Identifying the growing modes of Venus’ atmosphere using Bred Vectors

Numerical simulation of Venus’ atmosphere is useful to understand the dynamics of the system. The Venus atmospheric data assimilation system “ALEDAS-V” (Sugimoto et al. 2017) based on the Venus atmospheric general circulation model “AFES-Venus” (Sugimoto et al. 2014) has been used to simulate Venus’ atmosphere and generated some key phenomena such as the super-rotation, baroclinic waves, and thermal tides. To further understand the dynamics of Venus’ atmosphere, Bred Vectors (BV) are computed with the AFES-Venus model. This method can identify different growing modes of the system and has been used to study the dynamics of the Earth (Toth and Kalnay 1993, 1997) and Martian atmospheres (Greybush et al. 2013).  However, to our knowledge, there has been no similar study on Venus’ atmosphere. To conduct the breeding cycle, we first produced a five (Earth) year free run of the AFES-Venus model initialized from an idealized zonal wind profile. Next, the forecast states on January 01 and August 25 in the 5^th Earth year are used as the initial conditions for the control run and the perturbed run, respectively. These two initial conditions have the same sub-solar positions. To emphasize the active dynamics, the BV norm is defined by the temperature norm from the 60 km to 80 km altitudes, weighted by pressure and latitude. For the breeding cycle, a rescaling norm and rescaling interval are specified. During the breeding cycle, at every rescaling interval (including the initial time), if the norm is bigger than the specified rescaling norm, the BV is rescaled to the rescaling norm.

Different combinations of the parameters are tested. The BV amplitude generally remains stable throughout the whole year without significant seasonal variability (Figure 1), which is different from the Martian atmosphere. The growth rate of the BV amplitude can represent the characteristics of the instabilities. It is calculated by taking the natural logarithm of the ratio of the BV amplitude at the end of the time interval (before rescaling) to the amplitude at the beginning of the interval. It is then converted to the daily growth rate by dividing the ratio of the time interval to one day. When the rescaling norm is smaller or the rescaling interval is shorter, the average growth rate is higher (Figure 2). Further BV analysis will be conducted such as analyzing the BV structure by taking composite mean along the super-rotation and conducting BV breeding cycle without thermal tides. These results will be useful to understand the dynamics of the thermal tide, baroclinic waves, super-rotation, and other important features of Venus atmosphere.

 

Figure 1. The bred vector amplitude (K² ) time evolution from the experiments with different rescaling days (1, 2, 5, 10, 20) and the same rescaling norm (10 K² ).

 

Figure 2. The average daily growth rate of the bred vector amplitude (K² ) for different combination of rescaling norm and rescaling days. The average is taken from February to December.

 

Reference

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Sugimoto, N., M. Takagi, and Y. Matsuda, 2014: Baroclinic instability in the Venus atmosphere simulated by GCM. J. Geophys. Res. Planets, 119, 1950–1968, https://doi.org/10.1002/2014JE004624.

——, A. Yamazaki, T. Kouyama, H. Kashimura, T. Enomoto, and M. Takagi, 2017: Development of an ensemble Kalman filter data assimilation system for the Venusian atmosphere. Sci. Rep., 7, 9321, https://doi.org/10.1038/s41598-017-09461-1.

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