Explore the vast range of titles available.

Pakistan experiences several torrential rainfall events for many years due to interactive role of midlatitude atmospheric and warm moist monsoon circulations. Deadly events such as flash flooding, runoff, and mudslides may trigger by torrential rainfall events which pose significant threat to human assets. This study was undertaken based on three severe weather events (1988, 2010, and 2013) by using percentage departure to identify wet and dry years during the period 1985–2016, which occurred over northern Pakistan. Rainfall data were obtained from 14 synoptic stations of Pakistan Meteorological Department, which were analyzed at daily, monthly, and annual bases. The data sets of Climate Research Unit, Global Precipitation Climatology Centre, and Pakistan Meteorological Department are used to justify the Weather Research and Forecasting model simulation and for analyzing the various atmospheric and diagnostic variables. Results showed teleconnection between large‐scale circulation pattern and South Asian summer monsoon during the selected events. Detailed analysis revealed that during all three events, a westerly trough was moving in the north of Pakistan and well‐marked monsoon circulations were present at the lower levels. It was observed that the interaction of two weather systems triggered heavy rainfall over area of interest. In addition, high vorticity at all levels and extension of westerly trough to the surface resulted in heavy rainfall and the penetration of upper‐level circulations anomalies consists of blocking signature over Eurasia along with Tibetan anticyclone. Hence, the monsoon low‐pressure system provides moisture convergence and upper‐level westerly trough results in upper‐level divergence to enhance the extreme rainfall events. Key Points Interaction of westerly jet and well‐marked monsoon circulations triggers torrential rainfall events over upper parts of Pakistan High vorticity at all levels and extension of the westerly trough to the surface are the causes of heavy rainfall Penetration of upper‐level circulations anomalies consists of blocking signature over Eurasia along with Tibetan Plateau anticyclone

The impact of varying the tip clearance of each rotor on the performance of a counter-rotating axial compressor has been investigated based on numerical simulations. The main purpose was to investigate the sensitivity to the tip clearance of each of the two individual rotors and the corresponding aerodynamic mechanisms associated with the performance variation in this compressor. The results indicated that both the total pressure ratio and the efficiency decreased as the tip clearance was increased, and the sensitivity curve for peak efficiency for both rotors was found to be an approximately linear negative relationship with increasing tip clearance. The variations of peak efficiency and stability margin of Rotor 2 were more sensitive to changing tip clearance than Rotor 1. An optimum combination of tip gaps existed for this compressor, i.e. 0.5τ for Rotor 1 and 0.25τ for Rotor 2 (where τ represents the nominal tip clearance value). At this optimum configuration, the peak efficiency and stability margin were improved by 0.63% and 29.4%, respectively. The location of the onset of the tip leakage vortex was found to be shifted downstream when the tip clearance increased. The nature of the tip leakage flow for each rotor was found to be influenced by the variation of tip clearance in the other rotor. Rotor 2 showed a more significant impact on Rotor 1. Additionally, varying the combination of tip clearances changed which of the two rotors was the first to stall.

A global climatology of warm conveyor belts (WCBs) is presented for the years 1979–2010, based on trajectories calculated with Interim ECMWF Re-Analysis (ERA-Interim) data. WCB trajectories are identified as strongly ascending air parcels (600 hPa in 2 days) near extratropical cyclones. Corroborating earlier studies, WCBs are more frequent during winter than summer and they ascend preferentially in the western ocean basins between 25° and 50° latitude. Before ascending, WCB trajectories typically approach from the subtropics in summer and from more midlatitude regions in winter. Considering humidity, cloud water, and potential temperature along WCBs confirms that they experience strong condensation and integrated latent heating during the ascent (typically >20 K). Liquid and ice water contents along WCBs peak at about 700 and 550 hPa, respectively. The mean potential vorticity (PV) evolution shows typical tropospheric values near 900 hPa, followed by an increase to almost 1 potential vorticity unit (PVU) at 700 hPa, and a decrease to less than 0.5 PVU at 300 hPa. These low PV values in the upper troposphere constitute significant negative anomalies with amplitudes of 1–3 PVU, which can strongly influence the downstream flow. Considering the low-level diabatic PV production, (i) WCBs starting at low latitudes (<40°) are unlikely to attain high PV (due to weak planetary vorticity) although they exhibit the strongest latent heating, and (ii) for those ascending at higher latitudes, a strong vertical heating gradient and high absolute vorticity are both important. This study therefore provides climatological insight into the cloud diabatic formation of significant positive and negative PV anomalies in the extratropical lower and upper troposphere, respectively.

