Rafiq Hamdi, Cecille Villanueva-Birriel, Philippe Marbaix, Jean-Pascal van Ypersele, Hendrik Wouters, Sam Vanden Broucke, Nicole P.M. van Lipzig, Sébastien Doutreloup, Coraline Wyard, Chloé Scholzen, Xavier Fettweis, Steven Caluwaerts, and Piet Termonia

Abstract

Sub-daily precipitation extremes are high-impact events that can result in flash floods, sewer system overload, or landslides. Several studies have reported an intensification of projected short-duration extreme rainfall in a warmer future climate. Traditionally, regional climate models (RCMs) are run at a coarse resolution using deep-convection parameterization for these extreme events. As computational resources are continuously ramping up, these models are run at convection-permitting resolution, thereby partly resolving the small-scale precipitation events explicitly. To date, a comprehensive evaluation of convection-permitting models is still missing. We propose an evaluation strategy for simulated sub-daily rainfall extremes that summarizes the overall RCM performance. More specifically, the following metrics are addressed: the seasonal/diurnal cycle, temperature and humidity dependency, temporal scaling and spatio-temporal clustering. The aim of this paper is: (i) to provide a statistical modeling framework for some of the metrics, based on extreme value analysis, (ii) to apply the evaluation metrics to a micro-ensemble of convection-permitting RCM simulations over Belgium, against high-frequency observations, and (iii) to investigate the added value of convection-permitting scales with respect to coarser 12-km resolution. We find that convection-permitting models improved precipitation extremes on shorter time scales (i.e, hourly or two-hourly), but not on 6h-24h time scales. Some metrics such as the diurnal cycle or the Clausius-Clapeyron rate are improved by convection-permitting models, whereas the seasonal cycle appears robust across spatial scales. On the other hand, the spatial dependence is poorly represented at both convection-permitting scales and coarser scales. Our framework provides perspectives for improving high-resolution atmospheric numerical modeling and datasets for hydrological applications.

Restricted access
Ying-Hui Jia, Fang-Fang Li, Kun Fang, Guang-Qian Wang, and Jun Qiu

Abstract

Recently strong sound wave was proposed to enhance precipitation. The theoretical basis of this proposal has not been effectively studied either experimentally or theoretically. Based on the microscopic parameters of atmospheric cloud physics, this paper solved the complex nonlinear differential equation to show the movement characteristics of cloud droplets under the action of sound waves. The motion process of individual cloud droplet in a cloud layer in the acoustic field is discussed as well as the relative motion between two cloud droplets. The effects of different particle sizes and sound field characteristics on particle motion and collision are studied to analyze the dynamic effects of thunder-level sound waves on cloud droplets. The amplitude of velocity variation has positive correlation with Sound Pressure Level (SPL) and negative correlation with the frequency of the surrounding sound field. Under the action of low-frequency sound waves with sufficient intensity, individual cloud droplets could be forced to oscillate significantly. The droplet smaller than 40μm can be easily driven by sound waves of 50 Hz and 123.4 dB. The calculation of the collision process of two droplets reveals that the disorder of motion for polydisperse droplets is intensified, resulting in the broadening of the collision time range and spatial range. When the acoustic frequency is less than 100Hz (@ 123.4dB) or the Sound Pressure Level (SPL) is greater than 117.4dB (@ 50Hz), the sound wave can affect the collision of cloud droplets significantly. This study provides theoretical perspective of acoustic effect to the microphysics of atmospheric clouds.

Restricted access
Domingo Muñoz-Esparza, Hyeyum Hailey Shin, Teddie L. Keller, Kyoko Ikeda, Robert D. Sharman, Matthias Steiner, Jeff Rawdon, and Gary Pokodner

Abstract

Takeoff and landing maneuvers can be particularly hazardous at airports surrounded by complex terrain. To address this, the Federal Aviation Administration has developed a Precipitous Terrain classification, as a way to impose more restrictive terrain clearances in the vicinity of complex terrain and to mitigate possible altimeter errors and pilot control problems experienced while executing instrument approach procedures. The current Precipitous Point Value (PPV) algorithm relies on the terrain characteristics within a local area of 2 NM, and is therefore static in time. In this work, we investigate the role of meteorological effects leading to potential aviation hazards over complex terrain, namely turbulence, altimeter setting errors and density altitude deviations. To that end, we combine observations with high-resolution numerical weather forecasts within a 2° × 2° region over the Rocky Mountains in Colorado, containing three airports that are surrounded by Precipitous Terrain. Both available turbulence reports and model’s turbulence forecasts show little correlation with the PPV algorithm for the region analyzed, indicating that the static terrain characteristics cannot generally be used to reliably capture hazardous low-level turbulence events. Altimeter setting errors and density altitude effects are also found to be only very weakly correlated with the PPV algorithm. Altimeter setting errors contribute to hazardous conditions mainly during cold seasons, driven by synoptic weather systems, while density altitude effects are on the contrary predominantly present during the spring and summer months, and follow a very well-marked diurnal evolution modulated by surface radiative effects. These findings demonstrate the effectiveness of high-resolution weather forecast information in determining aviation-relevant hazardous conditions over complex terrain.

Open access