Temperature Advection Research Articles

Observed Sensitivity of Low-Cloud Radiative Effects to Meteorological Perturbations over the Global Oceans

AbstractUnderstanding how marine low clouds and their radiative effects respond to changing meteorological conditions is crucial to constrain low-cloud feedbacks to greenhouse warming and internal climate variability. In this study, we use observations to quantify the low-cloud radiative response to meteorological perturbations over the global oceans to shed light on physical processes governing low-cloud and planetary radiation budget variability in different climate regimes. We assess the independent effect of perturbations in sea surface temperature, estimated inversion strength, horizontal surface temperature advection, 700-hPa relative humidity, 700-hPa vertical velocity, and near-surface wind speed. Stronger inversions and stronger cold advection greatly enhance low-level cloudiness and planetary albedo in eastern ocean stratocumulus and midlatitude regimes. Warming of the sea surface drives pronounced reductions of eastern ocean stratocumulus cloud amount and optical depth, and hence reflectivity, but has a weaker and more variable impact on low clouds in the tropics and middle latitudes. By reducing entrainment drying, higher free-tropospheric relative humidity enhances low-level cloudiness. At low latitudes, where cold advection destabilizes the boundary layer, stronger winds enhance low-level cloudiness; by contrast, wind speed variations have weak influence at midlatitudes where warm advection frequently stabilizes the marine boundary layer, thus inhibiting vertical mixing. These observational constraints provide a framework for understanding and evaluating marine low-cloud feedbacks and their simulation by models.

Surface and Atmospheric Driven Variability of the Single‐Layer Urban Canopy Model Under Clear‐Sky Conditions Over London

AbstractUrban canopy models (UCMs) are parametrization schemes that are used to improve weather forecasts in urban areas. The performance of UCMs depends on understanding potential uncertainty sources that can generally originate from the (a) urban surface parameters, (b) atmospheric forcing, and (c) physical description. Here, we investigate the relative importance of surface and atmospheric driven model sensitivities of the single‐layer urban canopy model when fully interactive with a 1‐D configuration of the Weather Research and Forecasting model (WRF). The impact of different physical descriptions in UCMs and other key parameterization schemes of WRF is considered. As a case study, we use a 54‐hr period with clear‐sky conditions over London. Our analysis is focused on the surface radiation and energy flux partitioning and the intensity of turbulent mixing. The impact of changes in atmospheric forcing and surface parameter values on model performance appears to be comparable in magnitude. The advection of potential temperature, aerosol optical depth, exchange coefficient and roughness length for heat, surface albedo, and the anthropogenic heat flux are the most influential. Some atmospheric forcing variations have similar impact on the key physical processes as changes in surface parameters. Hence, error compensation may occur if one optimizes model performance using a single variable or combinations that have potential for carryover effects (e.g., temperature). Process diagrams help differences to be understood in the physical description of different UCMs, boundary layer, and radiation schemes and between the model and the observations.

A More General Paradigm for Understanding the Decoupling of Stratocumulus‐Topped Boundary Layers: The Importance of Horizontal Temperature Advection

AbstractMost prior studies on decoupling of a stratocumulus‐topped boundary layer (STBL) are focused on subtropics where cold air advection with moderate strength is dominant. This study expands across a wider spectrum of temperature advection spanning from moderately strong warm air advection to extremely strong cold air advection. A STBL undergoing warm advection is found to be more mixed than a STBL undergoing cold advection. This finding is consistent with the cold advection facilitating turbulent mixing in the boundary layer. When cold advection becomes sufficiently strong (<−5 K/day), the STBL becomes more stably stratified again because of emergence of the cumulus‐coupled STBL regime induced by the “deepening‐warming” mechanism. Such a “deepening‐warming” induced STBL decoupling, however, is still much weaker than that caused by warm advection flows (even weak ones), suggesting that the direction and strength of temperature advection must be considered for any STBL decoupling studies.

