Abstract
The role of warm conveyor belts (WCBs) and their associated positive low-level potential vorticity (PV) anomalies are investigated for extratropical cyclones in Northern Hemisphere winter, ...using ERA-Interim and composite techniques. The Spearman correlation coefficient of 0.68 implies a moderate to strong correlation between cyclone intensification and WCB strength. Hereby, cyclone intensification is quantified by the normalized maximum 24-h central sea level pressure deepening and WCB strength by the WCB air mass associated with the cyclone’s 24-h period of strongest deepening. Explosively intensifying cyclones typically have strong WCBs and pronounced WCB-related PV production in the cyclone center; they are associated with a WCB of type W2, which ascends close to the cyclone center. Cyclones with similar WCB strength but weak intensification are either diabatic Rossby waves, which do not interact with an upper-level disturbance, or cyclones where much of the WCB-related PV production occurs far from the cyclone center and thereby does not contribute strongly to cyclone deepening (WCB of type W1, which ascends mainly along the cold front). The category of explosively intensifying cyclones with weak WCBs is inhomogeneous but often characterized by a very low tropopause or latent heating independent of WCBs. These findings reveal that (i) diabatic PV production in WCBs is essential for the intensification of many explosive cyclones, (ii) the importance of WCBs for cyclone development strongly depends on the location of the PV production relative to the cyclone center, and (iii) a minority of explosive cyclones is not associated with WCBs.
The climatic condition that prevails at a glacier equilibrium line altitude (ELA) is often parameterized in terms of summer air temperature (T) and annual precipitation (P). This simple ...parameterization was initially proposed by Hans W:son Ahlmann. The physical background of the relationship between P and T on the equilibrium line, however, has been left unexplained since Ahlmann first questioned the mathematical form of the relationship. This relationship can be explained when the thermal and hydrological processes of the ELA formation are investigated. The present authors studied the energy exchange processes that prevail on the ELA during the melt season. The inclusion of solar radiation brings Ahlmann's hypothesis closer to energy balance, and improves his P/T diagram. By comparing the observed fluxes from the polar through the mid-latitude to the equatorial glaciers, it was found that these glaciers in different climatic regions share important similarities at the ELA. Further, it was found that the classic P/T curve originally proposed by Ahlmann in the early 20th century is a concise expression of the conservation principle of energy and mass at the ELA of glaciers, and takes the form of a polynomial of the fourth order.
The role of moisture for extratropical atmospheric dynamics is particularly pronounced within warm conveyor belts (WCBs), which are characterized by intense latent heat release and precipitation ...formation. Based on the WCB climatology for the period 1979–2010 presented in Part I, two important aspects of the WCB moisture cycle are investigated: the evaporative moisture sources and the relevance of WCBs for total and extreme precipitation. The most important WCB moisture source regions are the western North Atlantic and North Pacific in boreal winter and the South Pacific and western South Atlantic in boreal summer. The strongest continental moisture source is South America. During winter, source locations are mostly local and over the ocean, and the associated surface evaporation occurs primarily during 5 days prior to the start of the WCB ascent. Long-range transport and continental moisture recycling are much more important in summer, when a substantial fraction of the evaporation occurs more than 10 days before the ascent. In many extratropical regions, WCB moisture supply is related to anomalously strong surface evaporation, enforced by low relative humidity and high winds over the ocean. WCBs are highly relevant for total and extreme precipitation in many parts of the extratropics. For instance, the percentage of precipitation extremes directly associated with a WCB is higher than 70%–80% over southeastern North America, Japan, and large parts of southern South America. A proper representation of WCBs in weather forecast and climate models is thus essential for the correct prediction of extreme precipitation events.
The effects of global warming are strongly amplified in the Arctic, causing rapidly rising temperatures and ongoing dramatic loss of sea ice, which itself is subject to large interannual variability. ...We investigate changes in seasonal‐mean temperature and precipitation, its variability and extremes, using large‐ensemble climate model data with a representative concentration pathway 8.5 forcing scenario for historical (S2000) and end‐of‐century projections (S2100). Our results reveal regionally and seasonally dependent changes in Arctic interannual temperature and precipitation variability that are strongly linked to sea‐ice loss. We show a doubling in precipitation variability over the Arctic Ocean and a significant reduction in temperature variability in the Barents Sea. Extremely warm seasons in S2000 rank among the coldest seasons or become unrealistic in S2100. We further show the key role of large‐scale weather systems for shaping seasonal temperature and precipitation extremes in the Arctic which persists under climate warming.
Plain Language Summary
Rapidly rising temperatures and a dramatic reduction of sea ice in recent decades illustrate the impact of climate change in the Arctic. On top of this long‐term trend, Arctic seasonal‐mean conditions can vary substantially from one year to another. In this study, we use a climate model to simulate the Arctic climate at the end of the 21st century following a high‐emission scenario and compare it to simulations representative of the present‐day climate. In general, the Arctic will become warmer and wetter, but large regional and seasonal differences are observed in part owing to local changes of sea‐ice coverage. We find that at very high latitudes, the Arctic will soon experience conditions that are far outside the natural variability in today's climate. Further we show robust patterns of cyclones and anticyclones for seasonal temperature and precipitation extremes in both, present‐day and future Arctic climate.
