Synoptic Influence Research Articles

How Might the May 2015 Flood in the U.S. Southern Great Plains Induced by Clustered MCSs Unfold in the Future?

AbstractThe historic 22–26 May 2015 flood event in Texas and Oklahoma was caused by anomalous clustered mesoscale convective systems (MCSs) that produced record‐breaking rainfall and $3 billion of damage in the region. A month‐long regional convection‐permitting simulation is conducted to reconstruct multiple clustered MCSs that lead to this flood event. We further use the pseudo global warming approach to examine how a similar event may unfold in a warmer climate and the driving physical factors for the changes. Tracking of MCSs in observations and simulations shows that the historical simulation reproduces the salient characteristics of the observed MCSs. In a warmer climate under a high‐emission (SSP5‐8.5) scenario, the Southern Great Plains is projected to experience a near surface warming of 4–6 K, accompanied by enhanced moisture transport by the strengthened Great Plains low‐level jet. A warmer and moister lower troposphere leads to 36%–59% larger convective available potential energy, supporting wider and more intense convective updrafts and rainfall production. Consistently, MCSs have wider convective areas and stronger rainfall intensities, producing 50% larger rain volumes during the mature stage. Extreme (99.5%) MCS rainfall frequency and amount will increase by threefold. However, MCS stratiform rain area decreases as a result of elevated stratiform cloud bases that lead to stronger sublimation and evaporation of precipitation in response to warming, resulting in reduced weak‐to‐moderate surface precipitation. Results suggest that global warming greatly increases precipitation intensity of clustered MCS events under strong synoptic influence, with much higher potential to produce serious floods without additional climate adaptation.

Smoke in the river: an Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) case study

Abstract. The formation of a river of smoke crossing southern Africa was investigated during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) campaign in September 2017. A complementary set of global and mesoscale numerical simulations as well as ground-based, airborne and space-borne observations of the dynamics, thermodynamics and composition of the atmosphere are used to characterize the river of smoke in terms of timing and vertical extent of the biomass burning aerosol (BBA) layer. The study area was under the synoptic influence of a coastal low rooted in a tropical easterly wave and a high-pressure system over the continent and westerly waves at mid latitudes, one of which had an embedded cut-off low (CoL). The coastal low interacted with the second of two approaching westerly waves and ultimately formed a mid-level tropical temperate trough (TTT). The TTT created the fast-moving air mass transported to the southwestern Indian Ocean as a river of smoke. The CoL, which developed and intensified in the upper levels associated with the first (easternmost) westerly wave, remained stationary above northern Namibia prior to the formation of the TTT and was responsible for the thickening of the BBA layer. This shows that the evolution of the river of smoke is very much tied to the evolution of the TTT, while its vertical extent is related to the presence of the CoL. The mechanisms by which the CoL, observed over Namibia in the entrance region of the river of smoke, influences the vertical structure of the BBA layer is mainly associated with the ascending motion above the BBA layer. In the presence of the CoL, the top of the BBA layer over northern Namibia reaches altitudes above 8 km. This is much higher than the average height of the top of the BBA layer over the regions where the smoke comes from (Angola, Zambia, Zimbabwe, Mozambique), which is 5 to 6 km. The results suggest that the interaction between the TTTs and the CoLs which form during the winter may have a role in promoting the transport of BBA from fire-prone regions in the tropical band to the temperate mid latitudes and the southwestern Indian Ocean.

Atmospheric Boundary Layer—Free Troposphere Air Exchange in the North China Plain and its Impact on PM2.5 Pollution

