The Effect of Different Implementations of the Weak Temperature Gradient Approximation in Cloud Resolving Models

• Published in: Geophysical research letters 2023-12, Vol.50 (24), p.n/a
• Main Authors: Wong, N. Z., Kuang, Z.
• Format: Article
• Language: English
• Subjects: Approximation; Baroclinic mode; Convection; Mathematical analysis; Mathematical models; Modelling; Modes; Temperature gradients; Tropical circulation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL104350

The Weak Temperature Gradient (WTG) approximation is a popular method used to couple convection in limited‐area domain simulations with large‐scale dynamics. However, several different schemes have been created to implement this approximation, and these different WTG schemes show a wide range of different results in an idealized framework. Our investigation shows that different model behavior is caused by the treatment of the different baroclinic modes by the different WTG schemes. More specifically, we hypothesize that the relative strengths of the baroclinic modes plays a large role in these differences, and show that modifying these schemes such that they treat the baroclinic modes in a similar manner accounts for many of the significant differences observed. Plain Language Summary The Weak Temperature Gradient (WTG) approximation uses the fact that temperature gradients are weak in the tropics to simplify the interaction in the tropics between local convection and the broader‐scale tropical circulation. Several different schemes were created over the years to implement this approximation, and while they are broadly similar in many aspects, they also differ in the details. Although some previous studies aimed to quantify the differences between the implementations in various models, they did not delve into the reason behind these differences. We investigated the different model behaviors that result when different WTG schemes are utilized in an idealized model setup. We show through both mathematical analysis of the relevant equations and model runs implementing these different WTG schemes, that the resultant model behavior is dependent on how higher‐order baroclinic modes respond to temperature and buoyancy perturbations in the different WTG schemes. If we modify these schemes so that the strength of the response of higher‐order baroclinic modes is similar, many of these differences in model behavior observed will be reduced. Key Points Different implementations of the Weak Temperature Gradient result in divergent model behavior in idealized setups Divergent model behavior is caused by different treatment of baroclinic modes