A major open issue in tropical meteorology is how and why some tropical cyclones intensify under moderate vertical wind shear. This study tackles that issue by diagnosing physical processes of tropical cyclone intensification in a moderately sheared environment using a 20-member ensemble of idealized simulations. Consistent with previous studies, the ensemble shows that the onset of intensification largely depends on the timing of vortex tilt reduction and symmetrization of precipitation. A new contribution of this work is a process-based analysis following a shear-induced midtropospheric vortex with its associated precipitation. This analysis shows that tilt reduction and symmetrization precede intensification because those processes are associated with a substantial increase in near-surface vertical mass fluxes and equivalent potential temperature. A vorticity budget demonstrates that the increased near-surface vertical mass fluxes aid intensification via near-surface stretching of absolute vorticity and free-tropospheric tilting of horizontal vorticity. Importantly, tilt reduction happens because of a vortex merger process—not because of advective vortex alignment—that yields a single closed circulation over a deep layer. Vortex merger only happens after the midtropospheric vortex reaches upshear left, where the flow configuration favors near-surface vortex stretching, deep updrafts, and a substantial reduction of low-entropy fluxes. These results lead to the hypothesis that intensification under moderate shear happens if and when a “restructuring” process is completed, after which a closed circulation favors persistent vorticity spinup and recirculating warm, moist air parcels.

The radius of maximum wind (RMW) has been found to contract rapidly well preceding rapid intensification in tropical cyclones (TCs) in recent literature but the understanding of the involved dynamics is incomplete. In this study, this phenomenon is revisited based on ensemble axisymmetric numerical simulations. Consistent with previous studies, because the absolute angular momentum (AAM) is not conserved following the RMW, the phenomenon can not be understood based on the AAM-based dynamics. Both budgets of tangential wind and the rate of change in the RMW are shown to provide dynamical insights into the simulated relationship between the rapid intensification and rapid RMW contraction. During the rapid RMW contraction stage, due to the weak TC intensity and large RMW, the moderate negative radial gradient of radial vorticity flux and small curvature of the radial distribution of tangential wind near the RMW favor rapid RMW contraction but weak diabatic heating far inside the RMW leads to weak low-level inflow and small radial absolute vorticity flux near the RMW and thus a relatively small intensification rate. As RMW contraction continues and TC intensity increases, diabatic heating inside the RMW and radial inflow near the RMW increase, leading to a substantial increase in radial absolute vorticity flux near the RMW and thus the rapid TC intensification. However, the RMW contraction rate decreases rapidly due to the rapid increase in the curvature of the radial distribution of tangential wind near the RMW as the TC intensifies rapidly and RMW decreases.

The global perspective presented here is built on earlier papers discussing the dynamics of the upper branch of the Hadley cell in the two solsticial seasons. The role of the tropics is made explicit in a conceptual model that is presented and evaluated. The fluctuation of deep tropical convection in longitude and time is seen as crucial. The filamentary outflows from such convective events move westward and across the equator deep into the winter hemisphere. The horizontal tilt of the cross-equatorial flow implies a significant upper-tropospheric flux of westerly momentum from the winter tropics to the summer hemisphere. These properties are related to the cross-equatorial propagation of wave activity triggered by deep tropical convection in the summer hemisphere. The filaments carry with them near-equatorial values of absolute vorticity and potential vorticity. After turning anticyclonically, some filaments move eastward and poleward to the equatorial edge of the winter subtropical jet. There is strong evidence they can interact with the eddies on this jet and enhance their poleward westerly momentum flux. In the global perspective, tropical and extratropical systems and the interaction between them are all important for the dynamics of the upper branch of the Hadley cell.

Both the impacts of two types of El Niño on the western North Pacific (WNP) tropical cyclone (TC) activity and the seasonality in the relationship between genesis potential index (GPI) and El Niño–Southern Oscillation (ENSO) are investigated. The ENSO-induced GPI change over the northwestern (southeastern) part of the WNP is mostly attributed to the relative humidity (absolute vorticity) term, revealing a distinct meridional and zonal asymmetry in summer and fall, respectively. The seasonal change in ENSO (background states) from summer to fall is responsible for the seasonal change in GPI anomalies south of 20°N (over the northeastern part of the WNP). The downdraft induced by the strong upper-level convergence in the eastern Pacific (EP)-type El Niño and both the northwestward-shifted relative vorticity and northward-extended convection over the southeastern part of the WNP in the central Pacific (CP)-type El Niño lead to distinct TC impacts over East Asia (EA). The southward movement of genesis location of TCs and increased westward-moving TCs account for the enhanced strong typhoon activity for the EP-type El Niño in summer. In fall the downdraft and anomalous anticyclonic steering flows over the western part of the WNP remarkably decrease TC impacts over EA. The enhanced moist static energy and midlevel upward motion over the eastern part of the WNP under the northern off-equatorial sea surface temperature warming as well as longer passage of TCs toward EA are responsible for the enhanced typhoon activity for the CP-type El Niño. It is thus important to consider the seasonality and El Niño pattern diversity to explore the El Niño–induced TC impacts over EA.

