Atmos. Chem. Phys., 18, 15087–15104, 2018 https://doi.org/10.5194/acp-18-15087-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. The monsoon effect on energy and carbon exchange processes over a highland lake in the southwest of China Qun Du1, Huizhi Liu1,2, Lujun Xu1, Yang Liu1,2, and Lei Wang1 1State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 2University of Chinese Academy of Sciences, Beijing 100029, China Correspondence: Huizhi Liu ([email protected]) Received: 4 January 2018 – Discussion started: 30 January 2018 Revised: 3 September 2018 – Accepted: 2 October 2018 – Published: 19 October 2018 Abstract. Erhai Lake is a subtropical highland shallow lake ing a large impact of the monsoon on the heat and carbon on the southeast margin of the Tibetan Plateau, which is exchange processes over Erhai Lake. influenced by both South Asian and East Asian summer monsoons. Based on 4 years of continuous eddy covariance (EC) data over Erhai Lake, the monsoon effect on water– atmosphere exchange processes is investigated by comparing 1 Introduction the energy and CO2 flux patterns and their main drivers dur- ing pre-monsoon (March–April), monsoon (May–October) There are 304 million lakes globally and they are of signif- and post-monsoon (November–December) periods. The re- icant importance in determining local weather and climate sults show that the atmospheric properties display a large through complex physical, biochemical and biological inter- difference during the three different periods. There is a nega- actions (Cole et al., 2001; Downing et al., 2006; Shao et tive difference between water surface and air temperature (T ) al., 2015). Because of the substantial differences in underly- during the pre-monsoon period, while a positive T during the ing surface characteristics between lake surface and its sur- post-monsoon period. The diurnal sensible heat flux (Hs) is rounding land surface (i.e., albedo, roughness, heat capacity; larger during the post-monsoon period, while the latent heat O’Donnell et al., 2010), the carbon and energy exchange pro- flux (LE) is larger during the monsoon period. The monthly cesses over lakes are expected to respond different way to average Hs and heat storage (Q) in the lake remain nega- climate change. Lakes react rapidly to a change in the atmo- tive during the pre-monsoon period and the early monsoon spheric parameters and are able to modify the surrounding period, and they become positive in the middle monsoon pe- atmospheric circulation (Marie-Noëlle et al., 2012). Plenty riod, which indicates that the lake absorbs heat at first and of studies on water–atmosphere carbon and energy exchange releases it subsequently. LE plays a dominating role in the processes have been reported over high-latitude water bod- energy partitioning of the lake. The Bowen ratio is higher ies (Nordbo et al., 2011; Huotari et al., 2011; Mammarella et during the post-monsoon period. The uptake of CO2 flux is al., 2015). However, the characteristics of water–atmosphere observed in the middle of the day during monsoon and post- exchange processes differ for lake size, water depth, regional monsoon periods. The 1T is the main driver for Hs and the climate and geographical location (Liu et al., 2009). High- effect of 1T is increased as timescales are extended from altitude lakes are exposed to more extreme meteorological half-hourly to monthly. The wind speed has a weak effect on conditions and are more sensitive to variations in meteoro- Hs but a strong effect on LE and CO2 fluxes. Similar main logical forcing (Rueda et al., 2007). Shallow lakes respond drivers for Hs are found during the pre-monsoon and post- more quickly to changes in the atmospheric forcing due to a monsoon periods, which is also found for CO2 flux, indicat- smaller heat capacity (Liu et al., 2012; Zhang and Liu, 2013). Understanding the turbulent exchange processes between the lake surface and atmosphere and the response to atmospheric Published by Copernicus Publications on behalf of the European Geosciences Union. 15088 Q. Du et al.: The monsoon effect on lake–atmosphere exchanges properties is essential for improving numerical weather pre- fect the lake–air CO2 flux. Huotari et al. (2011) reported that diction and climate models (Dutra et al., 2010; Nordbo et al., the CO2 efflux was enhanced under persistent extratropical 2011). cyclone activities over high-latitude water bodies. The syn- The change in atmospheric properties over a water surface optic weather events associating with extratropical cyclones can cause large fluctuations in atmospheric forcing for lake– produced larger CO2 effluxes by bringing the bottom rich atmosphere interactions, and subsequently affects the turbu- CO2 water to the surface through upwelling, internal wave- lent exchange processes (Lenters et al., 2005; Liu et al., 2011; induced mixing and mixing by convection (Liu et al., 2016). Huotari et al., 