Characterizing quasi-biweekly variability of the Asian monsoon anticyclone using potential vorticity and large-scale geopotential height field

The spatial pattern of subseasonal variability of the Asian monsoon anticyclone is analyzed using long-term reanalysis data, focusing on the large-scale longitudinal movement. The air inside the anticyclone is quantified by a thickness-weighted low-PV (potential vorticity) area on an isentropic surface. It is shown that the longitudinal movement of the air inside the Asian monsoon anticyclone has a timescale of 1 to 2 weeks, which is shorter than the monthly dominant timescale of the variability in the anticyclone intensity. The movement of the anticyclonic air is suggested to be largely controlled by passive advection. The typical time evolution of the variability pattern, explained by two leading empirical orthogonal function (EOF) components of 100 hPa geopotential height, shows large-scale geopotential anomalies moving westward spanning from low to middle latitudes. This corresponds well with the rapid westward movement of low-PV air known as “eddy shedding” and following the eastward retreat of the anticyclonic air. The two EOF components can also explain the bimodal longitudinal distribution of geopotential maximum location.

Sea ice and atmospheric circulation shape the high-latitude lapse rate feedback

Arctic amplification of anthropogenic climate change is widely attributed to the sea-ice albedo feedback, with its attendant increase in absorbed solar radiation, and to the effect of the vertical structure of atmospheric warming on Earth’s outgoing longwave radiation. The latter lapse rate feedback is subject, at high latitudes, to a myriad of local and remote influences whose relative contributions remain unquantified. The distinct controls on the high-latitude lapse rate feedback are here partitioned into “upper” and “lower” contributions originating above and below a characteristic climatological isentropic surface that separates the high-latitude lower troposphere from the rest of the atmosphere. This decomposition clarifies how the positive high-latitude lapse rate feedback over polar oceans arises primarily as an atmospheric response to local sea ice loss and is reduced in subpolar latitudes by an increase in poleward atmospheric energy transport. The separation of the locally driven component of the high-latitude lapse rate feedback further reveals how it and the sea-ice albedo feedback together dominate Arctic amplification as a coupled mechanism operating across the seasonal cycle.

Characterizing quasi-biweekly variability of the Asian monsoon anticyclone using potential vorticity and large-scale geopotential field

The spatial pattern of subseasonal variability of the Asian monsoon anticyclone is analyzed using long-term reanalysis data, focusing on the large-scale longitudinal movement. The air inside the anticyclone is quantified by a thickness-weighted low PV area on an isentropic surface. It is shown that the longitudinal movement of the air inside the Asian monsoon anticyclone has a timescale of one to two weeks, which is shorter than the monthly dominant timescale of the variability in the anticyclone intensity. The movement of the anticyclonic air is suggested to be largely controlled by passive advection. The typical time evolution of the variability pattern, explained by two leading EOF components of 100 hPa geopotential height, shows large-scale geopotential anomalies moving westward spanning from low to middle latitudes. This corresponds well with the rapid westward movement of low-PV air known as eddy shedding and following eastward retreat of the anticyclonic air. The two EOF components can also explain the bimodal longitudinal distribution of geopotential maximum location.

A Comparative Study Between Short Life- Cut off Low and Long Life-Cut off Low Accompanied by Heavy Precipitation Storms Over Iraq: Case Study

A comparative study between Short Life Cut Off Low (SL-COL) extended for one day, and Long Life Cut Off Low (LL-COL) extended for ten days associated with successive rain storms over Iraq on 21 to 30 April, 2018. The study tracking the evolution stages of both COLs in different pressure surfaces at troposphere layer, and found that there are many dynamical processes effect on prolong the life of COL. These processes exchanged their roles between different pressure levels. In both cases the high potential vorticity (PV) anomaly at 315ᵒ K isentropic surface is responsible on the emergence COLs, and the convection processes at lower troposphere and latent heat at upper troposphere are responsible on COLs dissipation. The main reasons of long-life COL can be summaries as a high-pressure system below COL at the surface which preventing the convection process, the formation of Omega block that promoting the COL system and preventing its dissipation rapidly in spite of the intense convection processes due to tropical moist system at the surface.

