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User group experiments

Modifying the temperature tendency due to radiation

Main conclusions:

1. Increasing tendency from radiation (heating at daytime, cooling at night) in the upper levels over the North Atlantic (experiment rad_2_NA_top in Table) leads to higher mean sea level pressure values over the North Atlantic and it is compensated with lower MSLP almost everywhere else (Fig. 1).
2. Nullified tendency from radiation over the North Atlantic from 900 to 100 hPa (experiment rad_0_Karl) slightly deepens the cyclone (or shifts it eastwards). The output tendency from radiation remains below 1K/day after 48 hours (i.e. at 00 UTC; Fig. 2).

Explanation:

• The magnitude of temperature tendency due to radiation is small, a few K/day. Its modification has significant impact only if it occurs over a large area and during a longer period.

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Anchor fig1 fig1 Image Added Anchor fig2 fig2 Image Added Figure 1: Difference in mean sea level pressure (in hPa) between the experiment with no temperature tendency from radiation (rad_0_NA_top) and the experiment with doubled temperature tendency from radiation (rad_2_NA_top). Figure 2: Cross section for 48-hour forecast of temperature tendency from radiation (in K/day, with shading) and potential temperature (in K, with isolines) along the line highlighted in red in the top map. The graph shows the result of the experiment with zero temperature tendency from radiation (rad_0_Karl). The dotted rectangle represents the box where the tendency modification was applied.

Modifying the temperature tendency due to convection

Main conclusions:

1. Nullifying and doubling the temperature tendency due to convection in a box covering (and following) Karl in the first 48 hours (experiments con_0_Karl_48h and con_2_Karl_48h) leads to increased and decreased mean sea level pressure on the first day, respectively. The direction of the change is clear, but its magnitude is not proportional with the size of the scaling factor (confirmed also by the experiment pairs of con_0.5_def_24h and con_1.5_def_24h, con_0.5_Karl_48h and con_1.5_Karl_48h applying different scaling factors). The structure of the potential vorticity also coincides with that.
2. The temperature tendency due to convection can reach 3 K/day between 900 and 400 hPa in the 24- and 48-hour forecasts (Fig. 3), even if we remove (con_0_Karl_48h) or halve (con_0.5_Karl_48h) the tendency in a few time steps. It results in a small extra drying in the humidity tendency in the same layer (not shown).
3. Increasing the convective temperature tendency by 50 % in the first 24 hours in a small area close to the mean sea level pressure minimum of Karl (con_1.5_def_24h) leads to slight changes in the daily accumulated convective tendency: small increase in the lower levels and drop above 900 hPa (Fig. 3). The scaling has straightforward effect on the precipitation especially if it is applied during a 48-hour period (con_1.5_Karl_48h; Fig. 4).

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Anchor fig3 fig3  Image Added Figure 3: Vertical profile for 48-hour forecasts of temperature tendency from different sources (in K/day) over the area highlighted in red in the top right map. The top graph shows the result of the control experiment, the middle and the lower graphs show the results of the experiments where scaling factor of 0.5 and 1.5 was applied on the convective temperature tendency (con_0.5_Karl_48h and con_1.5_Karl_48h), respectively. Note the different scale of the x-axis.

Explanation:

• If the box moves along the trajectory of Karl (as in experiments con_{0|0.5|1.5|2}_Karl_48h), the tendency modification is applied over a given grid point only for few time steps.
• The convection scheme continuously acts in the model, so the temperature (humidity) tendency at step +24h is the sum of the tendencies resulted by the convection scheme during every 900 s (i.e. 15 minutes) between 00 UTC on 25 September and 00 UTC on 26 September 2016. Therefore, even if we remove the temperature tendency due to convection over a given area during a part of the day, the deep convection mechanisms considerably develop during the rest of the day.
• More intense convection processes deepen the cyclone and the opposite happens with a reduced convection influencing also the precipitation amount.

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Anchor fig4 fig4  Image Added Image Added Figure 4: 66-hour precipitation forecasts resulted by the experiments where scaling factor of 0.5 (left; con_0.5_Karl_48h) and 1.5 (right; con_1.5_Karl_48h) was applied on the convective temperature tendency.

Modifying the temperature tendency due to cloud processes

Main conclusions:

1. Halving the temperature tendency due to cloud processes in a box following Karl in the first 48 hours (experiment cloud_0.5_Karl_48h) increases the accumulated near-surface heating from the cloud microphysics and it is compensated by enhanced low-level cooling from the dynamics. Shallow convection seems to be less active resulting in a reduced drying near the surface. Between 800 and 400 hPa the deep convection gets more intense accompanied by more excessive heating and drying in this layer (Fig. 5). At the same time, the dynamical processes add moisture here.
2. Changing the temperature tendency due to cloud processes in a box covering Karl in the first 48 hours has significant impact on the precipitation over Scandinavia later (Fig. 6). Applying scaling factor of 1.5 (experiment cloud_1.5_Karl_48h) leads to an earlier precipitation maximum before 18 UTC 27 September 2016, while scaling factor of 0.5 (experiment cloud_0.5_Karl_48h) results in a delay in precipitation maximum.

Explanation:

• The stability changes due to the condensation heating in upper levels and changes in the evaporation (cooling) in the lower levels. Convection largely compensates the changes in the cloud tendencies.
• Dynamics also reacts to the changes in the heating, mainly balances the drying due to convection and cloud physics above the boundary layer. The heating generates lifting of air parcels and the lifting generates moistening.

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Anchor fig5 fig5   Image Added Figure 5: Vertical profile for 48-hour forecasts of humidity tendency from different sources (in K/day) over the area highlighted in red in the top right map. The top graph shows the result of the control experiment, the lower graph shows the result of the experiment where scaling factor of 0.5 was applied on the temperature tendency from cloud processes (cloud_0.5_Karl_48h). Note the different scale of the x-axis. Anchor fig6 fig6  Image Added Image Added Image Added Image Added Figure 6: 66-hour (top) and 72-hour (bottom) precipitation forecasts resulted by the experiments in which scaling factor of 0.5 (left; cloud_0.5_Karl_48h) and 1.5 (right; cloud_1.5_Karl_48h) was applied on the temperature tendency from cloud processes.

Acknowledgement

Grateful thanks to Peter Bechtold who helped in understanding the results and phrasing the explanations. We are also grateful to Richard Forbes who provided the box modifying code to the OpenIFS team.

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