Abstract

Long-lived coherent vortices located near the tropopause are often found over polar regions. Although these vortices are a commonly observed feature of the Arctic, and can have lifetimes longer than one month, little is known about the mechanisms that control their evolution. This paper examines mechanisms of intensity change for a cyclonic tropopause polar vortex (TPV) using an Ertel potential vorticity (EPV) diagnostic framework. Results from a climatology of intensifying cyclonic TPVs suggest that the essential dynamics are local to the vortex, rather than a consequence of larger-scale processes. This fact motivates a case study using a numerical model to investigate the role of diabatic mechanisms in the growth and decay of a particular cyclonic vortex. A component-wise breakdown of EPV reveals that cloud-top radiational cooling is the primary diabatic mechanism that intensifies the TPV during the growth phase. Increasing amounts of moisture become entrained into the vortex core at later times near Hudson Bay, allowing the destruction of potential vorticity near the tropopause due to latent heat release to become comparable to the radiational tendency to create potential vorticity.

Abstract

Characterized by radii as large as 800 km and lifetimes up to months, cyclonic tropopause polar vortices (TPVs) are coherent circulation features over the Arctic that are important precursors for surface cyclogenesis in high and middle latitudes. TPVs have been shown to be maintained by radiative processes over the Arctic owing to limited amounts of latent heating. This study explores the hypothesis that a downward extension of dry stratospheric air associated with TPVs results in an increase in longwave radiative cooling that intensifies the vortex.

Idealized numerical modeling experiments are performed to isolate physical interactions, beginning with radiative forcing in a dry atmosphere and culminating with multiple physical interactions between radiation and clouds that more accurately represent the observed environment of TPVs. Results show that longwave radiative cooling associated with a rapid decrease in water vapor concentration near the tropopause is primarily responsible for observed TPV intensification. These enhanced water vapor gradients result from a lower tropopause within the vortex that places dry stratospheric air above relatively moist tropospheric air. Cloud-top radiative cooling enhances this effect and also promotes the maintenance of clouds by destabilizing the region near cloud top. Shortwave radiation and latent heating offset the longwave intensification mechanism. Heating from shortwave radiation reduces the cloud water mixing ratio by preferentially warming levels above cloud tops.

Abstract

Tropopause polar vortices (TPVs) are commonly observed, coherent circulation features of the Arctic with typical radii as large as approximately 800 km. Intensification of cyclonic TPVs has been shown to be dominated by infrared radiation. Here the hypothesis is tested that while radiation alone may not be essential for TPV genesis, radiation has a substantial impact on the long-term population characteristics of cyclonic TPVs.

A numerical model is used to derive two 10-yr climatologies of TPVs for both winter and summer: a control climatology with radiative forcing and an experimental climatology with radiative forcing withheld. Results from the control climatology are first compared to those from the NCEP–NCAR reanalysis project (NNRP), which indicates sensitivity to both horizontal grid resolution and the use of polar filtering in the NNRP. Smaller horizontal grid resolution of 60 km in the current study yields sample-mean cyclonic TPV radii that are smaller by a factor of ~2 compared to NNRP, and vortex track densities in the vicinity of the North Pole are considerably larger compared to NNRP. The experimental climatologies show that winter (summer) vortex maximum amplitude is reduced by 22.3% (38.0%), with a net tendency to weaken without radiation. Moreover, while the number and lifetime of cyclonic TPVs change little in winter without radiation, the number decreases 12% and the mean lifetime decreases 19% during summer without radiation. These results suggest that dynamical processes are primarily responsible for the genesis of the vortices, and that radiation controls their maximum intensity and duration during summer, when the destructive effect of ambient shear is weaker.

