Ageostrophic Wind Research Articles

Implications for extreme midlatitude weather events of secondary flow associated with polar jets

AbstractIn the Earth's general circulation, polar jets act as baroclinic pumps of angular momentum and heat. Reanalysis datasets indicate that shear changes near jets induce upward displacements of the jet cores, suggesting a weakening thermodynamic pumping over the last 50 years. From secondary flow theory, a well‐established principle in fluid dynamics, an increased frequency of heatwaves and persistent winter storms is expected. The ageostrophic wind shear between 700 and 50 hPa indicates the strength of this secondary circulation. The weakening tendency during the reanalysis period also exists in multimodel simulations under the RCP8.5 emissions scenario for the 21st century. The reduction between the periods 2005–2025 and 2081–2100 reaches 18%, 5.3% and 19%, respectively, for the North America, Mid‐Europe and East Asia sectors of the Northern Hemisphere polar jet. Within this background, cold‐surge events are the result of synergic co‐working of several factors. The occurrence trend for transitional season winter extreme events also is examined. The winter extremes seemingly have larger temperature drops. However, in a warming climate, they emerge more rapidly from extreme cold states. The storm tracks, especially over North America, have equatorward extensions, indicating that winter storms can reach lower latitudes. Due to the temperature‐dependence of air viscosity, secondary flows decrease more slowly than the main zonal flow. This imposes an important adjustment to the traditional polar amplification effects on midlatitude winter extremes. During a warmer winter (the primary manifestation of a warmer climate), spatially uniform positive trends in cold extreme events are not expected. There are, however, regions experiencing more winter extremes. These regions show consistent patterns in both the reanalysis period and the remainder of the 21st century.

Evolution characteristics and mechanism of meso-γ-scale vortex in an atypical bow echo in the North China Plain

On June 25, 2020, an atypical bow echo (ATBE) accompanied by a meso-γ-scale vortex (MV) incurred the strongest gust (41.4 m s−1) recorded since 1957 in the North China Plain. In this article, we investigate the evolution characteristics of MV and its mechanism for the extreme wind by using the observation data of Doppler weather radar and automatic weather stations as well as the four-dimensional Variational Doppler Radar Analysis System (VDRAS). The results show that the MV in this process was initially born at a height of 0.9–2.4 km, and the contracting and stretching vertical vortex rapidly descended to surface from the 2.0 km height with the rotation speed increasing rapidly. Six minutes later, the extreme wind occurred. The generation mechanism of the MV is that, under the strong vertical wind shear and downdraft in the lower troposphere, the vertical vortex tube of MV was transformed from the horizontal vortex tube generated in the zone of perturbation temperature gradient. The influence mechanism of MV on the extreme wind is that, as the MV continued to intensify and extend downward, a negative perturbation low-pressure center formed in the lower troposphere near the MV. Meanwhile, the descending rear-inflow jet (RIJ) was blocked by the prefrontal upward motion and a positive perturbation high-pressure center formed at around 1 km height. The violent positive-negative couplets of perturbation pressure gradient produced a downward nonhydrostatic perturbation pressure gradient force (NPPGF) that further intensified the vertical downward velocity. In addition to the strong downdraft velocity, the Coriolis force also enhanced the formation of the extreme wind through the action of ageostrophic winds. Besides, this paper also compares the mechanism of this process with that of a typical bow echo MV and the extreme wind it affected.