AbstractThis study presents the first climatology of the atmospheric boundary layer (ABL)—free troposphere (FT) exchange during wintertime in a region covering the North China Plain (NCP) and further discusses its impact on PM2.5 pollution. The exchange flux is calculated based on a mass budget equation using the simulated meteorological field from 2013 to 2019. The mean ABL‐FT exchange is well related to topography and sea‐land distribution, manifesting as strong downward flux over the western/northwestern mountains, moderate exchange along the eastern coastline, and small upward flux over the NCP. Eight weather types are classified to analyze the synoptic influence on ABL‐FT exchange. Strong exchanges exist under the types of “low pressure trough,” “cold front,” and “high pressure” (about 63% of total days), characterized by the downward spatial mean flux of 0.019–0.026 kg m−2 s−1 over the whole research domain. But over the plain area, upward transport is dominant with the air flux from 0.004 to 0.013 kg m−2 s−1 under the first two weather types (about 32% of total days), corresponding to PM2.5 fluxes estimated in the range 0.30–0.54 μg m−2 s−1. This indicates the NCP being an effective pollution source for long‐range transport. We also find that weather types with large downward flux and straightforward winds can refresh the underneath ABL air in about 6 h, which explains the dispersion of air pollution. A typical pollution case illustrates convergence and uplift to the FT of the pollutants in the NCP and the possible influence on downstream air quality.

Heatwaves and Summer Urban Heat Islands: A Daily Cycle Approach to Unveil the Urban Thermal Signal Changes in Lisbon, Portugal

Lisbon is a European Mediterranean city, greatly exposed to heatwaves (HW), according to recent trends and climate change prospects. Considering the Atlantic influence, air temperature observations from Lisbon’s mesoscale network are used to investigate the interactions between background weather and the urban thermal signal (UTS) in summer. Days are classified according to the prevailing regional wind direction, and hourly UTS is compared between HW and non-HW conditions. Northern-wind days predominate, revealing greater maximum air temperatures (up to 40 °C) and greater thermal amplitudes (approximately 10 °C), and account for 37 out of 49 HW days; southern-wind days have milder temperatures, and no HWs occur. Results show that the wind direction groups are significantly different. While southern-wind days have minor UTS variations, northern-wind days have a consistent UTS daily cycle: a diurnal urban cooling island (UCI) (often lower than –1.0 °C), a late afternoon peak urban heat island (UHI) (occasionally surpassing 4.0 °C), and a stable nocturnal UHI (1.5 °C median intensity). UHI/UCI intensities are not significantly different between HW and non-HW conditions, although the synoptic influence is noted. Results indicate that, in Lisbon, the UHI intensity does not increase during HW events, although it is significantly affected by wind. As such, local climate change adaptation strategies must be based on scenarios that account for the synergies between potential changes in regional air temperature and wind.

Local and synoptic meteorological influences on daily variability in summertime surface ozone in eastern China

Abstract. Ozone pollution in China is influenced by meteorological processes on multiple scales. Using regression analysis and weather classification, we statistically assess the impacts of local and synoptic meteorology on daily variability in surface ozone in eastern China in summer during 2013–2018. In this period, summertime surface ozone in eastern China (20–42∘ N, 110–130∘ E) is among the highest in the world, with regional means of 73.1 and 114.7 µg m−3, respectively, in daily mean and daily maximum 8 h average. Through developing a multiple linear regression (MLR) model driven by local and synoptic weather factors, we establish a quantitative linkage between the daily mean ozone concentrations and meteorology in the study region. The meteorology described by the MLR can explain ∼43 % of the daily variability in summertime surface ozone across eastern China. Among local meteorological factors, relative humidity is the most influential variable in the center and south of eastern China, including the Yangtze River Delta and the Pearl River Delta regions, while temperature is the most influential variable in the north, covering the Beijing–Tianjin–Hebei region. To further examine the synoptic influence of weather conditions explicitly, six predominant synoptic weather patterns (SWPs) over eastern China in summer are objectively identified using the self-organizing map clustering technique. The six SWPs are formed under the integral influence of the East Asian summer monsoon, the western Pacific subtropical high, the Meiyu front, and the typhoon activities. On average, regionally, two SWPs bring about positive ozone anomalies (1.1 µg m−3 or 1.7 % and 2.7 µg m−3 or 4.6 %), when eastern China is under a weak cyclone system or under the prevailing southerly wind. The impact of SWPs on the daily variability in surface ozone varies largely within eastern China. The maximum impact can reach ±8 µg m−3 or ±16 % of the daily mean in some areas. A combination of the regression and the clustering approaches suggests a strong performance of the MLR in predicting the sensitivity of surface ozone in eastern China to the variation of synoptic weather. Our assessment highlights the importance of meteorology in modulating ozone pollution over China.