Key Points
Regionally dependent changes in seasonal temperature and precipitation variability are closely linked to local sea‐ice evolution
Seasons ranking among the warmest in present‐day climate are soon projected to rank among the coldest or become unrealistic
Robust dynamical relationship between weather system frequency and seasonal extremes persists in warming climate
Presently available information on the glacier equilibrium line altitude (ELA) is being collected and examined. The historical course of the world’s longest ELA series of 107 years at the ...Claridenfirn is reviewed together with climatic elements. Further, the changes in ELAs of 70 glaciers the world over are investigated, and a linear plane model for the speed of the ELA shift is proposed as a function of the changing rates of summer temperature and winter mass balance. The four glaciers in Europe, which diverge most from the plane, are investigated in detail. The cause of the divergence is likely due to be the change in solar global radiation. Although a precise quantification of the role of radiation is not possible at this stage for the entire world, the role of solar radiation is investigated for these glaciers. Globally viewed, ten, or 15% of the 70 investigated glaciers, are expected to lose their accumulation areas within the next ten years. Half of all studied glaciers will follow the same fate by the end of this century under the present climatic conditions. If climate change is accelerated, the disappearance of glaciers will occur sooner than presented in this study.
At the turn of the years 2015/2016, maximum surface temperature in the Arctic reached record‐high values, exceeding the melting point, which led to a strong reduction of the Arctic sea ice extent in ...the middle of the cold season. Here we show, using a Lagrangian method, that a combination of very different airstreams contributed to this event: (i) warm low‐level air of subtropical origin, (ii) initially cold low‐level air of polar origin heated by surface fluxes, and (iii) strongly descending air heated by adiabatic compression. The poleward transport of these warm airstreams occurred along an intense low‐level jet between a series of cyclones and a quasi‐stationary anticyclone. The complex 3‐D configuration that enabled this transport was facilitated by continuous warm conveyor belt ascent into the upper part of the anticyclone. This study emphasizes the combined role of multiple transport processes and transient synoptic‐scale dynamics for establishing an extreme Arctic warm event.
Key Points
A complex chain of extraordinary dynamical and physical processes resulted in an extreme wintertime Arctic warm event
The warm air came from three different regions and experienced fundamentally different temperature evolutions before arriving in the Arctic
Latent heating in ascending airstreams contributed to the setup of an unusual weather pattern that allowed the warm air to reach the Arctic
Abstract
Diabatic Rossby waves (DRWs) are low-tropospheric positive potential vorticity (PV) anomalies in moist and sufficiently baroclinic regions. They regenerate continuously by moist-diabatic ...processes and potentially develop into explosively intensifying cyclones. In this study a specific DRW-tracking algorithm is developed and applied to operational ECMWF analyses to compile a first climatology of DRWs in the Northern Hemisphere for the years 2001–10. DRWs are more frequent over the North Pacific than over the North Atlantic with on average 81 and 43 systems per year, respectively. Less than 15% of these systems intensify explosively, on average 12 per year over the Pacific and 5 over the Atlantic. DRWs are most frequent in summer but most of the explosively intensifying DRWs occur in autumn and winter. DRWs are generated typically between 30°–50°N over the eastern parts of the continents and the western/central parts of the oceans. They propagate fairly zonally along the midlatitude baroclinic zone. The generation of the initial low-tropospheric PV anomalies goes along with precipitation processes in characteristic flow patterns, which correspond to 1) flow around the subtropical high against the midlatitude baroclinic zone, 2) flow induced by an upper-level cutoff or a (tropical) cyclone against the baroclinic zone, 3) upper-level trough-induced ascent at the baroclinic zone, and 4) PV remnants of a tropical cyclone or a mesoscale convective system that are advected into the baroclinic zone where they start propagating as a DRW. In most cases, explosive intensification of DRWs occurs through interaction with a preexisting upper-level trough.
Abstract
The life cycle of a North Atlantic cyclone in December 2005 that included a rapid propagation phase as a diabatic Rossby wave (DRW) is investigated by means of operational analyses and ...deterministic forecasts from the ECMWF. A quasigeostrophic omega diagnostic has been applied to assess the impact of upper-level forcing during the genesis, propagation, and intensification phase, respectively. The system was generated in the Gulf of Mexico as a mesoscale convective vortex (MCV) influenced by vertical motion forcing from a nearby upper-level trough. The DRW propagation phase was characterized by a shallow, low-level, diabatically produced potential vorticity (PV) anomaly that rapidly propagated along the southern border of an intense baroclinic zone. No significant upper-level forcing could be identified during this phase of the development. Eventually, explosive intensification occurred as the region of vertical motion forced by an approaching upper-level trough reached the position of the DRW. The rapid intensification of 34 hPa in 24 h led to a mature extratropical cyclone in the central North Atlantic with marked frontal structures associated with a pronounced PV tower.
