Kjersti Konstali

When decomposing the atmospheric flow into a basic state and perturbations, the perturbations are generally interpreted as the contribution from chaotic non-linear weather. We explore the link between day-to-day weather and the climatological zonal mean perspective on zonal momentum in more detail by systematically linking eddy momentum fluxes to weather events. Specifically, we first decompose the full momentum flux divergence into contributions from mean flow and perturbations both in the time and zonal direction as well as their combinations, and then systematically relate synoptic jets, cyclones, and Rossby wave breaking events to the instantaneous momentum fluxes. We thus construct a step-by-step link between the time-zonal mean perspective on momentum flux convergence and the synoptic perspective.With this approach, we show that both the time and zonal averaging are a residual of a large compensation of momentum flux convergence and divergence. In both dimensions, the mean must be regarded as a residual that is at least an order of magnitude smaller than the original signal. Further, a large fraction of eddy momentum flux convergence and divergence occurs in association with weather features, with synoptic jets alone accounting for 60-80% of the convergence from the subtropics throughout the mid-latitudes. Rossby wave breaking, on the other hand, only features less than 30% of the momentum flux convergence in the midlatitudes.Finally, the attribution of the full-field momentum flux convergence is nearly indistinguishable from the attribution of eddy-momentum flux convergence, irrespective of whether the eddies are defined as perturbations in time, zonal direction, or the combination of both. The effect of stationary waves to the momentum fluxes is thus implicitly included in the selected transient weather events.

Due to their strong temperature gradients, fronts are a focal point of intense precipitation and gustiness related to extratropical cyclones. In addition, sustained condensational heating along trailing cold fronts has been shown to raise background baroclinicity, which can trigger secondary cyclogenesis and, consequentially, cyclone clustering.To study the driving mechanism, synoptic-scale characteristics, and impacts of fronts throughout their lifecycles, we have developed a front tracking algorithm. A frontal lifecycle is therein defined as a 4-dimensional space-time volume of strong gradients in equivalent potential temperature that is coherent in both time and space.Based on the climatology of frontal lifecycles, we identify distinct frontogenesis regions in the mid-latitudes. Frontogenesis typically occurs in the lee of meridionally oriented mountain ranges, such as the Rocky Mountains or the Andes, or along western boundary currents, such as the Gulf Stream or the Kuroshio. Fronts forming in these regions travel eastward along the storm tracks over a lifetime of one to two weeks. Such lifecycle characteristics distinguish mid-latitude fronts from stationary or short-lived airmass boundaries in lower latitudes, which are typically classified as fronts by conventional algorithms.We furthermore associate characteristic dynamic drivers of frontogenesis in the identified frontogenesis regions to lifecycle properties such as duration, strength, and the occurrence of one or multiple secondary cyclogenesis. Thus, investigating how fronts link large-scale atmospheric conditions with downstream storm activity in the Storm Track regions.

Weather features, such as extratropical cyclones (ETCs), atmospheric rivers (ARs), and fronts, contribute to substantial amounts of precipitation globally. We introduce a robust attribution method applicable at all latitudes present the first global climatology of the contributions from extratropical cyclones (ETCs), fronts, moisture transport axes (MTAs; AR-like features), and cold air outbreaks, as well as their combinations, to summer and winter precipitation as well as extreme precipitation using ERA5 and 10 ensemble members of the CESM2‐ LE. For the present climate, most of the precipitation in the midlatitudes relates to the combination of ETC, fronts, and MTAs (28%), while in polar regions most precipitation occurs within the ETC-only category (27%). Extreme precipitation events in all extratropical regions are predominantly associated with the combination of ETCs, fronts, and MTAs (46%). In the midlatitudes, the combination of ETCs, fronts, and MTAs occurs almost 4 times as often during extreme events compared to regular events.For the period 1960-2100 under the SSP3‐7.0 scenario, we find that CESM2‐LE adeptly represents the precipitation characteristics associated with the different combinations of weather features. The combinations of weather features that contribute most to precipitation in the present climate also contribute the most to future changes, both due to changes in intensity as well as frequency. While the increase in precipitation intensity dominates the overall response for total precipitation in the storm track regions, the precipitation intensity for the individual weather features does not necessarily change significantly. Instead, approximately half of the increase in precipitation intensity in the storm track regions can be attributed to a higher occurrence of the more intensely precipitating combinations of weather features, such as the co‐occurrence of extratropical cyclones, fronts, and moisture transport axes.Given that most of the extreme precipitation in the extratropics is associated with cyclones, fronts, and moisture transport axes, we also analyse the changes in precipitation characteristics associated with these weather features, as well as their combinations. We find that extreme precipitation associated with fronts increases substantially in the extratropics. Extreme precipitation associated with non‐frontal conditions, on the other hand, does not increase and even decreases in some regions. Hence, atmospheric fronts are the main driver of future extreme precipitation changes in the extratropics.

