Factors Controlling Stratocumulus Cloud Lifetime Over Coastal Land Mohamed S
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Cleveland State University EngagedScholarship@CSU Physics Faculty Publications Physics Department 8-1-2016 Factors Controlling Stratocumulus Cloud Lifetime Over Coastal Land Mohamed S. Ghonima University of California Thijs Heus Cleveland State University, [email protected] Joel R. Norris University of California Jan Kleissl University of California Follow this and additional works at: https://engagedscholarship.csuohio.edu/sciphysics_facpub Part of the Physics Commons How does access to this work benefit oy u? Let us know! Publisher's Statement © Copyright 2016 AMS Repository Citation Ghonima, Mohamed S.; Heus, Thijs; Norris, Joel R.; and Kleissl, Jan, "Factors Controlling Stratocumulus Cloud Lifetime Over Coastal Land" (2016). Physics Faculty Publications. 215. https://engagedscholarship.csuohio.edu/sciphysics_facpub/215 This Article is brought to you for free and open access by the Physics Department at EngagedScholarship@CSU. It has been accepted for inclusion in Physics Faculty Publications by an authorized administrator of EngagedScholarship@CSU. For more information, please contact [email protected]. VOLUME 73 JOURNAL OF THE ATMOSPHERIC SCIENCES AUGUST 2016 Factors Controlling Stratocumulus Cloud Lifetime over Coastal Land MOHAMED S. GHONIMA Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, California THIJS HEUS Department of Physics, Cleveland State University, Cleveland, Ohio JOEL R. NORRIS Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California JAN KLEISSL Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, California (Manuscript received 6 August 2015, in final form 10 March 2016) ABSTRACT The breakup of stratocumulus clouds over coastal land areas is studied using a combination of large-eddy simulations (LESs) and mixed-layer models (MLMs) with a focus on mechanisms regulating the timing of the breakup. In contrast with stratocumulus over ocean, strong sensible heat flux over land prevents the cloud layer from decoupling during day. As the cloud thins during day, turbulence generated by surface flux be- comes larger than turbulence generated by longwave cooling across the cloud layer. To capture this shift in turbulence generation in the MLM, an existing entrainment parameterization is extended. The MLM is able to mimic cloud evolution for a variety of Bowen ratios, but only after this modification of the entrainment parameterization. Cloud lifetime depends on a combination of the cloud-top entrainment flux, the Bowen ratio of the surface, and the strength of advection of cool ocean air by the sea breeze. For dry land surface conditions, the authors’ MLM suggests a breakup time a few hours after sunrise. For relatively wet land surface conditions, the cloud layer briefly breaks into partly cloudy conditions during midday, and the stra- tocumulus cloud reforms in the evening. 1. Introduction between cloud albedo and ocean surface albedo, the majority of research on stratocumulus clouds has fo- Stratocumulus clouds play an important role in Earth’s cused on observing, measuring, and modeling marine energy balance, hydrological cycle, and applications. stratocumulus clouds in the subtropics [Wood (2012) Because of the prevalence and high albedo of stratocu- and references therein] or as part of postfrontal sys- mulus clouds, small changes in stratocumulus cover and tems (Mechem et al. 2010a,2010b). Although re- thickness can lead to strong responses in Earth’s climate ceiving less attention, stratocumulus clouds are also (e.g., Randall et al. 1984). However, climate models are abundant over coastal land areas that are frequently unable to accurately model stratocumulus clouds (Bony well populated (Hilliker and Fritsch 1999; Iacobellis and Dufresne 2005). Because of the stark contrast and Cayan 2013; Mathiesen and Kleissl 2011). The stratocumulus topped boundary layer (STBL) over ocean and land is characterized by a strong tem- Corresponding author address: Mohamed Ghonima, University of California, San Diego, 9500 Gilman Dr., Bldg. EBU II, Room 304, La perature inversion that limits the vertical mixing of cool Jolla, CA 92093-0411. moist air in the boundary layer with warm dry air aloft E-mail: [email protected] (Klein and Hartmann 1993). This inversion forms when DOI: 10.1175/JAS-D-15-0228.1 Ó 2016 American Meteorological Society 2961 2962 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 73 warm dry air descending from aloft in a high pressure In this paper, we use large-eddy simulation (LES) and a cell interfaces with cool moist air near the surface. mixed layer model (MLM) to examine how cloud-top Turbulence in the STBL transports water from surface entrainment, surface