The performance of four operational deterministic ECMWF forecasts has been investigated for the DRW propagation and cyclone intensification. The forecasts showed a highly variable skill. Despite the fact that the DRW was initially well represented in all forecasts, two of them failed to capture the explosive intensification. By applying a DRW tracking tool, the low-level baroclinicity downstream of the DRW and the moisture supply to the south of the DRW could be identified as the key environmental parameters during DRW propagation. The subsequent cyclone intensification went wrong in two of the forecasts because of the missing interaction of the DRW and the upper-level trough. It is shown that this interaction can fail if the intensity of the DRW and/or the approaching upper-level wave are too weak, or in case of an erroneous structure of the upper-level trough leading to a phasing problem of the vertical interaction with the DRW. Therefore, the DRW intensification bears similar characteristics and forecast challenges as the extratropical reintensification of tropical cyclones.
Warm conveyor belts (WCBs) are important airstreams in extratropical cyclones. They can influence large-scale flow evolution by modifying the potential vorticity (PV) distribution during their ...cross-isentropic ascent. Although WCBs are typically described as slantwise-ascending and stratiform-cloud-producing airstreams, recent studies identified convective activity embedded within the large-scale WCB cloud band. However, the impacts of this WCB-embedded convection have not been investigated in detail. In this study, we systematically analyze the influence of embedded convection in an eastern North Atlantic WCB on the cloud and precipitation structure, on the PV distribution, and on larger-scale flow. For this reason, we apply online trajectories in a high-resolution convection-permitting simulation and perform a composite analysis to compare quasi-vertically ascending convective WCB trajectories with typical slantwise-ascending WCB trajectories. We find that the convective WCB ascent leads to substantially stronger surface precipitation and the formation of graupel in the middle to upper troposphere, which is absent for the slantwise WCB category, indicating the key role of WCB-embedded convection for precipitation extremes. Compared to the slantwise WCB trajectories, the initial equivalent potential temperature of the convective WCB trajectories is higher, and the convective WCB trajectories originate from a region of larger potential instability, which gives rise to more intense cloud diabatic heating and stronger cross-isentropic ascent. Moreover, the signature of embedded convection is distinctly imprinted in the PV structure. The diabatically generated low-level positive PV anomalies, associated with a cyclonic circulation anomaly, are substantially stronger for the convective WCB trajectories. The slantwise WCB trajectories lead to the formation of a widespread region of low-PV air (that still have weakly positive PV values) in the upper troposphere, in agreement with previous studies. In contrast, the convective WCB trajectories form mesoscale horizontal PV dipoles at upper levels, with one pole reaching negative PV values. On a larger scale, these individual mesoscale PV anomalies can aggregate to elongated PV dipole bands extending from the convective updraft region, which are associated with coherent larger-scale circulation anomalies. An illustrative example of such a convectively generated PV dipole band shows that within around 10 h the negative PV pole is advected closer to the upper-level waveguide, where it strengthens the isentropic PV gradient and contributes to the formation of a jet streak.
This suggests that the mesoscale PV anomalies produced by embedded convection upstream organize and persist for several hours and therefore can influence the synoptic-scale circulation. They thus can be dynamically relevant, influence the jet stream and (potentially) the downstream flow evolution, which are highly relevant aspects for medium-range weather forecast. Finally, our results imply that a distinction between slantwise and convective WCB trajectories is meaningful because the convective WCB trajectories are characterized by distinct properties.
Diabatic Rossby waves in the Southern Hemisphere Boettcher, Maxi; Wernli, Heini
Quarterly journal of the Royal Meteorological Society,
October 2015 Part B, Letnik:
141, Številka:
693
Journal Article
Recenzirano
Diabatic Rossby waves (DRWs) are a special type of low‐level cyclone with a self‐maintaining mechanism given an environment of moderate or strong baroclinicity and abundant moisture. Under certain ...conditions, these shallow cyclones associated with a cyclonic low‐level PV anomaly can serve as precursors for explosive cyclone intensification. The present study is the first to investigate DRWs in the Southern Hemisphere (SH). Two typical DRW case‐studies are presented and a sophisticated algorithm is used to detect DRWs in operational analysis data from 2001 to 2012. DRWs occur in all ocean basins and seasons with a maximum in summer. On average, about four DRWs are found per month and 12% intensify explosively. Thereby, they contribute 6% to the total set of explosively deepening extratropical cyclones in the SH. DRW tracks originate slightly closer to the Equator than normal extratropical cyclones, and they follow the convergence zones eastward and poleward. This results in a climatological DRW track density similar to a spiral, from the western South Pacific ending in the eastern South Indian Ocean, broken only by the Andes. Typical synoptic situations associated with DRW genesis are identified. The categories include (i) low‐level PV generation by upper‐level induced lifting, (ii) low‐level jet‐induced diabatic PV generation, and (iii) transformation of the low‐tropospheric PV anomaly from another system.
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