Iceland Simulation for 10 July 2018 02:00-06:00 UTC
This page describes a numerical simulation of flow over Iceland for 10 July
2018 02:00-06:00 UTC. The wind and temperature profiles were provided by
Steve Eckermann using the NAVGEM model. Although data was provided as a
function of space and time, the current simulation considers only the time
variation for a profile located near the highest point on Iceland. The
spatial variation is modest compared with the temporal variation and it is
much more difficult to incorporate spatial variation effects accurately in
the CGCAM model. Thus an initial simulation was performed by considering
only the temporal variation.
The results below show that the NAVGEM-generated wind profiles produce
a critical level for mountain waves at an altitude of about 20 km. Full
3D simulations confirm this as very little wave activity is seen above the
critical level. Accounting for spatial variation or NAVGEM data time interval
does not change this conclusion (a second wind/temperature NAVGEM data set
is avilable for 18:00-21:00 UTC on the same day). This fact presents a quandry
since PMC-Turbo observed interesting wave activity at an altitude of ~80 km
on this day. There are a few possible explanations for this discrepancy:
- The observed waves were generated by an agent other than mountain waves originating from Iceland.
- PMC-Turbo observed secondary waves resulting from turbulence generated
near the critical level at 20 km. The current simulation is too
coarse to resolve instability and turbulence at this altitude.
SEE Iceland SMW - There was a mistake in the date and/or time for either the PMC-Turbo or NAVGEM data.
Analysis of NAVGEM Winds
The plots below show the temporal variation for the NAVGEM wind profiles
over the interval 02:00-06:00 UTC at two locations: (1) a point close to
the highest mountain peak on Iceland (64.5° N 17° W) shown on the left and
(2) a point close to the PMC-Turbo baloon track (69.5° N 18° W), right. The
simulation discussed below uses the Iceland wind profile (left).
In order to see the critical level more clearly, the following two plots show profiles of the wind component in the direction of the surface wind. A critical level for mountain waves is the point where these profiles pass through zero. The critical level is seen to be close to 22 km for both locations and for all times considered.
Second Profile Set
A second set of profiles was also generated with NAVGEM for 10 July 2018
but for the time interval 18:00-21:00 UTC. These profiles are similar to
those displayed above and also show a critical level below 25 km. Here
we show just profiles of the wind component in the direction of the
surface wind.
Computational Domain
The mesh is uniform at 1000 m spacing in both lateral directions for a zone [-200:200] km by [-400:0] km, which covers the majority of Iceland. Gentle stretching is used from the high-resolution central region to the boundaries with rate ~2%. The vertical mesh has a nominal spacing of 1000 m from the surface to the domain top at 100 km. The vertical mesh spacing is somewhat finer near the surface of mountaneous terrain. In total, the mesh contains 630 x 570 x 100 points. Sponge layers are used on all external boundaries in order to absorb outgoing waves.
Wind Forcing
Forcing terms gradually introduce winds near the surface with the
objective of achieving the wind profile within a two hour period.
A hyperbolic tangent function is used in order to produce gentle
acceleration of the wind near the beginning and end of the forcing period.
The maximum forcing rate is equivalent to that of a linear ramp with a
duration of thirty minutes.
NOTE: A somewhat rapid wind ramp was used since the NAVGEM data only span a four hour period. This feature results in transient waves that do not need to obey the critical level. Thus high altitude waves are seen at early times (most notablly depicted in the x-y plane at 83.5 km from 0 to 2.5 hr), but these waves have mainly propagated out of the domain at by the end of the simulation.
NOTE: A somewhat rapid wind ramp was used since the NAVGEM data only span a four hour period. This feature results in transient waves that do not need to obey the critical level. Thus high altitude waves are seen at early times (most notablly depicted in the x-y plane at 83.5 km from 0 to 2.5 hr), but these waves have mainly propagated out of the domain at by the end of the simulation.