scholarly journals Intraseasonal Variability in a Cloud-Permitting Near-Global Equatorial Aquaplanet Model

2018 ◽  
Vol 75 (12) ◽  
pp. 4337-4355 ◽  
Author(s):  
Marat F. Khairoutdinov ◽  
Kerry Emanuel
Keyword(s):  
Intraseasonal Variability ◽  
Atmospheric Modeling ◽  
Sea Surface ◽  
Driving Mechanism ◽  
Madden Julian Oscillation ◽  
Grid Spacing ◽  
Beta Plane ◽  
Surface Heat Exchange ◽  
Primary Driver ◽  
Self Aggregation

Abstract Recent studies have suggested that the Madden–Julian oscillation is a result of an instability driven mainly by cloud–radiation feedbacks, similar in character to self-aggregation of convection in nonrotating, cloud-permitting simulations of radiative–convective equilibrium (RCE). Here we bolster that inference by simulating radiative–convective equilibrium states on a rotating sphere with constant sea surface temperature, using the cloud-permitting System for Atmospheric Modeling (SAM) with 20-km grid spacing and extending to walls at 46° latitude in each hemisphere. Mechanism-denial experiments reveal that cloud–radiation interaction is the quintessential driving mechanism of the simulated MJO-like disturbances, but wind-induced surface heat exchange (WISHE) feedbacks are the primary driver of its eastward propagation. WISHE may also explain the faster Kelvin-like modes in the simulations. These conclusions are supported by a linear stability analysis of RCE states on an equatorial beta plane.

Idealized large-eddy simulations of stratocumulus advecting over cold water. Part 1: Boundary layer decoupling

2021 ◽  
Author(s):  
Youtong Zheng ◽  
Haipeng Zhang ◽  
Daniel Rosenfeld ◽  
Seoung-Soo Lee ◽  
Tianning Su ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Surface Temperature ◽  
Cold Water ◽  
Temperature Inversion ◽  
Atmospheric Modeling ◽  
Sea Surface ◽  
Entrainment Rate ◽  
Surface Cooling ◽  
Near Surface ◽  
Large Eddy

AbstractWe explore the decoupling physics of a stratocumulus-topped boundary layer (STBL) moving over cooler water, a situation mimicking the warm air advection (WADV). We simulate an initially well-mixed STBL over a doubly periodic domain with the sea surface temperature decreasing linearly over time using the System for Atmospheric Modeling large-eddy model. Due to the surface cooling, the STBL becomes increasingly stably stratified, manifested as a near-surface temperature inversion topped by a well-mixed cloud-containing layer. Unlike the stably stratified STBL in cold air advection (CADV) that is characterized by cumulus coupling, the stratocumulus deck in the WADV is unambiguously decoupled from the sea surface, manifested as weakly negative buoyancy flux throughout the sub-cloud layer. Without the influxes of buoyancy from the surface, the convective circulation in the well-mixed cloud-containing layer is driven by cloud-top radiative cooling. In such a regime, the downdrafts propel the circulation, in contrast to that in CADV regime for which the cumulus updrafts play a more determinant role. Such a contrast in convection regime explains the difference in many aspects of the STBLs including the entrainment rate, cloud homogeneity, vertical exchanges of heat and moisture, and lifetime of the stratocumulus deck, with the last being subject to a more thorough investigation in part 2. Finally, we investigate under what conditions a secondary stratus near the surface (or fog) can form in the WADV. We found that weaker subsidence favors the formation of fog whereas a more rapid surface cooling rate doesn’t.

scholarly journals The Impacts of Horizontal Grid Spacing and Cumulus Parameterization on Subseasonal Prediction in a Global Convection-Permitting Model

2020 ◽  
Vol 148 (12) ◽  
pp. 4747-4765
Author(s):  
Nicholas J. Weber ◽  
Clifford F. Mass ◽  
Daehyun Kim
Keyword(s):  
Global Model ◽  
Forecast Skill ◽  
Cumulus Parameterization ◽  
Madden Julian Oscillation ◽  
Grid Spacing ◽  
Variable Resolution ◽  
Channel Simulation ◽  
Convective Parameterization ◽  
Tropical Precipitation ◽  
The Tropics

