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Article Is Available On- Dissipation Weather Clim. Dynam., 1, 405–426, 2020 https://doi.org/10.5194/wcd-1-405-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. The sensitivity of atmospheric blocking to upstream latent heating – numerical experiments Daniel Steinfeld1, Maxi Boettcher1, Richard Forbes2, and Stephan Pfahl3 1Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland 2European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom 3Institute of Meteorology, Freie Universität Berlin, Berlin, Germany Correspondence: Daniel Steinfeld ([email protected]) Received: 8 February 2020 – Discussion started: 17 February 2020 Revised: 15 June 2020 – Accepted: 28 July 2020 – Published: 24 August 2020 Abstract. Recent studies have pointed to an important role resent an important and challenging aspect of midlatitude of latent heating during cloud formation for the dynamics weather variability. Atmospheric blocking leads to persistent of anticyclonic circulation anomalies such as atmospheric changes in the large-scale circulation and blocks the west- blocking. However, the effect of latent heating on blocking erly flow (Rex, 1950; Woollings et al., 2018), often causing formation and maintenance has not yet been fully elucidated. anomalous, sometimes extreme, weather (Green, 1977) in a To explicitly study this cause-and-effect relationship, we per- situation of increased forecast uncertainty in weather models form sensitivity simulations of five selected blocking events (Pelly and Hoskins, 2003; Rodwell et al., 2013). with the Integrated Forecast System (IFS) global weather Despite its importance, there is currently no comprehen- prediction model in which we artificially eliminate latent sive theory of blocking (for a review see Tyrlis and Hoskins, heating in clouds upstream of the blocking anticyclones. This 2008). Several dynamical processes have been identified to elimination has substantial effects on the upper-tropospheric be conducive to blocking formation, such as planetary-scale circulation in all case studies, but there is also significant wave dynamics (e.g., Charney and DeVore, 1979; Hoskins case-to-case variability: some blocking systems do not de- and Valdes, 1989; Petoukhov et al., 2013), forcing by tran- velop at all without upstream latent heating, while for others sient eddies (e.g., Shutts, 1983; Luo et al., 2014), and Rossby the amplitude, size, and lifetime of the blocking anticyclones wave breaking (Pelly and Hoskins, 2003; Altenhoff et al., are merely reduced. This strong influence of latent heating 2008), with evidence that different processes can dominate on the midlatitude flow is due to the injection of air masses in different blocking cases (Nakamura et al., 1997; Drouard with low potential vorticity (PV) into the upper troposphere and Woollings, 2018; Steinfeld and Pfahl, 2019). Atmo- in strongly ascending “warm conveyor belt” airstreams and spheric blocking occurs when an air mass with anoma- the interaction of the associated divergent outflow with the lously low potential vorticity (PV) is advected poleward, upper-level PV structure. The important influence of diabatic related to a meridionally amplified flow (Nakamura and heating demonstrated with these experiments suggests that Huang, 2018), setting up a large-scale negative (anticyclonic) the accurate representation of moist processes in ascending PV anomaly in the upper troposphere at the level of the mid- airstreams in weather prediction and climate models is cru- latitude jet stream and a stable surface anticyclone under- cial for blocking dynamics. neath (Hoskins et al., 1985). Such large-scale advection of anticyclonic air masses into the blocking region typically oc- curs on the downstream side of developing baroclinic waves (e.g., Colucci, 1985; Mullen, 1987; Nakamura and Wallace, 1 Introduction 1993; Yamazaki and Itoh, 2012), which is the synoptic mech- anism behind the classical “eddy-mean flow” view, i.e., the The formation and maintenance of prolonged anticyclonic dynamical interaction between synoptic transient eddies and circulation anomalies, denoted as atmospheric blocking, rep- Published by Copernicus Publications on behalf of the European Geosciences Union. 