The sensitivity of atmospheric blocking to changes in upstream latent heating – numerical experiments

Abstract

<p><strong>Abstract.</strong> Recent climatological studies based on trajectory calculations have pointed to an important role of latent heating during cloud formation for the dynamics of anticyclonic circulation anomalies such as atmospheric blocking. However, the causal relationship between latent heating and blocking formation has not yet been fully elucidated. To explicitly study this causal relationship, we perform sensitivity simulations of five selected blocking events with a global weather prediction model in which we artificially eliminate latent heating in clouds upstream of the blocking anticyclones. This elimination has substantial effects on the upper-tropospheric circulation in all case studies, but there is also significant case-to-case variability: some blocking systems do not develop at all without upstream latent heating, while for others the amplitude of the blocking anticyclones is merely reduced. This strong influence of latent heating on the jet stream is due to the injection of air masses with low potential vorticity (PV) into the upper troposphere in strongly ascending <q>warm conveyor belt</q> airstreams, and the interaction of the associated divergent outflow with the upper-level PV structure. The important influence of diabatic heating demonstrated with these experiments suggests that an accurate parameterization of microphysical processes in weather prediction and climate models is crucial for adequately representing blocking dynamics.</p>

Integration-based Extraction and Visualization of Jet Stream Cores

“…In Figure 12a, we see a WCB approaching the jet stream, and in Figure 12b, we observe the situation two days later. We observe a displacement of the jet stream by the WCB as Steinfeld et al (2020) found while examining the same case as shown in Figure 12. As discussed before, WCBs can influence the jet in different ways: either they lead to local wind speed maxima (jet streaks, as in Fig.…”

The representation of winter Northern Hemisphere atmospheric blocking in ECMWF seasonal prediction systems

“…Dedicated studies involving specific processes, as performed by Steinfeld et al . (2020) for diabatic heating, are therefore recommended.…”

The movement of atmospheric blocking systems: can we still assume quasi-stationarity?

“…This velocity is calculated by comparing the intensity-weighted center of a 140 block at its date of onset to its date of decay and divide this distance in kilometers by the blocking duration. This method is consistent with the propagation velocity used by Steinfeld et al (2018). Blocks are categorised as winter or summer blocks based on their onset date being in respectively the months December, January or February, or June, July or August.…”

“…For each group, the coordinates of the weighted centres of the blocks based on their BI on the fourth day of their existence is used to show where the different groups of blocks occur most. The choice of the fourth day aligns with the division made by Steinfeld et al (2018) between the onset phase (day 1 and 2) and mature phase (day 3, 4 and 5) of the block. By focusing on the fourth day, we ensure that we assess the blocks during their mature phase.…”

“…More recently, Mokhov and Timazhev (2019) showed that different threshold values of maximum zonal propagation velocities led to different frequencies of blocking. Lastly, Steinfeld et al (2018) studied the distributions of the mean zonal propagation velocity during the lifetime of blocks and concluded that this velocity was highest in their onset phase and decreased during their mature phase. They also mention that their blocking algorithm detected some blocks with propagation velocities larger than 10 m s −1 (864 km d −1 ), but they doubted if they could be seen as classical blocks.…”