AbstractMonthlong simulations targeting four Madden–Julian oscillation events made with several global model configurations are verified against observations to assess the roles of grid spacing and convective parameterization on the representation of tropical convection and midlatitude forecast skill. Specifically, the performance of a global convection-permitting model (CPM) configuration with a uniform 3-km mesh is compared to that of a global 15-km mesh with and without convective parameterization, and of a variable-resolution “channel” simulation using 3-km grid spacing only in the tropics with a scale-aware convection scheme. It is shown that global 3-km simulations produce realistic tropical precipitation statistics, except for an overall wet bias and delayed diurnal cycle. The channel simulation performs similarly, although with an unrealistically higher frequency of heavy rain. The 15-km simulations with and without cumulus schemes produce too much light and heavy tropical precipitation, respectively. Without convection parameterization, the 15-km global model produces unrealistically abundant, short-lived, and intense convection throughout the tropics. Only the global CPM configuration is able to capture eastward-propagating Madden–Julian oscillation events, and the 15-km runs favor stationary or westward-propagating convection organized at the planetary scale. The global 3-km CPM exhibits the highest extratropical forecast skill aloft and at the surface, particularly during week 3 of each hindcast. Although more cases are needed to confirm these results, this study highlights many potential benefits of using global CPMs for subseasonal forecasting. Furthermore, results show that alternatives to global convection-permitting resolution—using coarser or spatially variable resolution—feature compromises that may reduce their predictive performance.

scholarly journals Connection between Sea Surface Anomalies and Atmospheric Quasi-Stationary Waves

2020 ◽  
Vol 33 (1) ◽  
pp. 201-212
Author(s):  
G. Wolf ◽  
A. Czaja ◽  
D. J. Brayshaw ◽  
N. P. Klingaman
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Southern Oscillation ◽  
Late Summer ◽  
Ice Cover ◽  
Sea Surface ◽  
Late Winter ◽  
Driving Mechanism ◽  
The North ◽  
The North Atlantic

AbstractLarge-scale, quasi-stationary atmospheric waves (QSWs) are known to be strongly connected with extreme events and general weather conditions. Yet, despite their importance, there is still a lack of understanding about what drives variability in QSW. This study is a step toward this goal, and it identifies three statistically significant connections between QSWs and sea surface anomalies (temperature and ice cover) by applying a maximum covariance analysis technique to reanalysis data (1979–2015). The two most dominant connections are linked to El Niño–Southern Oscillation and the North Atlantic Oscillation. They confirm the expected relationship between QSWs and anomalous surface conditions in the tropical Pacific and the North Atlantic, but they cannot be used to infer a driving mechanism or predictability from the sea surface temperature or the sea ice cover to the QSW. The third connection, in contrast, occurs between late winter to early spring Atlantic sea ice concentrations and anomalous QSW patterns in the following late summer to early autumn. This new finding offers a pathway for possible long-term predictability of late summer QSW occurrence.

scholarly journals A Parameterization Scheme for Air–Sea Surface Interface Fluxes: Design and Stand-Alone Experiments

2018 ◽  
Vol 75 (7) ◽  
pp. 2359-2383
Author(s):  
Haixiong Zhuang ◽  
Xiaojun Yang ◽  
Zhenling Wu
Keyword(s):  
Mixed Layer ◽  
Surface Wind ◽  
Experimental Studies ◽  
Ocean Surface ◽  
Atmospheric Modeling ◽  
Surface Interaction ◽  
Sea Surface ◽  
Bulk Temperature ◽  
Ocean Mixed Layer ◽  
Energy Fluxes

Abstract Observations show that sea surface temperature (SST) can vary up to several degrees in a day and sea surface energy fluxes up to a few hundreds of watts per square meter. For synoptic- and subsynoptic-scale atmospheric modeling, there remains a need for the parameterization of air–sea surface interaction using simple schemes. In this paper, such a simple scheme, Atmosphere–Ocean Surface Interaction Scheme (AOSIS), is presented so that the short time variations in SST and energy fluxes can be estimated using a small number of atmospheric and oceanic bulk quantities. The scheme consists of three components: a two-layer ocean temperature model, a wind-wave model, and a surface flux model. Numerical experiments show that the scheme performs well in simulating SST and the air–sea exchanges. Relative to other schemes, AOSIS shows the following improvements: 1) it simulates SST and the cool-skin and warm-layer effect of the ocean mixed layer without the input of ocean bulk temperature of the mixed layer as a prior condition, which is required by most one-layer models; 2) the depth of the ocean mixed layer is allowed to vary according to surface wind stress and buoyancy flux; and 3) a method for computing ocean surface roughness length is proposed, which accounts for the aerodynamic effect of wind-generated waves. For experimental studies, AOSIS can be used in stand-alone mode for the calculation of SST through a small number of bulk measurements. AOSIS can also be used as an interface between the atmosphere and ocean models to be coupled together.

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