Motivation
- Clouds’ effect on radiation budget depends on cloud base height (high clouds cause the earth’s surface to heat up, while low clouds cause cooling)
- Clouds are major source of uncertainty in weather and climate numerical prediction
- Cloud base height is also important to ensure air traffic safety
- Urban effects on clouds need thorough analysis
Sites and Instruments
10-year-long data series (01.01.2011 - 31.12.2020)
3 sites: Sofia University (SU), Sofia Airport (LBSF), National Institute of Meteorology and Hydrology (NIMH)
observations:
- surface - AWS (SU & LBSF) & weather observer (NIMH); profiles - ceilometers (SU - Lufft chm15k & LBSF - Vaisala cl31) & rawinsonde (NIMH)
- sources: SU (AWS + chm15k) departhment’s archive; LBSF (AWS + cl31) METAR; NIMH (surface) SYNOP; rawinsonde from University of Wyoming
- time resolution: SU chm15k 1 min; SU AWS 10 min; LBSF AWS + cl31 30min; NIMH surface 3 h & NIMH rawinsonde 24 h (at 14 LT)
Available Data
SU AWS - 99.97% of the data is available
SU CHM15k - 83.75% of the data is available
LBSF METAR (includes both AWS and CL31) - 93.24% of the data is available
NIMH SYNOP - 99.84% of the data is available
NIMH rawinsonde - 97.92% of the data is available
CBH retrieval
Column L
Ceilometers
CBH is retrieved by proprietary software from Lufft (CHM15k) and Vaisala (CL31). It should be noted that in METAR CBH data may have been modified by the observer to better represent cloudiness over the airport area.
Surface data
Lifted condensation level (LCL) is used as proxy of CBH. LCL is calculated following Romps 2017.
Column R
Rawinsonde
Thresholds based approach (adapted from Poore et al. 1995) is used in cloud base detection. We identify increased humidity layer where \(RH>RH_{min}\), if maximum RH within the layer is \(max(RH)\ge RH_{min} + RH_{jump}\). Poore et al. find that \(RH_{min}=82\%\) and \(RH_{jump}=3\%\) fit best with their data.
Results
Increasing RH thresholds reduce low-level clouds and fog/precipitation cases. Mid-level clouds detection shows weak sensitivity. Cases of CBH>6000m are uncommon.
CBH verification - rawinsonde gives systematicaly lower CBH than CHM15k in the city center for all RH thresholds. The smallest BIAS (-455m) is registered for RH min=88% and RH jump=6% …
CBH verification - CBH determined by rawinsonde and CL31 (airport) are much more similar. The best combination is RH min=85% and RH jump=5% that gives BIAS< -1m
In summer LCL at SU is lower than LCL at LBSF. SU AWS is situated in park, so local conditions (higher RH, lower air temperature) may explain the difference.
In summer CBH at SU is higher than CBH at LBSF. The urban heat/dry island and increased roughness intensify convection. Park effects are negligible as the atmosphere is not frozen.
Conclusion
1. It was shown that increasing RH thresholds for CBH detection based on rawinsonde profiles, reduces occurrence of low-level clouds and fog/precipitation. Mid-level clouds detection showed weak sensitivity to RH thresholds.
2. It was found that using threshold-based approach for obtaining CBH from upper-air measurements may produce plausible results if compared them against a ceilometer when both instruments are in the outskirts of the city.
3. The application of the LCL approach should be performed carefully as the obtained CBH proxies strongly depend on the local inhomogeneities intrinsic to the surface layer.
4. When we analyzed CBH <3000m derived from two ceilometers (the first located in the central part of the city and the second - in the suburbs) it was found that in summer CBH over the city center is several hundred meters higher. The finding may be due to the urban heat/dry island effect.
Acknowledgements
This work has been carried out in the framework of the National Science Program “Environmental Protection and Reduction of Risks of Adverse Events and Natural Disasters”, approved by the Resolution of the Council of Ministers 577/17.08.2018 and supported by the Ministry of Education and Science (MES) of Bulgaria (Agreement D01-363/17.12.2020). The study is also supported by Sofia University, Contract 80-10- 13/18.03.2021. We also acknowledge the COST Action CA18235 - PROfiling the atmospheric Boundary layer at European scale (PROBE).