Atmospheric circulation regime causing winter temperature whiplash events in North China

AbstractTemperature whiplash events, which are characterized by a rapid transition between persistent and extreme warm and cold conditions, usually damage natural systems and human communities and can even have catastrophic impacts. To more deeply understand the severe winter temperature whiplash events in North China, in this study, the atmospheric circulation regime that is primarily responsible for such events is investigated based on Japanese 55‐year reanalysis data. The results show that the drastic temperature shifts during winter temperature whiplash events are closely correlated with the southeastward propagation of the dipolar anomalous temperature in the middle and lower troposphere over the Eurasian continent. The dipolar structure of the temperature signals features an initial cold anomaly over the West Siberian Plain and a warm anomaly over East Asia during negative (extremely warm to extremely cold) events and the opposite pattern during positive (extremely cold to extremely warm) events. This dipolar temperature anomaly is tightly coupled with the southeastward development of an upper‐level wave‐like anomalous circulation pattern over the Eurasian continent, which is associated with a Rossby wave originating near the Kara Sea during negative events and in northern Europe during positive events. Based on the calculation of the temperature budget, the drastic temperature drop during negative events is mainly dominated by anomalous meridional temperature advection, and adiabatic heating due to vertical motion is negligible. In contrast, adiabatic heating due to sinking plays a dominant role in the drastic temperature increase during positive events, while anomalous meridional temperature advection also makes a substantial contribution to the temperature increase.

Contrasting Relationship between Wintertime Blocking Highs over Europe–Siberia and Temperature Anomalies in the Yangtze River Basin

Abstract Based on the Japanese 55-year reanalysis dataset, this study identifies 92 Europe–Siberia blocking high events (ESBs) over the 60 winters (November–March) from 1958/59 to 2017/18. According to the influence on the surface air temperature at 2 m over the middle and lower reaches of the Yangtze River, the ESBs are classified into three types: cold, neutral, and warm. Although cold-type ESBs are dominant, the number of warm-type ESBs is not negligible. The present study mainly focuses on the differences between cold-type and warm-type ESBs. Both the cold-type ESBs and the warm-type ESBs are characterized by height anomalies with a northwest–southeast-tilting dipole pattern over the Eurasian continent in the mid- and upper troposphere. However, the tilting dipole pattern of the warm type is located to the northwest of its cold-type counterpart, which reflects differences in the propagation of Rossby wave packets. The Siberian high is stronger in cold-type ESBs than in warm-type ESBs. The induced advection of the climatological mean air temperature by the anomalous meridional wind velocity in the lower troposphere accounts for the largest portion of the observed tendency of the air temperature for both ESB types. In addition, diabatic heating tends to counteract the local cooling tendency of air temperature over the Yangtze River region for the cold-type ESBs. Finally, cold-type ESBs are generally characterized by air parcels originating in the region to the north and northeast of the Tibetan Plateau, while warm-type ESBs are characterized by diverse trajectories.

Indian Ocean tripole mode and its associated atmospheric and oceanic processes

Differing from the Indian Ocean dipole (IOD) that has sea surface temperature anomalies (SSTAs) of opposing signs over the tropical southeastern and western Indian Ocean, a tripole pattern, characterized by positive (negative) SSTAs over the tropical central (southeastern and western) Indian Ocean, is observed and named the Indian Ocean tripole (IOT). This study proposes the concept of the IOT and further investigates the associated atmospheric and oceanic processes. Using empirical orthogonal function (EOF) analysis, the IOT (IOD) is represented by the third (second) leading mode of the monthly SSTAs in the tropical Indian Ocean, explaining about 8.2% (10.3%) of the total variance. The IOT peaks in boreal summer, while the IOD has its mature phase in boreal fall. The spatio-temporal differences, together with the significant separation of these two EOF patterns, illustrate that the IOT is independent of the IOD. Heat budget diagnoses indicate that the anomalous temperature over the southeastern and western Indian Ocean is mainly generated by the net heat flux during April–May and by the total ocean advection in June–August. In contrast, the anomalous temperature is mainly driven by the advection of the mean temperature by the anomalous current in April and the residual term in May–June over the central Indian Ocean, which is replaced by advection of the anomalous temperature by the mean zonal current in July.

What Leads to Persisting Surface Air Temperature Anomalies from Winter to Following Spring over Mid- to High-Latitude Eurasia?

AbstractSurface air temperature (SAT) anomalies tend to persist from winter to the following spring over the mid- to high latitudes of Eurasia. The present study compares two distinct cases of Eurasian SAT anomaly evolution and investigates the reasons for the persistence of continental-scale mid- to high-latitude Eurasian SAT anomalies from winter to following spring (termed persistent cases). The persisting SAT anomalies are closely associated with the sustenance of large-scale atmospheric circulation anomaly pattern over the North Atlantic and Eurasia, featuring a combination of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and the Scandinavian pattern, from winter to spring. The combined circulation anomalies result in SAT warming over most of mid- to high-latitude Eurasia via anomalous wind-induced temperature advection. The sustenance of atmospheric circulation anomaly pattern is related to the maintenance of the North Atlantic triple sea surface temperature (SST) anomaly pattern due to air–sea interaction processes. The Barents Sea ice anomalies, which form in winter and increase in spring, also partly contribute to the sustenance of atmospheric circulation anomalies via modulating thermal state of the lower troposphere. In the cases that notable SAT warming (cooling) in winter is replaced by pronounced SAT cooling (warming) in the subsequent spring—termed reverse cases—the North Atlantic SST anomalies become small and the Greenland Sea ice change is a response to atmospheric change in spring. Without the support of lower boundary forcing, the atmospheric circulation anomaly pattern experiences a reverse in the spatial distribution from winter to spring likely due to internal atmospheric processes.