Extreme precipitation is expected to increase at a faster rate with climate change than the global average precipitation. Given that most of the extreme precipitation in the extratropics is associated with cyclones, fronts, and moisture transport axes, we analyze the changes in precipitation characteristics associated with these weather features, as well as their combinations, in a climate model (CESM2‐LE) for the period 1950–2100. We find that extreme precipitation associated with fronts increases substantially in the extratropics. Extreme precipitation associated with non‐frontal conditions, on the other hand, does not increase and even decreases in some regions. Hence, atmospheric fronts are the main driver of future extreme precipitation changes in the extratropics.

The horizontal water vapour transport is mainly organised in narrow elongated filaments. These filaments are referred to with a variety of names depending on the context, e.g. tropical moisture export, atmospheric river, warm moist intrusion, warm conveyor belt, and feeder air stream. Despite the various names, these features share essential properties, such as their narrow elongated structure. Here, we propose an algorithm that detects these various lines of moisture transport in instantaneous maps of the vertically integrated water vapour transport. The detection algorithm extracts well-defined maxima in the water vapour transport and connects them to lines that we refer to as moisture transport axes. By only requiring a well-defined maximum in the vapour transport, we avoid imposing a threshold on the absolute magnitude of this transport or the total column water vapour. Consequently, the algorithm is able to trace moisture transport axes at all latitudes without requiring region-specific tuning or normalisation. We demonstrate that the algorithm can detect both atmospheric rivers and warm moist intrusions as well as tropical moisture exports, prominent monsoon air streams, and low-level jets with moisture transport. Atmospheric rivers sometimes consist of several distinct moisture transport axes, indicating the merging of several moisture filaments into one atmospheric river. We showcase the synoptic situations and precipitation patterns associated with the occurrence of the identified moisture transport axes in example regions in the low, mid, and high latitudes. As our detection algorithm performs seamlessly from the tropics across the mid-latitudes into the polar regions, our approach might turn out to be particularly useful to study moist interactions between the tropics and subtropics, mid-latitudes, and polar regions.

The water vapor transport in the extratropics is mainly organized in narrow elongated filaments. These filaments are referred to with a variety of names depending on the contexts, for example atmospheric river, warm moist intrusion, warm conveyor belt, and feeder air stream. Despite the various names, these features share essential properties, such as their narrow elongated structure. Here, we propose an algorithm that detects these various lines of moisture transport in instantaneous maps of the vertically integrated water vapor transport. The detection algorithm extracts well-defined maxima in the water vapor transport and connects them to lines that we refer to as moisture transport axes. By only requiring a well-defined maximum in the vapor transport, we avoid imposing a threshold in the absolute magnitude of this transport or the total column water vapor. Consequently, the algorithm is able to pick up moisture transport axes at all latitudes without requiring region-specific tuning or normalization. We demonstrate that the algorithm can detect both atmospheric rivers and warm moist intrusions, but also prominent monsoon air streams as well as low-level jets with moisture transport. Atmospheric rivers sometimes consist of several distinct moisture transport axes, indicating the merging of several moisture filaments into one atmospheric river. We showcase the synoptic situations and precipitation patterns associated with the occurrence of the identified moisture transport axes in example regions in the low, mid, and high latitudes.