Evidence for Gap Flows in the Birch Creek Valley, Idaho

Abstract A field study was conducted of flows in the Birch Creek Valley in eastern Idaho. There is a distinct topographic constriction in the Birch Creek Valley that creates two subbasins: an upper and lower valley. The data were classified into one of three groups based on synoptic influence (weak/absent, high wind speeds, and other evidence of synoptic influence). Gap flows commonly developed downwind of the constriction in association with the weak/absent group but also occurred in association with the two synoptic groups suggesting the potential for more diverse origins. In general, the frequency and strength of gap flows appeared to be linked to the development of the requisite thermal regime and minimization of any synoptically driven southerly winds that would suppress outflows. Gap flows were characterized by high wind speeds with jetlike vertical profiles along the axis of the lower valley. For all three groups the morning transition in the upper valley and western sidewall usually proceeded slightly ahead of the lower valley, consistent with the principles of the topographic amplification factor. The persistence of southerly winds in the lower valley past evening transition inhibited the development of gap flows, promoted strong nighttime inversions, and delayed the onset of morning transition relative to the upper valley. Nocturnal temperature inversions in the lower valley were largely eliminated with the onset of strong gap flows resulting in earlier morning transitions there. The form for a method of predicting gap flow wind speeds is proposed.

Gravity‐wave‐induced perturbations in marine stratocumulus

AbstractWe discuss the role of atmospheric gravity waves in modulating cloud radiative and dynamical properties over the southeast Pacific. Satellite imagery and satellite‐retrieved cloud properties during October 2008 illustrate three distinct episodes of horizontal propagation of gravity wave trains across the large‐scale stratocumulus (Sc) cloud deck capping the local marine boundary layer. In one period, 7–9 October 2008, the waves modulated cloud‐top‐height by up to 400 m peak‐to‐trough, propagating perpendicular to the synoptic boundary layer flow with phase speed 15.3 m s−1, period ∼1 h and horizontal wavelength 55 km. The gravity waves were observed to be non‐dispersive. These waves were first evident in the cloud deck near 30°S, 85°W during a 24 h period beginning at midday on 7 October 2008, and propagated northeastward toward the Peruvian coast for the following 48 h. During this time they induced both reversible and non‐reversible changes in cloud‐radiative and cloud‐dynamic properties, such that areas of clear sky developed in the troughs of passing wave‐fronts. These pockets of open cells persisted long after the passage of the gravity waves, advecting northwestward with the background wind. Using the analysis fields of the European Centre for Medium‐Range Weather Forecasts in conjunction with infrared and microwave satellite imagery, we show that these gravity waves emerged from a disturbed subtropical jet stream. The radiant of the waves was coincident in all cases with centres of large negative residuals in nonlinear balance, suggesting that geostrophic readjustment of sharply divergent flows associated with the disturbed jet provided a source for the wave energy. Conversely, gravity waves were not observed in more quiescent jet conditions. This case study highlights the important and irreversible effects that gravity waves propagating in the troposphere can have on cloud radiative properties (and hence surface radiation budgets) over a very wide area. It also highlights the importance of synoptic influence on Sc‐covered marine boundary layers. Copyright © 2012 Royal Meteorological Society

CMAQ predictions of tropospheric ozone in the U.S. southwest: Influence of lateral boundary and synoptic conditions

Phoenix, Arizona, has been an ozone nonattainment area for the past several years and it remains so. Mitigation strategies call for improved modeling methodologies as well as understanding of ozone formation and destruction mechanisms during seasons of high ozone events. To this end, the efficacy of lateral boundary conditions (LBCs) based on satellite measurements (adjusted-LBCs) was investigated, vis-à-vis the default-LBCs, for improving the predictions of Models-3/CMAQ photochemical air quality modeling system. The model evaluations were conducted using hourly ground-level ozone and NO2 concentrations as well as tropospheric NO2 columns and ozone concentrations in the middle to upper troposphere, with the ‘design’ periods being June and July of 2006. Both included high ozone episodes, but the June (pre-monsoon) period was characterized by local thermal circulation whereas the July (monsoon) period by synoptic influence. Overall, improved simulations were noted for adjusted-LBC runs for ozone concentrations both at the ground-level and in the middle to upper troposphere, based on EPA-recommended model performance metrics. The probability of detection (POD) of ozone exceedances (>75ppb, 8-h averages) for the entire domain increased from 20.8% for the default-LBC run to 33.7% for the adjusted-LBC run. A process analysis of modeling results revealed that ozone within PBL during bulk of the pre-monsoon season is contributed by local photochemistry and vertical advection, while the contributions of horizontal and vertical advections are comparable in the monsoon season. The process analysis with adjusted-LBC runs confirms the contributions of vertical advection to episodic high ozone days, and hence elucidates the importance of improving predictability of upper levels with improved LBCs.

