Mixing Layer Peculiarities at Urban Area (Funding: Bulgarian National Science Fund grant number DM 04/1 2016.)

The characteristics of mixing layer height (MLH) and its role to aerosol number concentration in Sofia, Bulgaria, are analyzed in this study. Monitoring of MLH diurnal evolution over the city is performed during ten days measurement campaign in summer of 2015 by Jenoptik ceilometer CHM15k. Radiosonde thermodynamic and wind profiles are used as a reference in reliability assessment of ceilometer determined MLHs and it is shown that ceilometer underestimate MLH by about 110m. The temporal variations of aerosols number concentration are investigated and correlated to MLH and ventilation coefficient (VC). The influence of MLH and VC evolution and car traffic intensity on air quality is investigated. Two clearly observed peaks in aerosol number concentration are found. The first peak (7-10 LT) is associated with morning rush hour in combination with low MLH. The second one is at 20-21 LT in the evening about 1-2 hours after the heavy traffic as a result of decreasing MLH during evening transition. The presented results emphasize important role of MLH in determination of surface particulate matter concentration.

The diurnal variation of summertime mixing layer height (MLH) over Sofia, Bulgaria retrieved from ceilometer profiles is compared to boundary layer height simulated by Advanced Research WRF (ARW) mesoscale numerical weather prediction model. Validation of both simulated by WRF and retrieved from ceilometer MLHs during five days in August 2015 is accomplished by using of radiosonde thermodynamic profiles as a reference. Although based on different tracer MLHs derived from ceilometer and radiosonde showed significant matching. Through sensitivity analysis of nine planetary boundary layer parametrization schemes in WRF is found that five of them: Yonsei University (YSU), quasi-normal scale elimination (QNSE), Bougeault-Lacarrre(BouLac), Mellor-Yamada Nakanishi and Niino Level 2.5 and 3 (MYNN2.5, MYNN3) outperform the rest. It is also shown that augmented surface heat capacity and roughness in the city significantly modify urban MLH growth rate, especially in early evening.

Mixing layer height (MLH) is a crucial parameter for air quality modelling that is still not routinely measured. Common methods for MLH determination use atmospheric profiles recorded by radiosonde but this process suffers from coarse temporal resolution since the balloon is usually launched only twice a day. Recently, cheap ceilometers are gaining popularity in the retrieval of MLH diurnal evolution based on aerosol profiles. This study presents a comparison between proprietary (Jenoptik) and freely available (STRAT) algorithms to retrieve MLH diurnal cycle over an urban area. The comparison was conducted in the summer season when MLH is above the full overlapping height of the ceilometer in order to minimize negative impact of the biaxial LiDAR’s drawback. Moreover, fogs or very low clouds which can deteriorate the ceilometer retrieval accuracy are very unlikely to be present in summer. The MLHs determined from the ceilometer were verified against those measured from the radiosonde, which were estimated using the parcel, lapse rate, and Richardson methods (the Richardson method was used as a reference in this study). We found that the STRAT and Jenoptik methods gave lower MLH values than radiosonde with an underestimation of about 150 m and 650 m, respectively. Additionally, STRAT showed some potential in tracking the MLH diurnal evolution, especially during the day. A daily MLH maximum of about 2000 m was found in the late afternoon (18–19 LT). The Jenoptik algorithm showed comparable results to the STRAT algorithm during the night (although both methods sometimes misleadingly reported residual or advected layers as the mixing layer (ML)). During the morning transition the Jenoptik algorithm outperformed STRAT, which suffers from abrupt changes in MLH due to integrated layer attribution. However, daytime performance of Jenoptik was worse, especially in the afternoon when the algorithm often cannot estimate any MLH (in the period 13–16 LT the method reports MLHs in only 15–30% of all cases). This makes day-to-day tracing of MLH diurnal evolution virtually impracticable. This problem is possibly due to its early version (JO-CloVis 8.80, 2009) and issues with real-time processing of a single profile combined with the low signal-to-noise ratio of the ceilometer. Both LiDAR-based algorithms have trouble in the evening transition since they rely on aerosol signature which is more affected by the mixing processes in the past hours than the current turbulent mixing.

Mixing layer height (MLH) is a crucial parameter for air quality modelling that is still not routinely measured. Common methods for MLH determination use atmospheric profiles recorded by radiosonde but they suffer from course temporal resolution since balloon launching is only twice a day. Recently cheap ceilometers are gaining popularity in the retrieval of MLH diurnal evolution based on aerosol profiles. This study presents a comparison of a proprietary (Jenoptik) and a free available (STRAT) algorithms to retrieve MLH diurnal cycle. The comparison is accomplished in summer season over urban area and radiosonde data is used to estimate MLHs according to parcel, lapse rate, and Richardson methods (the last algorithm is used as a reference in the study) in addition. It was found that STRAT and Jenoptik give lower MLH than radiosonde with an underestimation of about 150m and 650m respectively. Additionally, STRAT showed reasonable performance in tracking of MLH diurnal evolution. Daily MLH maximum of about 2000m was found in the late afternoon (18-19 LT). In contrast, Jenoptik algorithm showed more weaknesses, mainly attributed to its real-time operation and independent processing of a single profile. At night and during morning transition period, both lidar-based methods showed difficulties as MLH was often in the ceilometer’s incomplete overlapping zone so residual or advected aerosol layers aloft were misleadingly reported as mixing layer (ML).