Abstract

Arctic cyclones (ACs) are a primary driver of surface weather in the Arctic, contributing to heat and moisture transport and forcing short-term sea ice variability. Still, our understanding of the processes that drive ACs, particularly their large scales and long lifetimes, is limited. ACs are commonly associated with one or more cyclonic tropopause polar vortices (TPVs), potential vorticity (PV) anomalies in the upper troposphere and lower stratosphere that may spur baroclinic development in the surface system, though the exact processes that link the two have yet to be fully explored. In this study, we investigate physical links between TPVs, especially their mesoscale structure and moisture profiles, and ACs with idealized observing system simulation experiments (OSSEs). Starting with a nature run, we simulate different types of dropsonde observations over a TPV during the nascent phase of a nearby AC. The Model for Prediction Across Scales (MPAS) and the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter are then used to run experiments to test the impact of these detailed TPV observations. In addition to a control, five main experiments are conducted, assimilating new observations of temperature and humidity. All experiments reduce forecast errors at the surface and throughout the troposphere. Additional humidity observations alter vertical PV distributions, which in turn impact the development of the AC. Experiments with additional temperature observations exhibit improvements in TPV structure and surrounding PV features and produce stronger surface cyclones with skillful TPV forecasts for up to 36 h longer than the control.

Abstract

Tropopause polar vortices are coherent circulation features based on the tropopause in polar regions. They are a common feature of the Arctic, with typical radii less than 1500 km and lifetimes that may exceed 1 month. The Arctic is a particularly favorable region for these features due to isolation from the horizontal wind shear associated with the midlatitude jet stream, which may destroy the vortical circulation. Intensification of cyclonic tropopause polar vortices is examined here using an Ertel potential vorticity framework to test the hypothesis that there is an average tendency for diabatic effects to intensify the vortices due to enhanced upper-tropospheric radiative cooling within the vortices. Data for the analysis are derived from numerical simulations of a large sample of observed cyclonic vortices over the Canadian Arctic. Results show that there is on average a net tendency to create potential vorticity in the vortex, and hence intensify cyclones, and that the tendency is radiatively driven. While the effects of latent heating are considerable, they are smaller in magnitude, and all other diabatic processes have a negligible effect on vortex intensity.

Abstract

An upper-level cold bias in potential temperature tendencies of 10 K day⁻¹, strongest at the top of the model, is observed in Weather Research and Forecasting (WRF) model forecasts. The bias originates from the Rapid Radiative Transfer Model longwave radiation physics scheme and can be reduced substantially by 1) modifying the treatment within the scheme by adding a multilayer buffer between the model top and top of the atmosphere and 2) constraining stratospheric water vapor to remain within the estimated climatology in the stratosphere. These changes reduce the longwave heating rate bias at the model top to ±0.5 K day⁻¹. Corresponding bias reductions are also seen, particularly near the tropopause.

Abstract

Accurate predictions in numerical weather models depend on the ability to accurately represent physical processes across a wide range of scales. This paper evaluates the utility of model time tendencies, averaged over many forecasts at a given lead time, to diagnose systematic forecast biases in the Advanced Research version of the Weather Research and Forecasting (WRF) Model during the 2010 North Atlantic hurricane season using continuously cycled ensemble data assimilation (DA). Erroneously strong low-level heating originates from the planetary boundary layer parameterization as a consequence of using fixed sea surface temperatures, impacting the upward surface sensible heat fluxes. Warm temperature bias is observed with a magnitude 0.5 K in a deep tropospheric layer centered 700 hPa, originating primarily from the Kain–Fritsch convective parameterization.

This study is the first to diagnose systematic forecast bias in a limited-area mesoscale model using its forecast tendencies. Unlike global models where relatively fewer time steps typically encompass a DA cycling period, averaging all short-term forecast tendencies can require potentially large data. It is shown that 30-min averaging intervals can sufficiently represent the systematic model bias in this modeling configuration when initializing forecasts from an ensemble member that is generated using a DA system with an identical model configuration. However, the number of time steps before model error begins to dominate initial condition (IC) errors may vary between modeling configurations. Model and IC error are indistinguishable in short-term forecasts when initialized from the ensemble mean, a global analysis from a different model, and an ensemble member using a different parameterization.