In this study, we examine the role of curvature in modifying frontal stability. We first evaluate the classical criterion that the Coriolis parameter f multiplied by the Ertel potential vorticity (PV) q is positive for stable flow and that instability is possible when this quantity is negative. The first portion of this statement can be deduced from Ertel’s PV theorem, assuming an initially positive fq . Moreover, the full statement is implicit in the governing equation for the mean geostrophic flow, as the discriminant, fq , changes sign. However, for curved fronts in cyclogeostrophic or gradient wind balance (GWB), an additional term enters the discriminant owing to conservation of absolute angular momentum L . The resulting expression, (1 + Cu) fq < 0 or Lq < 0, where Cu is a nondimensional number quantifying the curvature of the flow, simultaneously generalizes Rayleigh’s criterion by accounting for baroclinicity and Hoskins’s criterion by accounting for centrifugal effects. In particular, changes in the front’s vertical shear and stratification owing to curvature tilt the absolute vorticity vector away from its thermal wind state; in an effort to conserve the product of absolute angular momentum and Ertel PV, this modifies gradient Rossby and Richardson numbers permitted for stable flow. This forms the basis of a nondimensional expression that is valid for inviscid, curved fronts on the f plane, which can be used to classify frontal instabilities. In conclusion, the classical criterion fq < 0 should be replaced by the more general criterion for studies involving gravitational, centrifugal, and symmetric instabilities at curved density fronts. In Part II of the study, we examine interesting outcomes of the criterion applied to low-Richardson-number fronts and vortices in GWB.

Extratropical regional-scale extreme precipitation events (EPEs) are usually associated with certain synoptic perturbations superimposed on slow-varying background circulations. These perturbations induce a dynamically forced ascent that destabilizes the atmospheric stratification and stimulates deep convection, which further drives the perturbation by releasing latent heat. This study identifies the characteristics of large-scale perturbations associated with summer EPEs in two representative regions, East China (ECN) and the southeastern United States (SUS), and analyzes the roles of dynamic forcings and diabatic heating using the quasigeostrophic omega equation. Composites of 39 events in each region show that the upper-level absolute vorticity advection and tropospheric warm advection promote dynamically forced ascent in EPEs, and the moisture advection premoistens the local environment. The background circulation and synoptic perturbations in ECN and the SUS have significant differences. The background vorticity, temperature, and moisture advection form the quasi-steady mei-yu front in ECN, which provides favorable conditions for heavy rainfall. In the SUS, weaker background ascents are forced mainly through vorticity advection. In the synoptic scale, the EPEs in ECN are triggered by short-wavelength wave trains, and in the SUS the EPEs are triggered by longer wavelength potential vorticity intrusions. Although the amplitudes of the dynamically forced ascent in the two regions are similar, diabatic heating contributes much more to the vertical motion in ECN than the SUS, which indicates that there is stronger diabatic heating feedback there. The stronger diabatic heating feedback in ECN appears to be due to stronger moisture advection, convective environments with more humidity, and stronger coupling between convection and large-scale dynamics.

As the ITCZ moves off the equator on an aquaplanet, the Hadley circulation transitions from an equinoctial regime with two near-symmetric, significantly eddy-driven cells to a monsoon-like regime with a strong, thermally direct cross-equatorial cell, intense low-latitude precipitation, and a weak summer hemisphere cell. Dynamical feedbacks appear to accelerate the transition. This study investigates the relevance of this behavior to monsoon onset by using primitive equation model simulations ranging from aquaplanets to more realistic configurations with Earth’s continents and topography. A change in the relationship between ITCZ latitude and overturning strength is identified once the ITCZ moves poleward of approximately 7°. Monsoon onset is associated with off-equatorial ascent in regions of nonnegligible planetary vorticity, and this is found to generate a vortex stretching tendency that reduces upper-level absolute vorticity. In an aquaplanet, this causes a transition to the cross-equatorial, thermally direct regime, intensifying the overturning circulation. Analysis of the zonal momentum budget suggests that a stationary wave, driven by topography and land–sea contrast, can trigger a similar transition in the more realistic model configuration, with the wave extending the ascent region of the Southern Hemisphere Hadley cell northward, and enhanced overturning then developing to the south. These two elements of the circulation resemble the East and South Asian monsoons.