Analysis of a Convective Storm Crossing Poyang Lake in China

A convective storm crossing Poyang Lake (PL) in China during 1200-1600 UTC on 13 May 2015 is examined. The results show that this storm occurs ahead of a 500-hPa trough with weak low-level temperature advection and a convectively stable layer between 925 and 850 hPa, and the tail of the storm is enhanced when its spearhead sweeps over PL after the sunset. Due to the heating and moistening of PL, the convectively stable layer over PL is destabilized; and instead, a deep (below 700 hPa) convectively unstable layer is organized. Moreover, both the radiative cooling and the storm-induced cooling result in a rapid air (near-surface) and land temperature decrease in the surrounding areas. Thus, a large lake-land temperature difference (about 6°C) occurs, which is conducive to generating land-lake breeze and enhancing the convergence of the low-level wind. Finally, the PL-induced deep convectively unstable layer and the enhanced low-level convergence jointly strengthen the crossing storm. To further confirm this, two simulations (with or without PL) are conducted with the Weather Research and Forecast (WRF) model. The simulation with PL successfully reproduces the evolution of the storm crossing PL, while the simulation without PL fails. In the simulation with PL, a high θse tongue at 850 hPa associated with the storm moves eastward and downward, and merges with the PL-induced lake boundary layer, forming a deep convectively unstable layer under 700 hPa. However, in the simulation without PL, the stable layer constantly maintains under 900 hPa. In addition, the 900-hPa wind difference between the simulations with and without PL shows a land-lake breeze circulation that strengths the convergence of the low-level wind.

Predictability of the Strong Ural blocking Event in January 2012 in the Subseasonal to Seasonal Models of Europe and Canada

The occurrence of a Ural blocking (UB) event is an important precursor of severe cold air outbreaks in Siberia and East Asia, and thus is significant to accurately predict UB events. Using subseasonal to seasonal (S2S) models of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the Environment and Climate Change Canada (ECCC), we evaluated the predictability of a persistent UB event on 18 to 26 January 2012. Results showed that the ECCC model was superior to the ECMWF model in predicting the development stage of the UB event ten days in advance, while the ECMWF model had better predictions than the ECCC model for more than ten days in advance and the decaying stage of the UB event. By comparing the dynamic and thermodynamic evolution of the UB event predicted by the two models via the geostrophic vorticity tendency equation and temperature tendency equation, we found that the ECCC model better predicted the vertical vorticity advection, ageostrophic vorticity tendency, the tilting effect, horizontal temperature advection, and adiabatic heating during the development stage, whereas the ECMWF model better predicted the three dynamic and the two thermodynamic terms during the decaying stage. In addition, during both the development and decaying stages, the two models were good (bad) at predicting the vortex stretching term (horizontal vorticity advection), with the PCC between both the predictions and the observations larger (smaller) than +0.70 (+0.10) Thus, we suggest that the prediction of the persistent UB event in the S2S model might be improved by the better prediction of the horizontal vorticity advection.

Evaluation of the Grell–Freitas Convective Scheme in the Hurricane Weather Research and Forecasting (HWRF) Model

Abstract The Developmental Testbed Center (DTC) tested two convective parameterization schemes in the Hurricane Weather Research and Forecasting (HWRF) Model and compared them in terms of performance of forecasting tropical cyclones (TCs). Several TC forecasts were conducted with the scale-aware Simplified Arakawa Schubert (SAS) and Grell–Freitas (GF) convective schemes over the Atlantic basin. For this sample of over 100 cases, the storm track and intensity forecasts were superior for the GF scheme compared to SAS. A case study showed improved storm structure for GF when compared with radar observations. The GF run had increased inflow in the boundary layer, which resulted in higher angular momentum. An angular momentum budget analysis shows that the difference in the contribution of the eddy transport to the total angular momentum tendency is small between the two forecasts. The main difference is in the mean transport term, especially in the boundary layer. The temperature tendencies indicate higher contribution from the microphysics and cumulus heating above the boundary layer in the GF run. A temperature budget analysis indicated that both the temperature advection and diabatic heating were the dominant terms and they were larger near the storm center in the GF run than in the SAS run. The above results support the superior performance of the GF scheme for TC intensity forecast.