Urbanization effect on soil temperature in Nanjing, China

Temporal and spatial changes in soil temperature were monitored in rural and urban areas within Nanjing city, China to investigate the influence of increasing urbanization. Diurnal changes in soil temperature as a function of depth (i.e., 10, 20, 30, 40, 50, 60, 100 and 150 cm below ground surface) were measured continuously for one year (from June 2009 to June 2010) at two locations. Soil temperature was also measured at selected times at 600 locations within urban and rural areas. The results show that both the urban and rural soil temperature profiles are strongly tied to the prevailing air temperature, while the synoptic influence on soil temperature occurs mostly at depths less than 60 cm. Based on spatial averages for the studied one year period, the urban soil is 1.21 °C warmer than the rural soil. The nature of the temporal and spatial changes in soil temperature varies according to whether a given location is rural or urban. The average daily, monthly and seasonal difference ranges from 0.29 to 3.03 °C, 0.62 to 2.0 °C and 0.84 to 1.51 °C, respectively, between urban and rural locations. Soil temperature variations generally decrease with increasing depth and increase with urbanization.

Parada cardíaca durante un curso de soporte vital

Temporal and spatial changes in soil temperature were monitored in rural and urban areas within Nanjing city, China to investigate the influence of increasing urbanization. Diurnal changes in soil temperature as a function of depth (i.e., 10, 20, 30, 40, 50, 60, 100 and 150 cm below ground surface) were measured continuously for one year (from June 2009 to June 2010) at two locations. Soil temperature was also measured at selected times at 600 locations within urban and rural areas. The results show that both the urban and rural soil temperature profiles are strongly tied to the prevailing air temperature, while the synoptic influence on soil temperature occurs mostly at depths less than 60 cm. Based on spatial averages for the studied one year period, the urban soil is 1.21 °C warmer than the rural soil. The nature of the temporal and spatial changes in soil temperature varies according to whether a given location is rural or urban. The average daily, monthly and seasonal difference ranges from 0.29 to 3.03 °C, 0.62 to 2.0 °C and 0.84 to 1.51 °C, respectively, between urban and rural locations. Soil temperature variations generally decrease with increasing depth and increase with urbanization.

Flow and pollution transport during Wagerup 2006: a case study

As part of a broader field campaign dubbed Wagerup 2006, a case study was carried out to determine the overnight pollution transport mechanisms and flow characteristics near Wagerup, Western Australia. The ambient conditions were characterized by stable stratification with little synoptic influence in the lower boundary layer. An elevated jet intrusion originating on a nearby escarpment slope was found to induce sufficient mixing causing elevated pollution plumes to reach ground level. Onset of mixing was accurately predicted by non-linear critical Richardson number estimates obtained in previous laboratory work. The increase in ground level temperatures brought about by shear induced mixing later prompted a sea-breeze like gravity current that was completely blocked by the escarpment and as a result pollutants were trapped against the escarpment slope. A notable side effect of the topographic blocking was the subsequent steady 360° rotation of ground level winds within an area of influence described by the Rossby deformation radius. Copyright © 2009 Royal Meteorological Society

연직관측자료를 통한 복잡 연안지역의 오존 이류특성

In order to clarify the vertical ozone distribution in planetary boundary layer of coastal area with complex terrain, an observation campaign was carried out around Gwangyang Bay with dense pollutant emission sources during two days from June, 4 2007. For this observation are Radiosonde, SODAR(SOnic Detection And Ranging) and Tethered ozone sonde were employed. The surface meteorological and photochemical observation data provided by AWS (Automatic Weather System) and AQMS (Air Quality Monitoring System) were also applied for analysis. Synoptic condition is strongly associated with lower level ozone distribution in complex terrain coastal area. Since mesoscale circulation induced by difference of characteristics of land and sea and orographic forcing is predominant under calm synoptic condition, vertical distribution of ozone is complicate and vertical ozone concentration greatly fluctuated. However in second day when synoptic influence become strong, ozone concentration in lower levels is vertically uniform regardless of observation level. This results in vertical observation indicates that vertical ozone distribution is often determined by synoptic condition and also affects surface ozone concentration.