406 D. Steinfeld et al.: The sensitivity of atmospheric blocking to upstream latent heating – numerical experiments the large-scale blocked flow (Berggren et al., 1949; Green, so, changes in the formation and maintenance of blocking in 1977; Shutts, 1983; Hoskins et al., 1983). these simulations can be attributed to altered LH upstream. While these studies focused on dry-adiabatic mechanisms The sensitivity experiments are presented as follows. Sec- and the isentropic advection of low-PV air, recent case tion2 describes the methodology, while Sect.3 introduces (Croci-Maspoli and Davies, 2009; Lenggenhager et al., 2019; one blocking event as an example with a synoptic overview. Maddison et al., 2019) and climatological (Pfahl et al., 2015; The results of the sensitivity experiments are presented in Steinfeld and Pfahl, 2019) studies based on air parcel tra- Sect.4, and our conclusions are summarized and discussed jectory calculations demonstrated that moist-diabatic pro- in Sect.5. cesses, and in particular latent heating (LH) during cloud formation in strongly ascending airstreams, play a signifi- cant role in the dynamics of blocking. The primary effect 2 Methods of latent heat release on blocking is the diabatic genera- 2.1 Model setup tion and amplification of upper-level negative PV anomalies (Pfahl et al., 2015). This amplification results from the injec- This work is based on numerical simulations with ECMWF’s tion of low PV into the upper troposphere in cross-isentropic global Integrated Forecast System (IFS) cycle 43R1, which ascending airstreams and the interaction of the diabatically was operational between November 2016 and July 2017. The enhanced divergent outflow with the upper-level PV struc- model is run at a cubic spectral truncation of TCo319, which ture at the tropopause (Steinfeld and Pfahl, 2019). For ex- corresponds to roughly 32 km grid spacing, and with 91 verti- ample, these diagnostic studies have shown that LH occurs cal hybrid pressure–sigma levels with a vertical resolution in predominantly in the warm conveyor belt (WCB; Wernli, the upper troposphere of about 10–25 hPa (ECMWF, 2016a). 1997; Methven, 2015) of extratropical cyclones and is gen- ECMWF operational analysis fields are used for initial con- erally most important during blocking onset and in more in- ditions. The 3-hourly output fields, including physical tem- tense and larger blocks. In addition, the repeated injection of perature tendencies, are interpolated to a regular grid at 1◦ diabatically heated low-PV air during the blocking life cy- horizontal resolution. cle, associated with a series of transient cyclones approach- In the IFS sub-grid-scale processes are represented by var- ing the block, can act to maintain blocks against dissipa- ious parametrization schemes (ECMWF, 2016b). The cloud tion. These findings complement the large body of previ- and large-scale precipitation microphysics scheme, based ous work that found LH to be important for the development on Tiedtke(1993), includes five prognostic variables (cloud of midlatitude weather systems, such as cyclones (Ahmadi- fraction, cloud liquid water, cloud ice, rain, and snow) with Givi et al., 2004; Binder et al., 2016), anticyclones (Quinting associated sources and sinks (Forbes et al., 2011; Forbes and and Reeder, 2017; Zschenderlein et al., 2020), Rossby waves Ahlgrimm, 2014; Ahlgrimm and Forbes, 2013). Convection (Grams et al., 2011; Röthlisberger et al., 2018), and Rossby is parametrized according to Tiedtke(1989) and Bechtold wave breaking (Zhang and Wang, 2018). et al.(2008), with a modified CAPE closure (Bechtold et al., Nevertheless, as these previous studies have used diagnos- 2013). tic methods to determine statistical relationships between LH and blocking, this does not necessarily mean that LH has a 2.2 Sensitivity experiments strong causal impact on blocking. The effect of LH on block- ing, and for Rossby wave dynamics at the tropopause in gen- The effect of cloud-diabatic heating on atmospheric blocking eral, is still not completely understood. It is a challenge to is investigated with sensitivity experiments by comparing the quantify the impact of LH, mainly because LH is strongly full-physics control simulation including LH (hereafter re- coupled to the dry dynamics of baroclinic waves and the as- ferred to as CNTRL) to the corresponding simulation without sociated adiabatic advection of PV (e.g., Kuo et al., 1990; LH (NOLH). LH is artificially turned off by multiplying the Teubler and Riemer, 2016). The question of whether LH crit- instantaneous temperature tendencies due to parameterized ically modifies the development of blocking, which is other- cloud and convection processes with a factor α D 0:0, but wise mostly affected by dry dynamics, and the investigation still allowing for moisture changes due to cloud and precip- of the corresponding cause-and-effect relationship is the fo- itation formation. Other nonconservative processes, such as cus of this study. radiative heating and turbulent mixing, which can also mod- The main objective of this paper is to study the sensitiv- ify PV (Spreitzer et al., 2019; Attinger et al., 2019), are not ity of atmospheric blocking to changes in upstream LH in altered. numerical model simulations. The effects of LH on the de- In
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