1. Department of Physics and Astronomy, University of Louisville, KY, USA
Physical Mechanisms of Seasonal Variation of DW1 from the Stratosphere to Thermosphere in eCMAM30
Hongping Gu and Jian Du
Department of Physics and Astronomy, University of Louisville, KY, USA
Motivation
The migrating diurnal tide (DW1) is one of the strongest-amplitude global scale perturbations at low latitudes of the middle and upper atmosphere. The physical mechanisms that determining the seasonal variations of DW1 are examined at three atmospheric altitudes – 45, 95 and 165 km, which represent stratosphere, mesosphere and thermosphere respectively.
Data and method
The data used in this analysis comes from a 30-year ( ) output from the extended Canadian Middle Atmosphere Model (eCMAM). The eCMAM is a general circulation model (GCM) extending from the Earth’s surface to ~220 km, and is recently nudged to ERA-Interim reanalysis data up to 1 hPa.
Global horizontal winds (U) and temperatures (T) are extracted from the model every 6 h. To obtain the amplitudes and phases of DW1 U and T, a 2-D Fourier transform in longitude and time is applied to U and T to get the spectrum density in the domain of wave frequency and zonal wave number. On the basis of the spectrum density corresponding to DW1 U and T, an inverse Fourier transform is then used to derive their amplitudes and phases.
Results (Stratosphere)
Figure 2. Seasonal variations of the zonal momentum budget (left panel) and thermodynamic budget (right panel) for DW1: the amplitudes of the left-hand-side (LHS) of the equations (top), the right-hand-side (RHS) (middle) and their difference (bottom) at 45 km. The units are m s-1 d-1 for the zonal momentum and K d-1 for the thermodynamic budget.
Figure 3a. Seasonal variations of the forcing from the classic terms (top left), advection (top right), curvature (bottom left), and total model physics (bottom right) for the zonal momentum budget of DW1 at 45 km. The unit is m s-1 d-1.
Figure 3b. Seasonal variations of the forcing from the adiabatic heating (top), advection (middle), and total diabatic heating (bottom) for the thermodynamic budget of DW1 at 45 km. The unit is K d-1.
Results (Mesosphere)
Conclusions
In the stratosphere, the pressure gradient force and Coriolis force are the dominant terms in the zonal momentum budget and exhibit similar seasonal variations as the tide whereas advection and model physics play insignificant role. The seasonal variation of DW1 in the thermodynamic budget is mainly controlled by the short-wave heating and adiabatic heating terms.
In the mesosphere, besides of the classic terms (PGF and Coriolis force), advection and curvature terms are also important for the seasonal variation of DW1. In the thermodynamic budget, both adiabatic and advection are important. All of the model physics and diabatic terms do not exhibit similar seasonal variations as the tide.
For the thermosphere, model physics (mainly ion drag) becomes the second largest term to the classic terms although advection is also important. The short-wave heating is the most important term in the thermodynamic budget besides adiabatic and advection terms.
This study shows the complexity of the main physical mechanisms modulating seasonal variation of tides at different regions of the atmosphere.
Table 1. Relative significances of the terms of the right-hand-side of the zonal momentum budget equation for three atmospheric layers. The term that has the same seasonal variation as the tide is shown in bold (the same as below).
Table 2. Relative significances of the terms of the right-hand-side of the thermodynamic budget equation for three atmospheric layers.
Acknowledgements
This research is supported by NSF CEDAR grant NO and NASA grant NO. NNX15AJ02G.
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Figure 2.
Figure 3a.
Figure 3b.
Results
Figure 1. Latitude-height structure of the DW1 amplitude in the stratosphere (bottom panel), mesosphere (middle panel) and thermosphere (top panel) for zonal wind U (left) and temperature T (right) in March from the eCMAM30 (climatological mean of 1979 – 2010). The units for zonal wind and temperature are in m s-1 and K, respectively.
Figure 1 shows that both U and T amplitudes show four distinct latitudinal structures with height.
Basic equations
Figure 4.
Figure 5a.
Figure 5b.
Figure 4. The comparison of time series of 2m height temperature on the observation point over the Lake Erie. Black line denotes the observation, blue line denotes the model result of WRF without lake, the red line denotes the model result that coupling with lake model.
Terms
Dominant
Secondary
Minor
Stratosphere
PGF
Coriolis
Advection
Curvature
Model physics
Mesosphere
Thermosphere
Figure 4. Same as Figure2, but at 95 km.
Figure 5a. Same as Figure3a, but at 95 km.
Figure 5b. Same as Figure3b, but at 95 km.
Results (Thermosphere)
Terms
Dominant
Secondary
Minor
Stratosphere
Diabatic heating
Adiabatic heating
Advection
Mesosphere
Thermosphere
Figure 6.
Figure 7a.
Figure 7b.
Figure 5. The compare picture of accumulated precipitation in Oct. (a) drawed from radar data, (b) model simulated result that used Noah that coupled with lake model, (c) model simulated result that used Noah but without lake model.
Figure 6. Same as Figures 2 and 4, but at 165 km.
Figure 7a. Same as Figures 3a and 5a, but at 165 km.
Figure 7b. Same as Figures 3b and 5b, but at 165 km.
(For diurnal tide)