Causes of the upper-level warm temperature anomaly (UWTA) associated with March–May heavy rainfall in Tanzania

This study diagnoses the cause of the upper-level warm temperature anomaly (UWTA) associated with March–May (MAM) heavy rainfall of 1980–2010 in Tanzania in terms of the empirical orthogonal function (EOF), singular value decomposition (SVD) and dynamical diagnosis. EOF reveals the dominance of enhanced UWTA over the entire study region with strong warming to the northern coast, central, south-western highlands (SWH) and western part of the country. SVD depicts a monopole positive co-variability between the UWTA of the entire study area and SSTA over the entire domain of the Indian Ocean with strong positive coefficients to the central equatorial and western Indian Ocean (WIO). The lead-lag correlation coefficients between the three months running mean for the normalized WIO SSTA and UWTA indices show that, the UWTA formation starts early before MAM with the highest peak in February–April (FMA). The ascending branch of the Walker circulation is the dominant primer for the vertical uplift of warm moist air from the WIO during MAM while convective coupled Kelvin waves (CCKWs) dominate in FMA season. In both seasons, strong positive zonal temperature advection anomalies over the WIO spread towards the study area thereby dominating diabatic cooling and further increase the upper-level tropospheric temperature. Thus, diabatic heating due to enhanced uplift of warm and moist air from enhanced warm SSTA of the WIO warms the upper-level troposphere (i.e., also the UWTA) of the study area through the zonal temperature advection.

Mechanisms of an extreme fog and haze event in the megacities of central and eastern China

In this study, the authors conduct a detailed analysis of a prolonged fog and haze (FAH) event in the megacities of central and eastern China in January 2013. The persistence and intensification mechanisms of high impact FAH weather are analyzed using surface and sounding data from national meteorological observation stations and national environmental pollutant observations. The large-scale circulation was characterized by the weak and stable polar vortex in Eurasia, a zonally elongated circulation spanning from the midlatitudes to high latitudes at 500 hPa, and the West Pacific Subtropical High extending further westward. At the surface, the FAH regions were under control by a weak easterly and southeasterly at the back or bottom of the surface high-pressure system, which was favorable for the accumulation of high aerosol pollutant concentrations. The formation and intensification of the FAH have an apparent correlation with the variation in meteorological variables and aerosols. The strengthening of the extreme fog was due to a combination of advection and radiation effects, among which alternative temperature advection played the key role in the maintenance and deterioration of FAH. The boundary layer height indicated the strengthening and weakening of the FAH. The persistence of weak vertical wind shear in the middle-lower layer and weak vertical motion were favorable dynamic factors for the FAH.

How could a difference of 0.5°C in global warming modify the mean and extreme climate conditions around Antarctica?

AbstractThis paper quantifies the potential benefit of limiting the global warming to 1.5°C above the preindustrial level on the high‐latitude climate of the Southern Hemisphere compared to 2.0°C and above. A set of Coupled Model Intercomparison Project Phase 5 global climate model experiments is used in order to determine the mean projected changes in surface air temperature, sea level pressure, surface winds and sea ice distribution over and around Antarctica. Alongside, changes in selected variables related to extremes (such as the annual number of extremely cold mornings and extremely cold days) are also quantified as a function of the Representative Concentration Pathway (RCP) considered reaching the given warming threshold. Physical mechanisms for the projected variations in sea ice are proposed, including surface air temperature, changes in low‐level air temperature advection and meridional winds. Results show that there would be significant differences in the atmospheric and sea ice variable changes depending on the warming threshold and RCP considered, and that indeed keeping the global warming under 1.5°C could have a noticeable impact particularly on the distribution of sea ice over the Southern Ocean.