A new perspective on Saturn’s polar vortices: Coupling the thermosphere and stratosphere

We argue that the observed temperature and altitude structure at the base of Saturn’s polar thermosphere implies the presence of a deep, warm-cored, anticyclonic vortex that extends deep into the mesosphere, and possibly into the stratosphere. This would be a unique structure in the solar system: a permanent, large-scale middle atmospheric feature that is a direct consequence of magnetosphere-atmosphere coupling. We investigate the plausibility of the suggestion that this deep vortex is linked to the observed non-seasonal polar hotspots in Saturn’s stratosphere. We develop a simple model of the dynamics and thermal structure of the middle atmosphere polar vortex, assuming vertical viscous transport is dominant and that the zonal winds are close to geostrophic balance. The model requires the existence of a small poleward ageostrophic wind which causes compressional heating, producing the required pressure gradients. This scenario is supported by a re-analysis of previously published thermospheric modelling results. Using this model we estimate the extent to which the thermospheric vortex penetrates into the deeper atmosphere. The results are very sensitive to the distribution of eddy viscosity. The models which best match the thermospheric temperatures produce a polar hotspot in the stratosphere that is hotter and broader than observed, indicating that our model, while a useful starting point, is incomplete. We also point out a puzzle concerning the half-widths of the polar hotspot and the polar altitude bulge at the base of the thermosphere. We are able to explain the greater width of the hotspot provided that there is enhanced downwards vertical transport of angular momentum closer to the pole, a situation that is expected given the strong convergence and downwelling predicted in this region. Finally, we speculate that the tropospheric polar cyclone could be connected to the anticyclonic vortex in the thermosphere.

The Importance of Local Thermal Circulations in PM2.5 Formation in a River Valley: A Case Study From the Lower Yangtze River, China

AbstractPoor air quality is often experienced in densely populated areas located alongside large rivers. The presence of a river valley can modify local meteorology, potentially interfering with the formation of air pollutants such as PM2.5. However, the mechanisms behind PM2.5 formation in river valleys and its impacts on surrounding regions remain poorly understood. This study investigates the formation mechanisms of PM2.5 in the Nanjing reach of the Yangtze River, considering its complex terrain and anthropogenic emissions, through numerical simulation. Simulated results revealed higher PM2.5 concentrations along the Yangtze River compared to the average concentrations in Nanjing, primarily driven by elevated nitrate level. Factors such as higher humidity, wind speed, ageostrophic wind field, and shallow boundary layer were identified as key contributors to the increased PM2.5 concentrations along the river. Our study demonstrates significant impact of local thermal circulations on PM2.5 formation, resulting from the interaction between the large water body of the Yangtze River and the urban area. This phenomenon was confirmed through sensitivity experiments, which showed attenuated local thermal circulations in the absence of the Yangtze River. Source apportionment and process analysis results confirm the dominant roles of local thermal circulation related horizontal and vertical advections, with diurnal fluctuations, in shaping PM2.5 variations along the river. Inorganic chemistry processes also played a non‐negligible role, particularly during polluted conditions. Local thermal circulations in river valleys are crucial for PM2.5 formation, highlighting the need for comprehensive strategies to tackle air pollution in similar regions.

The increased summer monsoon rainfall in Northwest India: Coupling with the Northwestern Arabian Sea warming and modulated by the Silk Road Pattern since 2000

This study investigates changes in rainfall patterns over Northwest India (NWI) and the warming of the Northwestern Arabian Sea (NWAS) since 2000. These changes are attributed to the Silk Road Pattern (SRP) phase change in the late 1990s. The SRP positive phase triggers geostrophic deformation over the Tibetan Plateau (TP) by inducing an anomalous circulation pattern along the westerly jet, at 200 hPa. This deformation along the westerly Jet leads to Jet streaks and thermally indirect ageostrophic circulation over TP. In addition, the intensified heating over the TP led to a westward shift of the South Asian High (SAH). Associated with this, the interaction of positive temperature advection and northward ageostrophic wind (Jet entrance) over south-central Asia generates a southward component of ageostrophic wind (Jet Exit), causing upper-level convergence at southeast TP, excites the sinking cold air thereby strengthen the surface pressure below, over TP. This amplifies the anomalous anticyclone effect over there resulting in strong easterly wind anomalies along the monsoon trough at 850 hPa, disrupting the monsoon circulation. These anomalies, combined with a poleward shifting moisture-laden Low-Level Jet (LLJ), contribute to the warming of the NWAS and increased rainfall over NWI. The NWAS has shown significant summer warming over the past two decades. The poleward shift of the LLJ and SRP-induced circulation changes also impact the ocean dynamics. Given the occurrence of severe floods in this region of the Indian subcontinent, these findings have broader implications for predicting and projecting monsoon variability and extremes.

