Estimation of planetary boundary layer height from radiosonde profiles over West Africa during the AMMA field campaign: Intercomparison of different methods
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Date
2019-11
Journal Title
Journal ISSN
Volume Title
Publisher
Scientific African
Abstract
Deducing realistic planetary boundary layer heights (PBLH) is crucial for weather, climate
and air quality models, despite its equivocal nature. In this paper, a comparative assessment of seven PBLH estimation methods has been performed, with radiosonde profiles
taken during the African Monsoon Multidisciplinary Analyses (AMMA) project campaign
from June, 2006 to July, 2007 over 18 locations in West Africa. First, PBLH was identified
from the radiosonde profiles as the location of minimum gradients in mixing ratio (q),
relative humidity (RH) and refractivity (N), and maximum gradient in potential temperature (θ). Other methods used to identify PBL tops were the statistical NS method and
bulk Richardson (Rib) method at a critical threshold of 0.25. Next, a reference method
(ho) was identified as the benchmark for PBL comparison. Visual inspection of the individual profiles allowed for assessing the ho method to yield reliable PBLH estimates. Thereafter, comparisons of the PBLH were performed for both convective and stable cases, with
the stable boundary layer (SBL) height being generally below 700 m a.g.l for all methods, whereas, convective boundary layer (CBL) heights ranged between 300 m a.g.l and
1400 m a.g.l across different regions of the study area. Contrarily, the NS method failed
to detect the SBL due to its inability to identify a dewpoint or virtual potential temperature discontinuity. Additionally, the Rib method, particularly in stable cases, yielded PBL
tops consistent with the reference method (ho) whenever the NLLJ was clearly defined
due to contribution of wind shear beneath the jet core to turbulent kinetic energy (TKE)
production. Afterwards, the study domain was split into three zones and the performance
of each method was assessed per each zone. Spatially, the CBL height was observed to
grow thicker in the north-east direction over the dry, arid regions, where integral values
of sensible heat rapidly converted by surface net radiation, has significant influence on the
growth of CBL throughout daytime. Other likely reasons for this observation are advection, orography and mechanical turbulence production. However, a reversal was observed
at night with the SBL being thinner in the dry, arid regions and rather relatively, higher
in the coastal regions. Finally, the statistical assessment, coupled with visual inspection of
the individual profiles, showed that the gradient methods (particularly N) outperformed
the Rib and NS methods, yielding very low biases as well as, high and statistically significant correlation co-efficients. These results are useful for enhancing the performance of
PBL models over the region. Possible limitations to the findings of this study are the different ascent times between the sites, as well as the number of ascents per site, which
∗ Corresponding author.
Description
This article is published in Scientific African and also available at https://doi.org/10.1016/j.sciaf.2019.e00228
Keywords
AMMA, DACCIWA, Planetary boundary laye, West Africa, Radiosounding
Citation
Scientific African 7 (2020) e00228