Seasonal Asymmetries in the Lag between Insolation and Surface Temperature

AbstractWe analyze the temporal structure of the climatological seasonal cycle in surface air temperature across the globe. We find that, over large regions of Earth, the seasonal cycle of surface temperature departs from an annual harmonic: the duration of fall and spring differ by as much as 2 months. We characterize this asymmetry by the metric ASYM, defined as the phase lag of the seasonal maximum temperature relative to the summer solstice minus the phase lag of the seasonal minimum temperature relative to winter solstice. We present a global analysis of ASYM from weather station data and atmospheric reanalysis and find that ASYM is well represented in the reanalysis. ASYM generally features positive values over land and negative values over the ocean, indicating that spring has a longer duration over the land domain whereas fall has a longer duration over the ocean. However, ASYM also features more positive values over North America compared to Europe and negative values in the polar regions over ice sheets and sea ice. Understanding the root cause of the climatological ASYM will potentially further our understanding of controls on the seasonal cycle of temperature and its future/past changes. We explore several candidate mechanisms to explain the spatial structure of ASYM including 1) modification of the seasonal cycle of surface solar radiation by the seasonal evolution of cloud thickness, 2) differences in the seasonal cycle of the atmospheric boundary layer depth over ocean and over land, and 3) temperature advection by the seasonally evolving atmospheric circulation.

The influence of the Madden-Julian oscillation on high-latitude surface air temperature during boreal winter

By analyzing NCEP-NCAR reanalysis daily data for 1979–2016, the modulation by Madden-Julian Oscillation (MJO) of the wintertime surface air temperature (SAT) over high latitude is examined. The real-time multivariate MJO (RMM) index, which divides the MJO into eight phases, is used. It is found that a significantly negative SAT anomaly over the northern high latitude region of (180°–60 °W, 60°–90 °N) lags the MJO convection for 1∼2 weeks in phase 3, in which the enhanced convective activity exists over the Indian Ocean. While a significantly positive SAT anomaly appears over the same region following the MJO phase 7, as the tropical heating shows an opposite sign. Analysis of the anomalous circulation indicates that the observed SAT signal is probably a result of the northeastward propagating Rossby wave train triggered by MJO-related tropical forcing through Rossby wave energy dispersion. By using an anomalous atmospheric general circulation model (AGCM), the significant effect of tropical forcing on organizing the extratropical circulation anomaly is confirmed. Analysis of a temperature tendency equation further reveals that the intraseasonal SAT anomaly is primarily attributed to the advection of the mean temperature by the wind anomaly associated with the anomalous circulation of the MJO-related variability.

Role of Local Atmospheric Forcing and Land–Atmosphere Interaction in Recent Land Surface Warming in the Midlatitudes over East Asia

AbstractSignificant summer land surface warming has been observed in the middle latitudes over East Asia, especially after the mid-1990s, which has evidently affected the East Asian weather and climate. Using multisource observations and reanalysis data during 1979–2013, this study explores the possible reasons for recent land surface warming over this region by considering atmospheric forcing and regional land–atmosphere interaction related to extratropical cyclones (ECs). Results show that there is a close relationship between land surface warming and weakened ECs over East Asia. Recent land surface warming was attributed to local atmospheric forcing and feedback of land–atmosphere interaction associated with weakened ECs. The abnormal large-scale circulation associated with anomalous ECs produced evident dynamic forcing on the land surface. Weakened ECs are usually accompanied by an abnormal high pressure system and anticyclonic circulation around Lake Baikal, which benefit the intensification of anomalous southerly wind in the rear of the anomalous anticyclone, leading to positive temperature advection and temperature increase over East Asia. Meanwhile, the anomalous adiabatic warming caused by abnormal descending motion associated with the anticyclonic anomaly also contributes to local warming. The feedback of local land–atmosphere interaction plays an important role in land surface warming. Weakened ECs increase both incident solar radiation and precipitation. The increased precipitation reduces the soil moisture and in turn weakens the surface evaporation and local cooling effect, resulting in land surface warming. Our findings are helpful for better understanding the mechanisms responsible for recent summer land surface warming over East Asia as well as its climatic effects.

A Framework for Understanding How Dynamics Shape Temperature Distributions

AbstractUnderstanding what physically sets the shape of temperature distributions will enable more robust predictions of local temperature with global warming. We derive the relationship between the temperature distribution shape and the advection of temperature conditionally averaged at each temperature percentile. This enables quantification of the shift of each percentile that is due to changes in the mean temperature, in horizontal temperature advection, and other processes (e.g., radiation and convection). We use this relationship to examine global model simulations in an idealized aquaplanet model with increasing carbon dioxide. Changes in the distribution with doubling and quadrupling of carbon dioxide are significant, and they are caused by different processes. We find that midlatitude temperature distributions can be explained mostly by the horizontal advection, except in the upper and lower 10% of the distribution.