A study of orographic effects on mountain-generated precipitation systems under weak synoptic forcing

¶Mountains profoundly impact precipitation systems in Taiwan, particularly in areas occupying roughly two-thirds of the island’s landmass. This study examines the terrain structures possibly affecting the formation of rainfall systems in northern Taiwan by analyzing radar data, surface rainfall data, and simulation results from MM5 (Fifth-Generation NCAR/Penn State Mesoscale Model) under a weak synoptic influence condition. More specifically, this study analyzes precipitation systems formed in three different days with different ambient wind directions (i.e., southwesterly, southerly and south-southeasterly flows) in a low Froude number regime in Mei-Yu (or Baiu) season.

Observations of Flow and Turbulence in the Nocturnal Boundary Layer over a Slope

Measurements were conducted on an eastern slope of the Salt Lake Basin (SLB) as a part of the Vertical Transport and Mixing Experiment (VTMX) conducted in October 2000. Of interest was the nocturnal boundary layer on a slope (in particular, katabatic flows) in the absence of significant synoptic influence. Extensive measurements of mean flow, turbulence, temperature, and solar radiation were made, from which circulation patterns on the slope and the nature of stratified turbulence in katabatic winds were inferred. The results show that near the surface (,25‐50 m) the nocturnal flow is highly stratified and directed downslope, but at higher levels winds strongly vary in magnitude and direction with height and time, implying the domination of upper levels by air intrusions. These intrusions may peel off from different slopes surrounding the SLB, have different densities, and flow at their equilibrium density levels. The turbulence was generally weak and continuous, but sudden increases of turbulence levels were detected as the mean gradient Richardson number ( ) dropped to Rig about unity. With a short timescale fluctuated on the order of a few tens of seconds while modulating with Rig a longer (along-slope internal waves sloshing) timescale of about half an hour. The mixing efficiency (or the flux Richardson number) of the flow was found to be a strong function of , similar to that found in laboratory Rig experiments with inhomogeneous stratified shear flows. The eddy diffusivities of momentum and heat were evaluated, and they showed a systematic variation with when scaled with the shear length scale and the rms Rig vertical velocity of turbulence.

A Simple Model of the Australian West Coast Trough

Abstract The Australian west coast trough forms near the west coast in the easterly flow over Australia in the warmer months of the year. Its development and movement is the major synoptic influence on the weather in those months, particularly in the production of extreme maximum temperatures and subsequent “cool changes.” This paper begins with a brief discussion of the climatology of the trough, followed by a case study. The main focus is on the development and interpretation of a simple dynamical model of the trough. The model is an adaptation of the linear diabatic equatorial β-plane models of Matsuno and of Gill for the particular situation of the trough. The study leads to a new hypothesis on the role of the sea breeze in the dynamics of the trough.

A Kinetic Energy Budget over North America During a Period of Short Synoptic wave Development

Abstract A kinetic energy budget over North America is computed using standard rawinsonde data during 11–24 April 1970, a period of repeated short synoptic wave development. A comparative analysis with energy transformation documented for periods of intermediate synoptic wave influence is presented. In both the abort and intermediate wave periods, horizontal flux convergence is an important energy source. However, the two periods differ in both their kinetic energy generation and dissipation. In contrast to the intermediate wave case, the short wave period is dominated by negative generation of kinetic energy. In addition, the vertically-integrated dissipation, determined as the residual term in the kinetic energy budget, is substantially smaller for the short wave case. This latter result, which occurs primarily because the residual quantity is positive in the upper troposphere, is hypothesized to be associated with subgrid-to-grid-scale kinetic energy exchange. Of particular interest is a secondary wave...