Exploratory cloud-resolving simulations of boundary-layer Arctic stratus clouds

[1] Radiative heating and cooling occurs throughout most of a stratocumulus cloud layer. Shortwave (SW) heating of individual drops can be strong enough that drop temperatures at equilibrium deviate by as much as 6°C from the surrounding environment. These large temperature differences lead to substantial errors when the classical equation for vapor growth is used. A new form of the vapor growth equation is derived for cases of strong radiative heating. The new equation is used to assess the equilibrium supersaturation (s eq ) state of drops and cloud condensation nuclei (CCN) within a stratocumulus cloud. Longwave (LW) cloud top cooling has two primary effects on s eq . It tends to reduce s eq for large drops to values as low as À6%, providing for drop growth at subsaturations. It also tends to reduce the critical supersaturation and size at which larger CCN activate to become growing cloud drops. In contrast, LW heating of cloud base suppresses the growth of cloud drops and CCN. Larger CCN (sizes between 0.5 and 5 mm) lose their Köhler curve maximum, indicating restricted CCN growth. Solar heating produces s eq up to 20% for drops with sizes between 500 and 1000 mm, indicating strong evaporation of drizzle drops. Since solar absorption increases more slowly with drop size than LW emission, a minimum appears in the Köhler curves at sizes between 20 and 100 mm, suggesting a preferred size range for vapor growth in SW heated clouds. Like LW heating, SW heating causes suppression of growth for larger CCN.

Section: Methodsmentioning

confidence: 99%

Radiative influences on drop and cloud condensation nuclei equilibrium in stratocumulus

Marquis

Harrington

2005

“…RAMS has been used for cloud-resolving model (CRM) simulations of arctic stratus clouds over sea ice in warm [Olsson et al, 1998] and transition [Harrington et al, 1999] several times that of a LES domain, as these may effect the cloud evolution and BL structure.…”

Section: Numerical Simulationsmentioning

confidence: 99%

Dynamics and energetics of the cloudy boundary layer in simulations of off‐ice flow in the marginal ice zone

Olsson

Harrington

2000

Abstract. The case under congideration occurred on March 4, 1993, and was observed as part of the Radiation and Eddy Flux Experiment (REFLEX II) 1993 observational campaign northwest of Spitsbergen. The off-ice flow on this day brought very cold surface air temperatures (-35øC) over a relatively warm ocean surface. The resultant latent and sensible surface heat fluxes produced intense convection and a thermal internal boundary layer (TIBL) which deepened with distance from the ice edge. Two-dimensional cloudresolving model (CRM) simulations were performed to determine the impact of various cloud parameterizations on the structure and evolution of the TIBL. The model was able to reproduce the observed thermal structure of the boundary layer to within the acknowledged limitations of the CRM approach. Sensitivity studies of cloud type showed that inclusion of mixed-phase microphysics had a large impact of BL depth and structure. Radiative heating of the cloud near cloud base and cooling near cloud top along with latent heat release were found to be significant sources of turbulence kinetic energy even in the present case where very strong surface heat fluxes occur. Ice-phase precipitation processes rapidly depleted the BL of condensate, weakening the radiative thermal forcing. A further consequence of condensate depletion in the mixed-phase cloud was a less humid boundary layer that was able to maintain a larger surface latent heat flux and continuously extract heat through condensation and deposition. Not surprisingly, the presence of clouds had a profound impact on the radiative budget at the surface, with the cloudy BL reducing surface radiative losses more that 60% over clear-sky values. Inclusion of the ice phase significantly affected the radiative budget as compared to purely liquid clouds, illustrating the importance of ice-phase-radiative couplings for accurate simulations of arctic clouds and boundary layer dynamics. IntroductionThe marginal ice zone (MIZ) is that region within which, more or less, total sea-ice coverage gives way to a generally ice-free ocean.

“…[7] Although some of these models were based on the explicit bin-resolving microphysics [e.g., Khvorostyanov, 1991Khvorostyanov, , 1995Bakan et al, 1995;Olsson et al, 1998;Harrington et al, 1999;Jiang et al, 2000Jiang et al, , 2001Olsson and Harrington, 2000;Khvorostyanov et al, 2001], most of the aforementioned simulation studies used bulk microphysical parameterizations. In bulk microphysical parameterizations, assumptions must be made about the temperature at which ice nucleation occurs, the threshold for the so-called autoconversion of cloud water to rain or snow, the relative humidity threshold for nucleation, and the relaxation times for condensation and sublimation.…”

Section: Introductionmentioning

confidence: 99%

“…These simulations include boundary layer stratus formed during the summertime [Buikov et al, 1981;Khvorostyanov, 1995;Olsson et al, 1998], and transition seasons [Harrington et al, 1999;Jiang et al, 2000]; cloud streets formed during a cold air outbreak [Bakan et al, 1995]; and a two-layer cloud system consisting of cirrus and altostratus [Khvorostyanov et al, 2001]. These simulations showed high sensitivity of the simulated cloud properties to the cloud microphysics.…”

Section: Introductionmentioning

confidence: 99%

A springtime cloud over the Beaufort Sea polynya: Three‐dimensional simulation with explicit spectral microphysics and comparison with observations

Khvorostyanov

Curry

Gültepe³

et al. 2003

[1] This paper addresses the formation of a cloud system associated with an arctic polynya in Beaufort Sea during springtime. Data were obtained as a part of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Arctic Clouds Experiment from the Canadian Convair 580 aircraft during 25 April 1998. These data include in situ observations of cloud microphysics and meteorological variables and remote measurements from satellite and airborne lidar. A three-dimensional cloud-resolving model with explicit bin-resolving cloud microphysics is used to simulate the atmospheric boundary layer and cloud evolution associated with the polynya. After initialization with the aircraft sounding profiles, a quasi-steady polynya-induced atmospheric boundary layer (ABL) and a cloud system form. Strong turbulence in the ABL occurs above the polynya, the internal thermal boundary layer (ITBL) develops and grows upward in the downwind direction, and the ABL downwind of the polynya is separated into two decoupled layers. The cloud forms in the middle of the polynya, and its upper boundary lifts downwind while the lower boundary lowers and reaches the surface beyond the polynya southern boundary as a light fog. Farther to the south, the fog evaporates and transforms into an elevated cloud plume that extends for several tens of kilometers downwind. This cloud morphology is in good agreement with the lidar observations, which showed a cloud layer extending for more than 100 km downwind, and with the numerous previous observations. The simulated microphysical properties of a mixed cloud are similar to those observed. A new ice nucleation scheme resulted in cloud plume gradual crystallization along the wind and eventual transformation into diamond dust. Detailed evaluation of the water budget, supersaturation, and crystal size spectra showed that the ice crystal supersaturation relaxation times are 10-60 min; thus deposition on the crystals is rather slow, and only 1-5% of the available vapor is deposited on the crystals. The large ice crystal supersaturation relaxation time explains the relatively slow growth and gravitational fallout of the ice crystals, as well as the extensive propagation downwind of the plume.