Amplified warming of seasonal cold extremes relative to the mean in the Northern Hemisphere extratropics

Abstract. Cold extremes are anticipated to warm at a faster rate than both hot extremes and average temperatures for much of the Northern Hemisphere. Anomalously warm cold extremes can affect numerous sectors, including human health, tourism and various ecosystems that are sensitive to cold temperatures. Using a selection of global climate models, this paper explores the accelerated warming of seasonal cold extremes relative to seasonal mean temperatures in the Northern Hemisphere extratropics. The potential driving physical mechanisms are investigated by assessing conditions on or prior to the day when the cold extreme occurs to understand how the different environmental fields are related. During winter, North America, Europe and much of Eurasia show amplified warming of cold extremes projected for the late 21st century, compared to the mid-20th century. This is shown to be largely driven by reductions in cold air temperature advection, suggested as a likely consequence of Arctic amplification. In spring and autumn, cold extremes are expected to warm faster than average temperatures for most of the Northern Hemisphere mid-latitudes to high latitudes, particularly Alaska, northern Canada and northern Eurasia. In the shoulder seasons, projected decreases in snow cover and associated reductions in surface albedo are suggested as the largest contributor affecting the accelerated rates of warming in cold extremes. The key findings of this study improve our understanding of the environmental conditions that contribute to the accelerated warming of cold extremes relative to mean temperatures.

Improved ENSO Prediction Skill Resulting From Reduced Climate Drift in IAP‐DecPreS: A Comparison of Full‐Field and Anomaly Initializations

AbstractWhen initiated from observational conditions, coupled climate models used in seasonal predictions generally experience climate drifts. How climate drift affects El Niño–Southern Oscillation (ENSO) prediction is important but not clearly understood. Here, we investigate this issue by comparing seasonal hindcasts using two distinct initialization approaches, namely, anomaly and full‐field initializations, based on a climate prediction system named IAP‐DecPreS. The differences between the two approaches are mainly evident in the drift behavior. We find that the hindcasts based on anomaly initialization (Hindcast‐A) have higher ENSO prediction skill compared to those based on full‐field initialization (Hindcast‐F). The climate drifts are largely reduced in the Hindcast‐A as expected. In contrast, the Hindcast‐F features a growing warming of the equatorial central eastern Pacific with increasing lead times. To investigate the impact of drift on the prediction, the 1997/1998 and 2015/2016 El Niño cases are analyzed. At a 7‐month lead, the Hindcast‐A reasonably predicts the two events, while the Hindcast‐F shows large errors in both evolution and amplitude. Budget analyses show that the underestimation of warming tendency in the Hindcast‐F is caused by cooling effects of excessive anomalous surface shortwave radiative flux and anomalous temperature advection by mean horizontal currents, both of which are associated with the climate drift. Our results imply that the use of the AI scheme can improve ENSO predictions through a reduction in climate drift in IAP‐DecPreS. The drifts can dynamically influence ENSO predictions, and their impact cannot be thoroughly removed via the empirical bias correction. Thus, reducing drift impacts is necessary.

Foehn-cold pool interactions in the Inn Valley during PIANO IOP2.

A case‐study is presented of a south foehn emanating from the Wipp Valley, Austria, which encountered a cold‐air pool (CAP) in the Inn Valley near the city of Innsbruck. The analysis is based on data collected during the second Intensive Observation Period of the Penetration and Interruption of Alpine Foehn (PIANO) field experiment. Foehn was initiated on 3 November 2017 by an eastward moving trough and terminated in the afternoon of 5 November 2017 by a cold front passage. On two occasions, reversed foehn flow deflected at the mountain ridge north of Innsbruck penetrated to the bottom of the Inn Valley. The first breakthrough occurred in the afternoon of 4 November 2017. It was transient and locally limited to the northwest of the city. The second (final) breakthrough occurred in the morning of 5 November 2017 and was recorded by all surface stations in the vicinity of Innsbruck. It started with a foehn air intrusion to the northeast of Innsbruck and continued with the westward propagation of the foehn–CAP boundary along the valley. Subsequently observed northerly winds above the city were caused by an atmospheric rotor. A few hours later and prior to the cold front passage, the CAP pushed back and lifted the foehn air from the ground. During both nights, shear flow instabilities formed at the foehn–CAP interface, which resulted in turbulent heating of the CAP and cooling of the foehn. However, this turbulent heating/cooling was partly compensated by other mechanisms. Especially in the presence of strong spatial CAP heterogeneity during the second night, heating in the CAP was most likely overcompensated by negative horizontal temperature advection.