Chlorine partitioning in the lowermost Arctic vortex during the cold winter 2015/2016

Activated chlorine compounds in the polar winter stratosphere drive catalytic cycles that deplete ozone and methane, whose abundances are highly relevant to the evolution of global climate. The present work introduces a novel dataset of in situ measurements of relevant chlorine species in the lowermost Arctic stratosphere from the aircraft mission POLSTRACC–GW-LCYCLE–SALSA during winter 2015/2016. The major stages of chemical evolution of the lower polar vortex are presented in a consistent series of high-resolution mass spectrometric observations of HCl and ClONO2. Simultaneous measurements of CFC-12 are used to derive total inorganic chlorine (Cly) and active chlorine (ClOx). The new data highlight an altitude dependence of the pathway for chlorine deactivation in the lowermost vortex with HCl dominating below the 380 K isentropic surface and ClONO2 prevailing above. Further, we show that the Chemical Lagrangian Model of the Stratosphere (CLaMS) is generally able to reproduce the chemical evolution of the lower polar vortex chlorine budget, except for a bias in HCl concentrations. The model is used to relate local measurements to the vortex-wide evolution. The results are aimed at fostering our understanding of the climate impact of chlorine chemistry, providing new observational data to complement satellite data and assess model performance in the climate-sensitive upper troposphere and lower stratosphere region.

The Momentum Budget in the Stratosphere, Mesosphere, and Lower Thermosphere. Part I: Contributions of Different Wave Types and In Situ Generation of Rossby Waves

A momentum budget is examined in the stratosphere, mesosphere, and lower thermosphere using simulation data over ~11 years from a whole-atmosphere model in terms of the respective contributions of gravity waves (GWs), Rossby waves (RWs), and tides. The GW forcing is dominant in the mesosphere and lower thermosphere (MLT), as indicated in previous studies. However, RWs also cause strong westward forcing, described by Eliassen–Palm flux divergence (EPFD), in all seasons in the MLT and in the winter stratosphere. Despite the relatively coarse model resolution, resolved GWs with large amplitudes appear in the MLT. The EPFD associated with the resolved GWs is eastward (westward) in the summer (winter) hemisphere, similar to the parameterized GW forcing. A pair of positive and negative EPFDs are associated with the RWs and GWs in the MLT. These results suggest that the RWs and resolved GWs are generated in situ in the MLT. Previous studies suggested that a possible mechanism of RW generation in the MLT is the barotropic/baroclinic instability. This study revisits this possibility and examines causes of the instability from a potential vorticity (PV) viewpoint. The instability condition is characterized as the PV maximum at middle latitudes on an isentropic surface. Positive EPFD for RWs is distributed slightly poleward of the PV maximum. Because the EPFD equals the PV flux, this feature indicates that the RW radiation acts to reduce the PV maximum. The PV maximum is climatologically maintained in both the winter and summer mesospheres, which is caused by parameterized GW forcing.

Fast descent routes from within or near the stratosphere to the surface at Fukuoka, Japan, studied using ⁷Be measurements and trajectory calculations