Sensitivity of boundary layer parameterization schemes in a marine boundary layer jet and associated precipitation during a coastal warm-sector heavy rainfall event

The sensitivity of planetary boundary layer (PBL) parameterization schemes in a marine boundary layer jet and associated precipitation is investigated in this study. Six PBL parameterization schemes in the Weather Research and Forecasting Model, including YSU, MYJ, MYNN, ACM2, BouLac, and UW schemes, are examined in simulating a marine boundary layer jet (BLJ) over South China Sea and associated coastal precipitation during a warm-sector heavy rainfall event (19–20 May 2015) near the coast of South China. The results show that YSU, MYJ, MYNN, and BouLac schemes can generally reproduce the coastal warm-sector heavy rainfall with 6-h accumulated precipitation exceeding 50 mm, but not for the ACMs and UW schemes. No convection initiation occurs in the ACM2 run, while rainfall is located to further north with weaker intensity in the UW run. Meanwhile, weakest and strongest BLJs are simulated in the ACM2 and UW runs, respectively. In the ACM2 run, the weaker BLJ with the maximum wind speed less than 17 m s−1 induces weaker convergence and lifting in the upwind side of the coastal terrain as well as less water vapor transport to the coastal area, which thus inhibit convection initiation. On the contrary, the too strong BLJ in the UW run with large area of wind speed greater than 18 m s−1 causes the northward movement of convection along with cold pools, and rainfall moves further north accordingly. The differences in BLJs’ strength among PBL schemes are attributed to varying simulated low-level vortex on the northern side of the BLJ through veering ageostrophic winds. The intensity of the simulated low-level vortex is affected by variations in boundary layer mixing over land and associated vertical temperature stratification under different PBL schemes.

Interannual variability of the double easterly jets over the tropical western Pacific and their effects on tropical cyclone genesis

AbstractDifferent from other upper‐tropospheric jet stream that owns the single jet axis, the tropical easterly jet (TEJ) generally splits into two branches over the tropical western Pacific where the TEJ inflows from June to September, with the north branch locating near 20°N and the south branch appearing near the equator. However, how these double easterly jets change from year to year and whether they have effects on the tropical cyclones (TCs) over the tropical western Pacific are still unknown. Observational evidence has demonstrated that the double easterly jets in the TEJ's inflow region exhibit an out‐of‐phase relationship in the interannual time scale and is significantly related to the TCs genesis. Corresponding to a stronger south branch and a weaker north branch, the accelerated (decelerated) easterly wind for the south (north) branch would induce a northerly (southerly) ageostrophic wind, thus triggering an upper‐tropospheric divergence between the double easterly jets. This divergence can further lead to anomalous ascending flows over the western North Pacific, which turns out to be the primary cause that increases the TCs number over there. The results highlight that the out‐of‐phase variation of the double easterly jets could dynamically modulate the occurrence of the TCs over the western North Pacific by inducing the secondary vertical circulation.

Mesoscale Moisture Transport in Determining the Location of Daytime Convection Initiations Clustered in Time and Space Over Southern China

