Potential Effects of Cirrus Cloud on Planetary Boundary Layer Height and Aerosol Loading in Metro Manila Using Ground-Based High Spectral Resolution Lidar

Date of Award

12-2021

Document Type

Thesis

Degree Name

Master of Science in Atmospheric Science

First Advisor

Lyndon Mark P. Olaguera, PhDMaria Obiminda L. Cambaliza, PhD

Abstract

This study investigated the potential influence of cirrus clouds on the planetary boundary layer (PBL) height and aerosol loading within the PBL, during daytime for no-rain days over an urban mixed background site in Metro Manila from January to December 2019. A unique opportunity wherein collocated cloud, aerosol, and radiation measurements from a ground-based High Spectral Resolution Lidar (HSRL), Beta Attenuation Monitor, and broadband Sunshine Pyranometers, respectively at the Manila Observatory (MO; 14.64°N, 121.08°E) were obtained from the CAMP2Ex Weather and CompoSition Monitoring observation campaign. Seasonal and diurnal variations of cirrus optical depth (!) from daytime (0600-1800 Philippine Time (PHT)) and their effects on the PBL height (PBLH) and aerosol loading within the PBL were characterized. The conditions from December to May 2019 are characterized by thinner cirrus (!mean = 0.19) and higher average PBLHmax of 1600 meters while June to November 2019 are characterized by thicker cirrus (!mean = 0.28) and lower average PBLHmax of 1200 m.

Irradiances play a key role in bridging the cirrus and PBLH seasonal differences. Opaque cirrus clouds (! > 0.3) lead to higher diffuse irradiance fraction than thin cirrus (0.03 < ! < 0.3), indicating a higher fraction of the total irradiances being scattered by the cirrus cloud. As higher cirrus ! leaves less direct incoming solar radiation available to reach the surface, it also leads to lower PBLH. Case studies reveal that other PBLH forcing mechanisms such as soil moisture also affect the energy transfer from the surface to the PBL. A tropical depression attenuated cirrus outflow case from 29-Oct- 2019 show that the combination of thick cirrus (! not quantified due to HSRL signal attenuation), high soil moisture, and strong downdrafts lead to the lowest net radiation reaching the surface (RN of 250 W/m2) with only 4% partitioned to sensible heat flux (SHF) leading to PBLHmax below 1000 m. Meanwhile, the combination of high RN of 720 W/m2 for clear or thin-cirrus conditions together with drier surfaces from 24-Dec- 2019 lead to higher magnitude of updrafts and maximum SHF > 150 W/m2. These indicate high instability and more vigorous vertical mixing which leads to higher PBLH > 1500 m.

The HSRL aerosol column and PBLH retrievals together with PM2.5 (Particulate Matter with aerodynamic diameter less than 2.5 !m) surface concentration measurements also provided new information on the air quality above the surface. PM2.5 surface concentrations can be representative of the immediate 500-m aerosol column in terms of diurnal variability with a Pearson correlation R = 0.63. PBLH and near surface aerosol loading have a significant inverse relationship (R = -0.37). Further analysis revealed the compounding effects of winds and PBLH on the observed high surface aerosol loading especially in the morning. Shallow PBLH and wind speeds both constrain and stagnate the fresh emissions to a small PBL volume, which leads to poor air quality in the site. The findings of this study contribute to the understanding of the cloud and boundary layer interactions that have implications on the meteorology and extent of poor air quality in urban areas in the Philippines.

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