Abstract

Tropopause polar vortices (TPVs) are long-lived, coherent vortices that are based on the dynamic tropopause and characterized by potential vorticity anomalies. TPVs exist primarily in the Arctic, with potential impacts ranging from surface cyclone generation and Rossby wave interactions to dynamic changes in sea ice. Previous analyses have focused on model output indicating the importance of clear-sky and cloud-top radiative cooling in the maintenance and evolution of TPVs, but no studies have focused on local observations to confirm or deny these results. This study uses cloud and atmospheric state observations from Summit Station, Greenland, combined with single-column experiments using the Rapid Radiative Transfer Model to investigate the effects of clear-sky, ice-only, and all-sky radiative cooling on TPV intensification. The ground-based observing system combined with temperature and humidity profiles from the European Centre for Medium-Range Weather Forecasts’s fifth major global reanalysis dataset, which assimilates the twice-daily soundings launched at Summit, provides novel details of local characteristics of TPVs. Longwave radiative contributions to TPV diabatic intensity changes are analyzed with these resources, starting with a case study focusing on observed cloud properties and associated radiative effects, followed by a composite study used to evaluate observed results alongside previously simulated results. Stronger versus weaker vertical gradients in anomalous clear-sky radiative heating rates, contributing to Ertel potential vorticity changes, are associated with strengthening versus weakening TPVs. Results show that clouds are sometimes influential in the intensification of a TPV, and composite results share many similarities to modeling studies in terms of atmospheric state and radiative structure.

Abstract

Tropopause polar vortices (TPVs) are coherent circulations that occur over polar regions and can be identified by a local minimum in potential temperature and local maximum in potential vorticity. Numerous studies have focused on TPVs in the Arctic region; however, no previous studies have focused on the Antarctic. Given the role of TPVs in the Northern Hemisphere with surface cyclones and other extreme weather, and the role that surface cyclones can play on moisture transport and sea ice breakup, it is important to understand whether similar associations exist in the Southern Hemisphere. Here, characteristics of TPVs in the Antarctic are evaluated for the first time under the hypothesis that their characteristics do not significantly differ from those of the Northern Hemisphere. To improve understanding of Antarctic TPV characteristics, this study examines TPVs of the Southern Hemisphere and compares them to their Northern Hemisphere counterparts from 1979 to 2018 using ERA-Interim data. Common characteristics of TPVs including frequency, locations, lifetimes, strength, and seasonality are evaluated. Results indicate that topography correlates to the geographic distribution of TPVs and the locations of local maxima TPV occurrence, as observed in the Northern Hemisphere. Additionally, TPVs in the Southern Hemisphere exhibit seasonal variations for amplitude, lifetime, and minimum potential temperature. Southern Hemisphere TPVs share many similar characteristics to those observed in the Northern Hemisphere, including longer summer lifetimes. The association of Southern Hemisphere TPVs and surface cyclone frequency is explored, and it appears that TPVs have a precursory role to surface cyclones, as seen in the Northern Hemisphere.

Abstract

Due in part to sparse conventional observation coverage in the Antarctic region, atmospheric studies in this part of the globe often rely more heavily on numerical models. Model representation of atmospheric processes in the Antarctic remains inferior to representation in the Northern Hemisphere midlatitudes. Poor representation may be related to inaccurate model analyses that do not optimally utilize the limited observation network. Here, the ensemble Kalman filter (EnKF) data assimilation (DA) technique is employed in lieu of variational DA techniques to investigate impacts on model analysis accuracy. This DA technique [provided by the Data Assimilation Research Testbed (DART)] is coupled with a polar-modified, mesoscale numerical model that together compose Antarctic-DART (A-DART). A-DART is cycled with DA and run over a 1-month period, assimilating only conventional observations. Results show relatively good agreement between A-DART and observations. Comparison with radiosonde temperature and geostationary satellite wind observations shows large differences between RMSE and ensemble spread in the upper troposphere. The analysis increment shows large values in the eastern Atlantic–western Indian Oceans associated with geostationary satellite wind observations. Further evaluation determines that geostationary satellite wind observations may be biased in this region. Overall, this baseline demonstration of ensemble-based modeling applied in the Antarctic produced short-term forecasts that were competitive with two operational modeling systems while assimilating on the O(10⁶) fewer observations. A-DART is capable of assimilating additional observations for a variety of applications. This study highlights the capability of applying this ensemble-based DA technique for process and forecast studies in an observation-sparse region.