By using high concentrations of 7Be as an indicator, we clarify fast descent routes from within or near the stratosphere to Earth's surface, with the study site being in Fukuoka, Japan. Most routes arise from high latitudes through the following processes. First, the descent associated with a tropopause fold occurs, followed by southward movement with slow descent at the rear side of a strong trough. Because this motion occurs along an isentropic surface, the descending air parcels nearly conserve the potential temperature. As an extension, a strong descent associated with a sharp drop in the isentropic-surface height occurs at the southern edge of the trough; this transports air parcels to low altitudes. This process involves irreversible phenomena such as filamentation and cutoff of potential vorticity. Finally, upon meeting appropriate near-surface disturbances, parcels at low altitudes are transported to Earth's surface.In some cases, parcels descend within midlatitudes. In such routes, because the potential temperature is much higher at high altitudes than at low altitudes, descent with conservation of the potential temperature is impossible, and the potential temperature decreases along the trajectories through mixing.The prevalence of the high-latitude route is explained as follows. In the midlatitude route, because parcels at high and relatively low altitudes mix, the high concentrations of 7Be included in high-altitude parcels are difficult to maintain. Therefore, for parcels to arrive at low altitudes in the midlatitude while maintaining high concentrations of 7Be, i.e., conserving the potential temperature, their area of origin should be high altitudes in high latitudes where the potential temperature is almost the same as that in the arrival area.In spring, tropopause folds are frequent in high latitudes, disturbances in the southward transport of parcels are strong, and disturbances occur by which parcels descend to the surface. Therefore, high concentrations of 7Be occur most frequently in spring.

Fast descent routes from within or near the stratosphere to Earth's surface

By using high concentrations of 7Be as an indicator, we clarify fast descent routes from within or near the stratosphere to Earth's surface, with the study site being in Fukuoka, Japan. Most routes arise from high latitudes through the following processes. First, the descent associated with a tropopause fold occurs, followed by southward movement with slow descent at the rear side of a strong trough. Because this motion occurs along an isentropic surface, the descending air parcels nearly conserve the potential temperature. As an extension, a strong descent associated with a sharp drop in the isentropic-surface height occurs at the south edge of the trough; this transports air parcels to low altitudes. This process involves irreversible phenomena such as filamentation and cutoff of potential vorticity. Finally, upon meeting appropriate near-surface disturbances, parcels at low altitudes are transported to Earth's surface. In some cases, parcels descend within mid-latitudes. In such routes, because the potential temperature is much higher at high altitudes than at low altitudes, strong descent with conservation of the potential temperature is impossible, and the potential temperature decreases. In these cases, the entire flow does not move downward; instead, only part of the flow in a diffluent field descends. When parcels descend, they push low isentropic surfaces, and their potential temperature decreases upon mixing with parcels having low potential temperature in the lower layers. The prevalence of the high-latitude route is explained as follows. In the mid-latitude route, because parcels at high and relatively low altitudes mix, the high concentrations of 7Be included in high-altitude parcels are difficult to maintain. Therefore, for parcels to arrive at low altitudes in the mid-latitude while maintaining high concentrations of 7Be, i.e., conserving the potential temperature, their area of origin should be high altitudes in high latitudes where the potential temperature is almost the same as that in the arrival area. Moreover, the initial descent must occur, because parcels cannot descend in the stratosphere when moving from high to mid-latitudes; parcels must already have descended from the stratosphere to the troposphere in high latitudes for effective descent with the movement to mid-latitudes. In spring, tropopause folds are frequent in high latitudes, disturbances in the southward transport of parcels are strong, and disturbances occur by which parcels descend to the surface. Therefore, high concentrations of 7Be occur most frequently in spring.

Aspects of Potential Vorticity Fluxes: Climatology and Impermeability

Some aspects of the dynamics of generalized potential vorticity (PV) density P = ω ⋅ ∇χ are discussed with the main emphasis on P fluxes, where ωa is absolute vorticity and χ is a scalar. The impermeability theorem claims that there is no net P flux across a χ surface. Various forms of the flux are presented that mostly cross χ surfaces. As these fluxes are as dynamically relevant as the one chosen for the theorem, P fluxes through a surface element are inherently multivalued and there is no best choice on physical grounds. Nevertheless, the net P flux is unique for closed surfaces. This point is illustrated by P integrals over the volume between the earth’s surface and an isentropic surface. Reanalysis data are used to present mean advective and some nonadvective P fluxes for χ = θ in height coordinates. The extratropical tropopause appears to be supported by advective P fluxes. A satisfactorily closed P budget cannot, however, be presented.