Peng Wu

Date of Award

January 2015

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Atmospheric Sciences

First Advisor

Baike Xi

Second Advisor

Xiquan Dong

Abstract

In this study, a new method has been developed to retrieve the marine boundary layer (MBL) cloud microphysical properties, which provides a complete diurnal variation of MBL cloud properties for 19-month dataset at the Azores. All nighttime monthly means of cloud liquid water path (LWP) exceed their daytime counterparts with an annual mean LWP of 140 g m-2, which is ~30.9 g m-2 larger than the daytime mean. The seasonal and diurnal variations of cloud LWC and optical depth basically follow the variation of LWP. There are, however, no significant day-night differences and diurnal variations in cloud-droplet effective radius (re) and number concentration (Nt). The corresponding surface measured cloud condensation nuclei (CCN) number concentration (NCCN) (at Supersaturation S=0.2%) exhibit a semidiurnal variation. Surface NCCN increases from around sunrise (0300-0600 LT) to late afternoon, which strongly correlates with surface wind speed (r=0.76) from 0300 to 1900 LT. The trend in hourly mean Nt is consistent with NCCN variation from 0000 to 0900 LT, but not for afternoon and evening with an averaged ratio (Nt / NCCN) of 0.35 during the entire study period.

Using potential temperature method and sounding data, all cloud samples were then classified into coupled and decoupled conditions. A schematic diagram is given to demonstrate the coupled and decoupled MBL vertical structures and how they associate with non-drizzle, virga and rain events. Out of a total of 30432 5-min samples (both

daytime and nighttime), 9888 samples can be identified, with 22.2% as coupled and 77.8% as decoupled; 32.7% as non-drizzle and 67.3% as drizzle (47.8% as virga, 19.5% as rain); 40.6% as daytime and 59.4% as nighttime. The averaged thickness of decoupled cloud layer (400 m) is deeper than that of coupled cloud layer (330 m), and its LWP (135.1 g m-2) and re (12.7 μm) values are higher than coupled ones (116.4 g m-2, 11.9 μm) too. Conversely, decoupled stratocumuli have lower Nt (80.6 cm-3) and NCCN (180.9 cm-3) than coupled stratocumuli (102.2 cm-3, 210.8 cm-3). The MBL cloud properties under non-drizzle and virga conditions are similar to each other, but significantly different to those of rain.

To further investigate the effect of drizzle on the MBL clouds, drizzle properties below cloud base have been retrieved using lidar and radar observations. For all the cloud and drizzle samples, without considering coupled/decoupled conditions, the drizzle occurrence is 42.6% with a maximum of 55.8% in winter and a minimum of 35.6% in summer. Out of a total of 13092 daytime 5-min samples, 5580 samples can be identified as drizzling cloud, the annual means of drizzle liquid water path (LWPd), effective radius (rd) and number concentration (Nd) for the rain (virga) samples are 5.48 (1.29) g m-2, 68.7 (39.5) μm, and 0.14 (0.38) cm-3. The seasonal mean LWPd values are less than 4% of the MWR-retrieved LWP values. The annual mean differences in cloud-droplet effective radius with and without drizzle are 0.12 and 0.38 µm, respectively, for the virga and rain samples. Therefore, the impact of drizzle on cloud property retrievals is insignificant over ARM Azores site

Download

Share

COinS