Bowen ratio, and advection by to the cloud layer and promotes entrainment of dry air the sea breeze contribute to cloud breakup during the from the free troposphere. Nocturnally, cloud-top morning transition of stratocumulus over coastal land. longwave radiative cooling is the primary driver of tur- We will first use LES of some idealized cases to obtain a bulence in the boundary layer, often resulting in a well- reference state of the cloud evolution that reasonably mixed layer that acts to maintain the stratocumulus portrays the breakup of coastal stratocumulus clouds cloud layer (Lilly 1968). During the day, solar absorp- (section 3). Next, we describe modifications to the MLM tion within the cloud counteracts the longwave cooling, to more accurately represent how changes in cloud-base thereby reducing the convective mixing. Thus, strato- and cloud-top temperature affect longwave radiation and cumulus clouds over the ocean usually attain maximum to more accurately represent the combined effect of sur- coverage and thickness just before dawn, minimum cov- face and cloud-driven buoyancy on turbulence and en- erage and thickness in the early afternoon, and exhibit a trainment (section 4). Finally, the MLM is applied in marked diurnal cycle (Wood et al. 2002). conjunction with the LES to understand how STBL tur- The stratocumulus diurnal cycle is even stronger over bulence, entrainment, cloud liquid water path (LWP), and land. Since land surfaces have a lower heat capacity than cloud thickness respond to the varying initial profiles, wet ocean, a significant portion of incoming solar radiation is and dry land surfaces, large-scale advection, and sub- reemitted as sensible and latent heat flux. The stronger sidence (section 5). surface flux leads to substantial warming and moistening of the STBL over land compared to ocean. Furthermore, 2. Characterization of coastal stratocumulus during day the surface buoyancy flux becomes the main driver of turbulence in the STBL, offsetting the decrease Over midlatitude coastal land areas, stratocumulus in turbulence generated by longwave cooling. Hence, clouds usually form at night owing to the advection of cool cloud-top entrainment velocities over land can exceed ocean air onto the coast. They are especially prevalent 2 10 mm s 1 during midday (Mechem et al. 2010b), in during the summer months in the Northern Hemisphere comparison to entrainment velocities that rarely exceed owing to strong climatological anticyclones over the ad- 2 5mms 1 over ocean (Stevens 2002). Consequently, jacent ocean. Using normalized global horizontal irradi- warming through solar absorption, surface sensible heat ance (GHI) observations [percentage of instantaneous flux, and entrainment of drier warmer air aloft are the clear-sky irradiance computed based on Ineichen and main causes of daytime cloud dissipation over land. This Perez (2002)] from a Li-200SZ (LiCor, Nebraska) pyr- strong mixing is in stark contrast to the decoupling fre- anometer at the University of California, San Diego, we quently observed in the STBL over the ocean. observe that the averaged irradiance is lowest during Less research attention has been given to stratocumu- mornings in the months of May–September, compared to lus over land than over ocean, and of those studies, the the rest of the year, corresponding with the occurrence of majority have focused on postfrontal continental strato- coastal stratocumulus clouds (Fig. 1a). Clouds begin to cumulus clouds (Kollias and Albrecht 2000; Zhu et al. dissipate farthest inland first in the morning and at pro- 2001; Ghate et al. 2010; Fang et al. 2014a,b; Mechem et al. gressively later times closer to the coast, where the clouds 2010a; Mechem et al. 2010b). Stratocumulus clouds over often survive into the afternoon (Fig. 1b; Skupniewicz coastal land differ from clouds in the continental interior et al. 1991). Schwartz et al. (2014) analyzed six decades of in two major ways. First, coastal stratocumulus clouds airport observations to find that during the summer usually form in the summer when there is a strong in- months low clouds, including stratocumulus clouds, re- version capping the STBL, whereas continental clouds spond to large-scale forcings, such as Pacific decadal os- often occur during the cool season when downwelling cillation, coherently across the entire west coast of North solar radiation is smaller (Mechem et al. 2010a). Second, America and across a wide range of time scales. horizontal advection by the sea breeze plays an impor- Similar to the stratocumulus clouds occurring over the tant role in modulating coastal stratocumulus clouds. ocean, stratocumulus clouds over coastal land attain The transport of air from and to the ocean and the di- maximum coverage at sunrise. After