2011; Z. Li et al., 2015). The southeasterly The windy and stormy days increased 16 % of the annual wind with warm moist air masses reduced and inverted the CO2 effluxes over Ross Barnett reservoir in central Missis- vertical temperature difference between water surface and sippi, USA (Liu et al., 2016). A 15-year long study found that atmosphere to be negative over a large high-latitude saline the amount of precipitation had a large effect on dissolved lake (Qinghai Lake) on the northeastern Qinghai–Tibetan organic carbon (DOC) concentrations in rivers (Pumpanen et Plateau (QTP) in China (Li et al., 2016). The cold fronts and al., 2014). The waterside convection was believed to cause the meteorological properties of the air masses behind cold the higher CO2 fluxes during night compared to day (Pod- fronts (e.g., windy, cold and dry) significantly promoted tur- grajsek et al., 2014). bulent exchange of sensible heat (Hs) and latent heat (LE) Erhai Lake is a subtropical highland shallow lake on the through enhanced turbulent mixing (thermally and mechan- southeast margin of TP, which is influenced by both the ically), whereas southerly winds with warm and humid air South Asian and East Asian summer monsoons. The sum- masses generally suppressed turbulent exchanges of Hs and mer monsoon induces an abrupt change in large-scale atmo- LE over a mid-latitude large reservoir in Mississippi (Liu et spheric circulation and convective activity over Asia, and car- al., 2009, 2012). In response to the changes in the weather ries in air mass with distinct atmospheric properties (i.e., air conditions, the heat balance over a large tropical reservoir in temperature, wind direction, relative humidity; Li and Yanai, Brazil is substantially altered, and the heat loss can be twice 1996; Lau and Yang, 1997; Zhou et al., 2012). The seasonal or 3-fold greater during cold-front days than that during the reversals of atmospheric properties caused by summer mon- non-cold-front days (Curtarelli et al., 2014). Consistent diur- soon circulation play an important role in regulating land– nal peaks in LE flux during the afternoon were observed as atmosphere heat and water exchange processes (Flohn, 1957; a result of strong, dry winds coinciding with peak water sur- Hsu et al., 1999). The land–atmosphere exchange processes face temperatures over a small subtropical reservoir in Aus- are found to be closely related to the onset and retreat of sum- tralia (McGloin et al., 2015). An increasing sensible heat flux mer monsoon (Zhang et al., 2012). It is reported that most over the lake retarded the cooling of lower atmosphere (be- of the available energy was transformed into Hs before the low 500 m) and weakened the vertical potential temperature arrival of monsoonal winds, whereas LE increased and ex- gradient over the lake, while increasing wind speed and verti- ceeded Hs after the onset of monsoonal wind (Xu et al., 2009; cal wind shear further facilitated the buoyancy flux to exert a Mauder et al., 2007; Li et al., 2016). Considering the substan- higher heat convection efficiency when cold air arrived over tial difference in atmospheric properties during the monsoon Ngoring Lake in the Tibetan Plateau (TP; Li et al., 2017). and non-monsoon periods, the water–atmosphere carbon and The CO2 emissions from lakes are traditionally measured energy exchange processes are expected to display large dif- by non-continuous or indirect methods, e.g., floating cham- ferences. However, few studies have reported the variation in ber (Riera et al., 1999) and boundary layer transfer tech- heat and carbon fluxes over lakes during different monsoonal niques (Cole and Caraco, 1998). The uncertainty in the float- periods, and the effect of the monsoon on water–atmosphere ing chamber method is that the flux it measures only repre- heat and carbon is not clear. sents a very small area and it produces biases because the Four years of continuous EC measurements from 2012 to disturbances in the water–air surface (Vachon et al., 2010). 2015 have been obtained over Erhai Lake. The summer mon- The boundary layer method estimates CO2 flux by the dif- soon generally bursts in May and retreats in October. Ac- ference in CO2 concentration between the water and atmo- cording to the activity of summer monsoon, three monsoon sphere and the gas transfer velocity, which is traditionally periods are defined, including pre-monsoon (March–April), parameterized only by wind speed (Cole and Caraco, 1998). monsoon (May–October) and post-monsoon (November– However, it has been reported that different processes includ- December) periods. We hypothesize that the contrasting at- ing convection, microwave breaking and stratification could mospheric properties during these three different monsoon influence the gas transfer velocity (Zappa et al., 2001; Eu- periods play an important role in modulating the turbulent gster et al., 2003; Podgrajsek et al., 2014).