TY - JOUR
T1 - An Axisymmetric Limit for the Width of the Hadley Cell on Planets With Large Obliquity and Long Seasonality
AU - Guendelman, I.
AU - Kaspi, Y.
N1 - We thank Rei Chemke for fruitful conversations and help with the model configuration. We also thank the reviewers that helped improve this manuscript. Data sets used in this manuscript are available on https://doi.org/10.5281/zenodo.1442928. The authors acknowledge support from the Minerva Foundation with funding from the Federal German Ministry of Education and Research and from the Weizmann institute Helen Kimmel Center for Planetary Science.
PY - 2018/12/28
Y1 - 2018/12/28
N2 - Hadley cells dominate the meridional circulation of terrestrial atmospheres. The solar system terrestrial atmospheres, Venus, Earth, Mars, and Titan, exhibit a large variety in the strength, width, and seasonality of their Hadley circulation. Despite the Hadley cell being thermally driven, in all planets, the ascending branch does not coincide with the warmest latitude, even in cases with very long seasonality (e.g., Titan) or very small thermal inertia (e.g., Mars). In order to understand the characteristics of the Hadley circulation in cases of extreme planetary characteristics, we show both theoretically, using axisymmetric theory, and numerically, using a set of idealized GCM simulations, that the thermal Rossby number dictates the character of the circulation. Given the possible variation of thermal Rossby number parameters, the rotation rate is found to be the most critical factor controlling the circulation characteristics. The results also explain the location of the Hadley cell ascending branch on Mars and Titan.Plain Language Summary The Hadley circulation is a thermally driven circulation, meaning that air raises at warm latitudes and descends at colder ones. As the solar forcing is seasonal this cell has a seasonal cycle as well, with typically the winter cell being stronger. Previous studies showed that under planetary conditions where the maximum temperature at solstice is at the summer pole, the ascending branch does not necessarily follow the maximum surface temperature; however, slowing down the rotation rate allows the ascending branch to follow the warmest latitude. In this study, we aim to explain this rotation rate dependence and the Hadley circulation on planets that exhibit strong seasonality like Titan and Mars, by using both theoretical arguments based on angular momentum conservation and idealized 3D model simulations. We find that the rotation rate is the main factor controlling the ascending branch of the circulation.
AB - Hadley cells dominate the meridional circulation of terrestrial atmospheres. The solar system terrestrial atmospheres, Venus, Earth, Mars, and Titan, exhibit a large variety in the strength, width, and seasonality of their Hadley circulation. Despite the Hadley cell being thermally driven, in all planets, the ascending branch does not coincide with the warmest latitude, even in cases with very long seasonality (e.g., Titan) or very small thermal inertia (e.g., Mars). In order to understand the characteristics of the Hadley circulation in cases of extreme planetary characteristics, we show both theoretically, using axisymmetric theory, and numerically, using a set of idealized GCM simulations, that the thermal Rossby number dictates the character of the circulation. Given the possible variation of thermal Rossby number parameters, the rotation rate is found to be the most critical factor controlling the circulation characteristics. The results also explain the location of the Hadley cell ascending branch on Mars and Titan.Plain Language Summary The Hadley circulation is a thermally driven circulation, meaning that air raises at warm latitudes and descends at colder ones. As the solar forcing is seasonal this cell has a seasonal cycle as well, with typically the winter cell being stronger. Previous studies showed that under planetary conditions where the maximum temperature at solstice is at the summer pole, the ascending branch does not necessarily follow the maximum surface temperature; however, slowing down the rotation rate allows the ascending branch to follow the warmest latitude. In this study, we aim to explain this rotation rate dependence and the Hadley circulation on planets that exhibit strong seasonality like Titan and Mars, by using both theoretical arguments based on angular momentum conservation and idealized 3D model simulations. We find that the rotation rate is the main factor controlling the ascending branch of the circulation.
U2 - 10.1029/2018GL080752
DO - 10.1029/2018GL080752
M3 - مقالة
SN - 0094-8276
VL - 45
SP - 13213
EP - 13221
JO - Geophysical Research Letters
JF - Geophysical Research Letters
IS - 24
ER -