Investigating the Impacts of Daytime Boundary Layer Clouds on Surface Energy Fluxes and Boundary Layer Structure During CHEESEHEAD19. Issue 5 (3rd March 2022)

• Record Type: Journal Article
• Authors: Sedlar, J.; Riihimaki, L. D.; Turner, D. D.; Duncan, J.; Adler, B.; Bianco, L.; Lantz, K.; Wilczak, J.; Hall, E.; Herrera, C.; Hodges, Gary B.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-03
• Subjects: Surface energy budget -- Clouds -- Land‐atmosphere -- Boundary layer -- Heterogeneous surface; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD036060
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Studies of land‐atmosphere interactions under a clear sky and low cumulus cloud conditions are common from long‐term observatories like at the southern great plains. How well the relationships and responses of surface radiative and turbulent heat fluxes determined from these investigations hold for more heterogeneous surfaces in other climate regimes, however, is uncertain. In this study, detailed observations of the surface energy budget and daytime boundary layer properties are analyzed using measurements from the Chequamegon Heterogenous Ecosystem Energy‐Balance Study Enabled by a High‐Density Extensive Array of Detectors 2019 (CHEESEHEAD19) field campaign, July‐October 2019, across a heterogeneous forested landscape of northern Wisconsin. A cloud regime framework is employed to classify consecutive periods of clear skies from lower atmosphere stratiform and cumulus clouds. A seasonal transition from low cumulus to low stratiform periods occurred, together with a diurnal pattern in cloudy or clear sky period dominance. Radiative forcing was highly dependent on sky conditions, leading to changes in the redistribution efficiency of radiative energy by the surface turbulent heat fluxes. During CHEESEHEAD19, small Bowen ratios dominated with daytime latent heat fluxes three times as large as sensible heat fluxes for all sky conditions studied; the forested region, therefore, falls within an energy‐limited regime. The depth of the daytime mixed layer depended upon the sky condition and thermodynamic setting; deeper mixed layers occurred during periods of low cumulus and not clear skies. Profiles of vertical velocity were found to have enhanced variance under low cumulus compared to clear sky periods, suggesting potential for cloud feedbacks on boundary layer structure and surface energy fluxes. Plain Language Summary: This study investigates how different cloud regimes influence the exchange of energy at Earth's surface over a highly heterogeneous forested landscape in northern Wisconsin. Clouds directly modify the solar and infrared radiation reaching the surface. In turn, the modifications to radiation affect how turbulence near the surface is generated and its magnitude. The net result of these energy fluxes determines the warming and cooling processes at the surface, with direct implications on the development of local weather systems. From the observations in northern Wisconsin, an apparent partition in energy fluxes between two commonly observed lower atmosphere cloud types is found. A seasonal pattern in the occurrence of these cloud types was observed. Because of these cloud‐specific preferences, turbulence generated near the surface was larger during the first half of the 3‐month field campaign. Enhanced surface fluxes supported a deeper boundary layer for the shallow cumulus cloud conditions compared to overcast conditions. These fluxes were even larger than periods when skies were clear; however clear sky conditions were most frequent during the morning when surface energy fluxes were generally increasing with time during this development stage of the convective mixed layer. Turbulent fluxes associated with evaporation at the surface dominated over dry fluxes, regardless of the overhead sky conditions. These findings represent important differences to heavily study climatological regions where grasslands make up the primary surface characteristics. Key Points: Detailed surface energy fluxes and boundary layer structure responses to three boundary layer sky conditions are examined: low stratiform, low cumulus, clear Turbulent and radiative flux relationships were separable by cloud regime; latent fluxes dominated over sensible fluxes by a factor of 3 (low Bowen ratios dominated) Boundary layers were deeper during low cumulus compared to clear sky periods; surface‐atmosphere interactions are complex over the heterogenous forest landscape