Evaluation of climate model simulations of boundary layers in the Southern hemisphere
Date
2022
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Abstract
There is a constant need to improve the simulations of weather and climate across different timescales. In this study we evaluate the simulations of Southern Hemispheric boundary layers by a variable-resolution global atmospheric model named the conformal-cubic atmospheric model (CCAM). To do the evaluation, we firstly determine a climatology of present-day ABL attributes from atmospheric soundings at three weather stations located in the high-latitudes (Syowa), mid-latitudes (Marion Island), and subtropical continent (Irene). We then proceed to evaluate how the CCAM simulates attributes of the ABL climatology over the three locations.
Observational radiosonde data indicates that there persistently exists a temperature inversion over Irene at 00h00 UTC, throughout all the months of the year. It was found that this inversion is more intense during the autumn and winter seasons, while it is not as pronounced during the spring and summer seasons. Stability analysis over Marion Island indicated that the nearsurface atmosphere is neutrally stratified during all the seasons at 12h00 UTC. The radiosonde data over Syowa also indicated the presence of surface-based inversion at 00h00 UTC for all the months of the year. The surface-based inversion was also observed for 12h00 during all the months except the summer months, May, October and November.
It was also found that monthly variability within the different seasons are prominently observed during the transitional seasons of autumn and spring, especially for thermodynamic variables. An interesting feature which was also observed over Irene and Syowa throughout all the seasons is a bulge in relative humidity above the surface. For Irene this is true for all the months of the year at 12h00 UTC, while for Syowa this bulge is observed for both 00h00 UTC and 12h00 UTC for all the months of the year. It is currently not known why this bulge exists, but a hypothesise is that it is due to the presence of low-level jet (LLJ).
In terms of the model evaluations, it was also found that there is persistent positive bias of moisture variables – dewpoint temperature and relative humidity. This is especially true for the spring and summer months over Irene, while autumn and winter months indicate pronounced negative bias near the surface. Over Marion Island the positive bias of relative humidity is even more pronounced and persists throughout the four seasons at both 00h00 UTC and 12h00 UTC hours. This may be an indicator that the CCAM model is too moist above the surface over the two stations.
Irene ABLH values based on calculations from observations were found to be lower than values simulated by the CCAM at 12h00 UTC throughout the four seasons, with an overestimation of more than 900 m during spring. On the contrary, the CCAM underestimated 4 ABLH values at 00h00 UTC for all the four seasons over Irene, with differences being only as much as 210 m for the winter season. Over Marion Island, CCAM consistently underestimated values of the ABLH for both 00h00 UTC and 12h00 UTC. An exception occurred at 00h00 UTC during the winter season, which indicates an overestimation of the ABLH by 550 m. Over Syowa, the differences between modelled and observed ABLH were not as pronounced as in both Irene and Marion Island. The smaller differences can be attributed to the presence of the surface-based inversion CCAM depicts fairly well.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, University of the Witwatersrand, Johannesburg, 2022
Keywords
Climate Model Simulations, Climate Modelling, Southern Hemispheric boundary layers