AbstractConvective initiation (CI) episodes are defined as a series of CIs that occur in clusters in time and space, and their prediction remains a challenge due to their complex mechanisms. In the present study, high‐resolution convection‐permitting simulations and observations were conducted to investigate the source of inhomogeneous instability for a cluster of daytime CIs gathering in a northeast‐southwest oriented region in South China where the convergence boundary for lifting was not evident. In the region of the CI episode, low‐level water vapor was enhanced significantly, which thus induced the heterogeneity of convective available potential energy. The enhanced moisture and instability in the CI episode region were caused by a mesoscale anticyclonic moisture channel. Mesoscale moisture transport formed under both the inertial oscillation of ageostrophic winds and the mesoscale high‐pressure disturbances in the daytime. The revealed mechanism is further confirmed by sensitivity experiments, where mesoscale anticyclonic transport is highly sensitive. Water vapor transport was affected by the evolution of mesoscale pressure disturbances and dominated the location and shape of the CI episode. Stronger mesoscale pressure disturbances resulted in the subsequent mesoscale anticyclonic airflow coming from various high‐pressure systems and conveying moisture farther east, and therefore, a CI episode occurred farther east by dozens to hundreds of kilometers. The climatology of the daytime CI episode also showed that the various mesoscale moisture transports confined moisture within the CI episode regions under weak convergence.

Improving the Understanding of Atmospheric River Water Vapor Transport Using a Three‐Dimensional Straightened Composite Analysis

AbstractIrregular shapes of atmospheric rivers (ARs) hamper easy AR composite analyses and understandings of AR's moisture transport mechanisms. We develop a method to composite AR‐related variables from a reanalysis data set. By averaging a large number of samples, the three‐dimensional structure and some evolutionary features of a typical North Pacific AR are revealed. An AR is typically located along and in advance of the surface cold front of an extratropical cyclone. A mesoscale secondary circulation is observed in the cross sections of the AR corridor, where both geostrophic and ageostrophic winds make indispensable contributions to the moisture fluxes. Geostrophic moisture advection across the cold front within the Equatorward half of the AR is created by baroclinicity of the system and serves as the primary moisture source for the AR. Moisture fluxes from the warm sector of the cyclone are primarily due to boundary layer ageostrophic winds and are more important within the poleward half of the AR, particularly during the genesis stage. Faster AR movement compared with low‐level winds enables the AR to collect downwind moisture. In an AR‐relative view, this is represented as an easterly flow, consistent with the “feeder‐airstream” concept, and emphasizes the role of local moisture recycling. Moisture transport within the Equatorward half is mostly due to geostrophic advection of the propagating AR‐cyclone couple in an Earth‐relative view. Driven by intensifying geostrophic winds, ARs tend to reach peak moisture transport intensity about 2 days after genesis. Then, reduced moisture level and influxes from lateral boundaries prevent further intensification.

Uplift Mechanism of Coastal Extremely Persistent Heavy Rainfall (EPHR): The Key Role of Low‐Level Jets and Ageostrophic Winds in the Boundary Layer

AbstractExtremely persistent heavy rainfall (EPHR) events frequently occur in southern China. The mechanisms underlying their generation and development are not well understood, and this leads to difficulties in accurately forecasting these events. In this paper, hourly precipitation grid data and ERA‐5 reanalysis data are used to examine the relationships between precipitation, ageostrophic winds, and low‐level jets (LLJ). The results suggest that ageostrophic winds in the boundary layer mainly exhibit confrontational and asymptotic convergence, with the former being accompanied by double LLJ, which transport more vapor and provide favorable uplift conditions, thus producing stronger precipitation and wider rainbands. The different cooperated relationships between LLJ and ageostrophic winds play an important role in the development of EPHR events, and greater account needs to be taken of these relationships when forecasting.

Multiscale Processes of Heavy Rainfall over East Asia in Summer 2020: Diurnal Cycle in Response to Synoptic Disturbances

