Browsing by Author "Aryee, J.N.A."

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    Estimation of planetary boundary layer height from radiosonde profiles over West Africa during the AMMA field campaign: Intercomparison of different methods
    (Scientific African, 2019-11) Amekudzi, L.K.; Aryee, J.N.A.; Preko, K.; Atiah, W.A.; S.K., Danuor
    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.
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    Overview of surface to near‑surface atmospheric profles over selected domain during the QWeCI project
    (Meteorology and Atmospheric Physics, 2018-07-04) Amekudzi, L.K.; Aryee, J.N.A.; Atiah, W. A.; Osei, M. A.; Agyapong, E.
    Abstract Assessing the evolution of surface to near-surface atmospheric fuxes is key to improving our understanding of their interactions, while further advancing climate applications. In this paper, an overview of the diurnal to seasonal evolution of some surface to near-surface atmospheric fuxes, coupled with their interactions, have been provided. Fluxes of downwelling and upwelling radiation (SW↓, SW↑, LW↓, LW↑), soil heat fux (ΔH), relative humidity (RH), rainfall (RR) and surface air temperature (T), measured from two diferent locations (Owabi and KNUST) and at a temporal resolution of 10 min, encompassing the quantifying weather and climate impact (QWeCI) Project period (2011–2013), were used to assess their relationship on diurnal to seasonal scales. First, diurnal assessments of the various profles were performed. These provided information on the relatively active daytime, with the earth surface exposed to substantial SW↓, initiating rising and sinking thermals which subsequently increased T and ΔH, with reductions in RH until few hours after midday, beyond which a reversal was observed. Also, ΔH from the vegetative terrain (Owabi) was found to be directed into the surface at daytime, and released from the sub-surface layer back into the atmosphere at night time, compensating the energy loss by LW↑ from the surface. Furthermore, rainfall (RR) in both locations were found to be generally convective and occurring mostly between 1500 GMT and 2300 GMT. The relationship between net radiation (RN) and RR is presently statistically unclear, although rainfall peaks were found to be occurring at low RN and relatively warmer T, accompanied by high RH. Thereafter, seasonal assessments were performed to capture the monthly-averaged diurnal variabilities in the measured surface to near-surface parameters. These showed heightened daytime T, ΔH and RN, coupled with relatively low RH within the dry seasons, and more reduced profles within the monsoon season. Additionally, countrywide assessments were performed using ERA-5 datasets which showed similarities with the in situ data. However, convective rains over the domain were not fully resolved in ERA-5. Nonetheless, the fndings of this study are essential to understanding surface energy balance processes in tropical, humid climates, which is important for various climate-impact modeling applications and policy formulations over the region.