Weather features, such as extratropical cyclones, atmospheric rivers (ARs), and fronts, contribute to substantial amounts of precipitation globally and are associated with different precipitation characteristics. However, future changes in these characteristics, as well as their representation in climate models, remain uncertain. We attribute 6‐hourly accumulated precipitation to cyclones, moisture transport axes (AR‐like features), fronts, and cold air outbreaks, and the combinations thereof in 10 ensemble members of the CESM2‐LE between 1960 and 2100 under the SSP3‐7.0 scenario. We find that, despite some biases in both precipitation and weather features, CESM2‐LE adeptly represents the precipitation characteristics associated with the different combinations of weather features. The combinations of weather features that contribute most to precipitation in the present climate also contribute the most to future changes, both due to changes in intensity as well as frequency. While the increase in precipitation intensity dominates the overall response for total precipitation in the storm track regions, the precipitation intensity for the individual weather features does not necessarily change significantly. Instead, approximately half of the increase in precipitation intensity in the storm track regions can be attributed to a higher occurrence of the more intensely precipitating combinations of weather features, such as the co‐occurrence of extratropical cyclones, fronts, and moisture transport axes.

The water vapor transport in the extratropics is mainly organized in narrow elongated filaments. These filaments are referred to with a variety of names depending on the contexts, for example atmospheric river, warm moist intrusion, warm conveyor belt, and feeder air stream. Despite the various names, these features share essential properties, such as their narrow elongated structure. Here, we propose an algorithm that detects these various lines of moisture transport in instantaneous maps of the vertically integrated water vapor transport. The detection algorithm extracts well-defined maxima in the water vapor transport and connects them to lines that we refer to as moisture transport axes. By only requiring a well-defined maximum in the vapor transport, we avoid imposing a threshold in the absolute magnitude of this transport or the total column water vapor. Consequently, the algorithm is able to pick up moisture transport axes at all latitudes without requiring region-specific tuning or normalization. We demonstrate that the algorithm can detect both atmospheric rivers and warm moist intrusions, but also prominent monsoon air streams as well as low-level jets with moisture transport. Atmospheric rivers sometimes consist of several distinct moisture transport axes, indicating the merging of several moisture filaments into one atmospheric river. We showcase the synoptic situations and precipitation patterns associated with the occurrence of the identified moisture transport axes in example regions in the low, mid, and high latitudes.

The water vapor transport in the extratropics is mainly organized in narrow elongated filaments. These filaments are referred to with a variety of names depending on the contexts, for example atmospheric river, warm moist intrusion, warm conveyor belt, and feeder air stream. Despite the various names, these features share essential properties, such as their narrow elongated structure. Here, we propose an algorithm that detects these various lines of moisture transport in instantaneous maps of the vertically integrated water vapor transport. The detection algorithm extracts well-defined maxima in the water vapor transport and connects them to lines that we refer to as moisture transport axes. By only requiring a well-defined maximum in the vapor transport, we avoid imposing a threshold in the absolute magnitude of this transport or the total column water vapor. Consequently, the algorithm is able to pick up moisture transport axes at all latitudes without requiring region-specific tuning or normalization. We demonstrate that the algorithm can detect both atmospheric rivers and warm moist intrusions, but also prominent monsoon air streams as well as low-level jets with moisture transport. Atmospheric rivers sometimes consist of several distinct moisture transport axes, indicating the merging of several moisture filaments into one atmospheric river. We showcase the synoptic situations and precipitation patterns associated with the occurrence of the identified moisture transport axes in example regions in the low, mid, and high latitudes.