Abstract Multiscale processes from synoptic disturbances to diurnal cycles during the record-breaking heavy rainfall in summer 2020 were examined in this study. The heavy rainfall consisted of eight episodes, each lasting about 5 days, and were associated with two types of synoptic disturbances. The Type-1 episodes featured a northwestward extending western Pacific subtropical high (WPSH), while the Type-2 episodes had approaching midlatitude troughs with southward retreat in the WPSH. Each heavy rainfall episode had 2–3 occurrences of nocturnal low-level jets (NLLJs), in close association with intense rainfall in the early morning. The NLLJs formed partly due to the geostrophic wind by increased pressure gradients under both types of synoptic disturbances. The NLLJs were also driven by the ageostrophic wind that veered to maximum southerlies at late night due to the boundary-layer inertial oscillation. The diurnal amplitudes of low-level southerlies increased remarkably after the onset of Type-1 episodes, in which the extending WPSH provided strong daytime heating from solar radiation. By contrast, the wind diurnal amplitudes were less changed after the onset of Type-2 episodes. The NLLJs strengthened the mesoscale mean ascent, net moisture flux convergence, and convective instability in elevated warm moist air, which led to the upscale growth of MCSs at the northern terminus of the LLJ after midnight. The MCSs-induced Meiyu rainband was re-established in Central China during the Type-1 episodes with the increased diurnal variations. The findings highlight that the regional diurnal cycles of low-level winds in response to synoptic disturbances can strongly regulate mesoscale convective activities in a downscaling manner, and thus produce heavy rainfall.

Effect of Upper-Air Moistening by Northward Ageostrophic Winds Associated with a Tropical Cyclone on the PRE Enhancement

Effect of Upper-Air Moistening by Northward Ageostrophic Winds Associated with a Tropical Cyclone on the PRE Enhancement

Divergent Responses of Summer Precipitation in China to 1.5°C Global Warming in Transient and Stabilized Scenarios

AbstractResponses of summer precipitation in China to global 1.5°C warming with transient and stabilized pathways are investigated through the analysis of the Common Earth System Model climate simulations. Precipitation increases more significantly in southeast China in the stabilized scenario compared with the transient one. The difference is primarily attributable to dynamic factors related to circulation changes and driven by differences of land‐ocean thermal contrast between the two scenarios. The stabilized warming favors conditions of larger ocean warming and smaller land warming, leads to enhanced meridional temperature gradient and ultimately speeds up the jet stream over East Asia. Ageostrophic wind at the entrance of the accelerated jet strengthens ascending motion and precipitation along the coast of Asia from southeast China to India. The enhanced heating over India further induces a Kelvin‐wave response with an anomalous anticyclone that contributes to the enhanced westward shifted of the western North Pacific Subtropical High. Southerly winds at the west flank of the anticyclone transport moisture northward from tropical oceans, and strengthen precipitation in southeast China.

Orographically induced spontaneous imbalance within the jet causing a large-scale gravity wave event

Abstract. To better understand the impact of gravity waves (GWs) on the middle atmosphere in the current and future climate, it is essential to understand their excitation mechanisms and to quantify their basic properties. Here a new process for GW excitation by orography–jet interaction is discussed. In a case study, we identify the source of a GW observed over Greenland on 10 March 2016 during the POLSTRACC (POLar STRAtosphere in a Changing Climate) aircraft campaign. Measurements were taken with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument deployed on the High Altitude Long Range (HALO) German research aircraft. The measured infrared limb radiances are converted into a 3D observational temperature field through the use of inverse modelling and limited-angle tomography. We observe GWs along a transect through Greenland where the GW packet covers ≈ 1/3 of the Greenland mainland. GLORIA observations indicate GWs between 10 and 13 km of altitude with a horizontal wavelength of 330 km, a vertical wavelength of 2 km and a large temperature amplitude of 4.5 K. Slanted phase fronts indicate intrinsic propagation against the wind, while the ground-based propagation is with the wind. The GWs are arrested below a critical layer above the tropospheric jet. Compared to its intrinsic horizontal group velocity (25–72 m s−1) the GW packet has a slow vertical group velocity of 0.05–0.2 m s−1. This causes the GW packet to propagate long distances while spreading over a large area and remaining constrained to a narrow vertical layer. A plausible source is not only orography, but also out-of-balance winds in a jet exit region and wind shear. To identify the GW source, 3D GLORIA observations are combined with a gravity wave ray tracer, ERA5 reanalysis and high-resolution numerical experiments. In a numerical experiment with a smoothed orography, GW activity is quite weak, indicating that the GWs in the realistic orography experiment are due to orography. However, analysis shows that these GWs are not mountain waves. A favourable area for spontaneous GW emission is identified in the jet by the cross-stream ageostrophic wind, which indicates when the flow is out of geostrophic balance. Backwards ray-tracing experiments trace into the jet and regions where the Coriolis and the pressure gradient forces are out of balance. The difference between the full and a smooth-orography experiment is investigated to reveal the missing connection between orography and the out-of-balance jet. We find that this is flow over a broad area of elevated terrain which causes compression of air above Greenland. The orography modifies the wind flow over large horizontal and vertical scales, resulting in out-of-balance geostrophic components. The out-of-balance jet then excites GWs in order to bring the flow back into balance. This is the first observational evidence of GW generation by such an orography–jet mechanism.

Kinematic processes contributing to the intensification of anomalously strong North Atlantic jets

AbstractAnomalously strong North Atlantic jets, defined in this study as jets with wind speeds exceeding 100 m·s−1, are notable due to their potential to induce high‐impact weather. This study examines the kinematic processes that contribute to the intensification of anomalously strong North Atlantic jets, as well as the variability in those processes across a large number of events. Anomalously strong jets are objectively identified during September–May 1979–2018 within the Climate Forecast System Reanalysis and composited to reveal the synoptic‐scale flow evolution associated with jet intensification. The analysis demonstrates that anomalously strong North Atlantic jets are most frequent during the winter compared with the fall and spring, and that their development is preceded by low‐level warm‐air advection, poleward moisture advection, and moist ascent within the warm conveyor belt of a surface cyclone beneath the equatorward jet‐entrance region. A diagnosis of the irrotational and nondivergent components of the ageostrophic wind within the near‐jet environment reveals that both wind components facilitate jet intensification via their nonnegligible contributions to negative potential vorticity (PV) advection and PV frontogenesis in the vicinity of the dynamic tropopause. Weather Research and Forecasting (WRF) model simulations of a jet event from December 2013 with and without latent heating further suggest that the ageostrophic wind field within the near‐jet environment is substantially modulated by latent heating. The foregoing results indicate that a diagnosis of jet intensification during anomalously strong jet events is dependent on an accurate representation of the cumulative effects of latent heating within the near‐jet environment.

The Crucial Role of Synoptic Pattern in Determining the Spatial Distribution and Diurnal Cycle of Heavy Rainfall over the South China Coast

AbstractThe South China coast suffers frequent heavy rainfall every warm season. Based on the objective classification method of principal components analysis, the key role of the synoptic pattern in determining the heavy rainfall processes that occurred over the South China coast in the warm season during 2008–18 is examined in this study. We found that heavy rainfall occurs most frequently under three typical synoptic patterns (P1–P3 hereafter) characterized by strong low-level onshore winds. P1 and P3 feature a prevailing southwesterly monsoonal flow in the lower troposphere, with heavy rainfall frequently occurring over the inland windward region in the afternoon associated with the orographic lifting and solar heating. The onshore wind of P3 is stronger than P1 as the western Pacific subtropical high extends more westward to 122°E, which induces stronger low-level convergence along the coastline than P1 when the ageostrophic wind veers from the offshore to onshore direction in the early morning. Hence, a secondary early morning rainfall peak can be found along the coastline. P2 is characterized by a low-level vortex located over the southwest portion of south China. Heavy rainfall under P2 usually initiates over the western part of the coastal region in the morning and then propagates inland in the afternoon. Overall, the synoptic patterns strongly determine the spatial distribution and diurnal cycle of heavy rainfall over the South China coast. This heavy rainfall is closely related to the diurnally varying low-level onshore winds rather than the low-level jets, as well as the different interactions between the low-level onshore winds and the local orography, coastline, and land–sea breeze circulations under different synoptic patterns.

Storm Filomena 8 January 2021

Storm Filomena at 1200 utc on 8 January 2021, showing a sub‐section of the WRF model domain. The model’s low centre was 998hPa at this time; schematic warm and cold surface fronts are shown conventionally. Wind speeds of over 40ms−1 at 300hPa are shaded pale red and those greater than 60ms−1 at 200hPa are shaded pale blue. The flow vectors depict the ageostrophic wind and the bold contour shows divergence equal to 4 × 10−5s−1, both at 300hPa. The axis of the cross‐section shown in Figure 2 is marked by the dashed line and the approximate location of Madrid by ‘M’.

Nocturnal Destabilization Associated with the Summertime Great Plains Low-Level Jet

AbstractThe Great Plains low-level jet (LLJ) has long been associated with summertime nocturnal convection over the central Great Plains of the United States. Destabilization effects of the LLJ are examined using composite fields assembled from the North American Mesoscale Forecast System for June and July 2008–12. Of critical importance are the large isobaric temperature gradients that become established throughout the lowest 3 km of the atmosphere in response to the seasonal heating of the sloping Great Plains. Such temperature gradients provide thermal wind forcing throughout the lower atmosphere, resulting in the establishment of a background horizontal pressure gradient force at the level of the LLJ. The attendant background geostrophic wind is an essential ingredient for the development of a pronounced summertime LLJ. Inertial turning of the ageostrophic wind associated with LLJ provides a westerly wind component directed normal to the terrain-induced orientation of the isotherms. Hence, significant nocturnal low-level warm-air advection occurs, which promotes differential temperature advection within a vertical column of atmosphere between the level just above the LLJ and 500 hPa. Such differential temperature advection destabilizes the nighttime troposphere above the radiatively cooled near-surface layer on a recurring basis during warm weather months over much of the Great Plains and adjacent states to the east. This destabilization process reduces the convective inhibition of air parcels near the level of the LLJ and may be of significance in the development of elevated nocturnal convection. The 5 July 2015 case from the Plains Elevated Convection at Night field program is used to demonstrate this destabilization process.

Northward Ageostrophic Winds Associated with a Tropical Cyclone. Part 2: Moisture Transport and Its Impact on PRE

Northward Ageostrophic Winds Associated with a Tropical Cyclone. Part 2: Moisture Transport and Its Impact on PRE

Diurnal Effects of Regional Soil Moisture Anomalies on the Great Plains Low-Level Jet

AbstractThe Great Plains (GP) low-level jet (GPLLJ) contributes to GP warm season water resources (precipitation), wind resources, and severe weather outbreaks. Past research has shown that synoptic and local mesoscale physical mechanisms (Holton and Blackadar mechanisms) are required to explain GPLLJ variability. Although soil moisture–PBL interactions are central to local mechanistic theories, the diurnal effect of regional soil moisture anomalies on GPLLJ speed, northward penetration, and propensity for severe weather is not well known. In this study, two 31-member WRF-ARW stochastic kinetic energy backscatter scheme ensembles simulate a typical warm season GPLLJ case under CONUS-wide wet and dry soil moisture scenarios. In the GP (24°–48°N, 103°–90°W), ensemble mean differences in sensible heating and PBL height of 25–150 W m−2 and 100–700 m, respectively, at 2100 UTC (afternoon) culminate in GPLLJ 850-hPa wind speed differences of 1–4 m s−1 12 hours later (0900 UTC; early morning). Greater heat accumulation in the daytime PBL over dry soil impacts the east–west geopotential height gradient in the GP (synoptic conditions and Holton mechanism) resulting in a deeper thermal low in the northern GP, causing increases in the geostrophic wind. Enhanced daytime turbulent mixing over dry soil impacts the PBL structure (Blackadar mechanism), leading to increased ageostrophic wind. Overnight geostrophic and ageostrophic winds constructively interact, leading to a faster nocturnal GPLLJ over dry soil. Ensemble differences in CIN (~50–150 J kg−1) and CAPE (~500–1000 J kg−1) have implications for severe weather predictability.