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An improved understanding of atmospheric perturbations within and above a forest during a wildland fire has relevance to many aspects of wildland fires including fire spread, smoke transport and dispersion, and tree mortality. In this study, the ARPS-CANOPY model, a version of the Advanced Regional Prediction System (ARPS) model with a canopy parameterization, is utilized in a series of idealized numerical experiments to investigate the influence of vertical canopy structure on the atmospheric response to a stationary sensible heat flux at the ground (“fire heat flux”), broadly consistent in magnitude with the sensible heat flux from a low-intensity surface fire. Five vertical canopy structures are combined with five fire heat flux magnitudes to yield a matrix of 25 simulations. Analyses of the fire-heat-flux-perturbed ucomponent of the wind, vertical velocity, kinetic energy, and temperature show that the spatial pattern and magnitude of the perturbations are sensitive to vertical canopy structure. Both vertical velocity and kinetic energy exhibit an increasing trend with increasing fire heat flux that is stronger for cases with some amount of overstory vegetation than cases with exclusively understory vegetation. A weaker trend in cases with exclusively understory vegetation indicates a damping of the atmospheric response to the sensible heat from a surface fire when vegetation is most concentrated near the surface. More generally, the results presented in this study suggest that canopy morphology should be considered when applying the results of a fire-atmosphere interaction study conducted in one type of forest to other forests with different canopy structures. Perturbation of the atmosphere by sensible heat from a surface fire is sensitive to vertical canopy structureCaution is suggested when extrapolating study findings across forests with different canopy structure

Intense precipitation over a short duration is a major cause of flash floods. Using hourly rainfall data from the North America Land Data Assimilation System Phase 2 from 1979 to 2013, we compared the differences in the response of the subdaily extreme precipitation occurrences across the contiguous U.S. to strong anomalous warming over the eastern equatorial Pacific known as El Niño and over the central equatorial Pacific known as El Niño Modoki. For both types of anomalous equatorial Pacific warming, the teleconnection is much stronger in the cold season (November through April) than warm season (May through October). During the warm season, while both types correspond to an increase in subdaily extreme precipitation in areas of Texas and a decrease in some areas of the northern Plains, El Niño is associated with a significant increase in the northern Rockies and El Niño Modoki is associated with a decrease in the Intermountain West. During the cold season, the overall patterns are similar between the two types, with positive anomalies over much of the southern and negative anomalies over the northern U.S. However, large regional differences exist, with El Niño exerting a much stronger influence on subdaily extreme precipitation in the Atlantic and Gulf states as well as California, while the influence of El Niño Modoki is slightly broader over the Midwest. These differences can be largely explained by the vertical velocity, moisture convergence, and jet stream patterns induced by El Niño and El Niño Modoki episodes. Significant regional differences exist in the occurrences of subdaily extreme rainfall in the U.S. during El Niño and El Niño ModokiCold‐season extreme rainfall occurrences in the U.S. respond more strongly to El Niño than to El Niño ModokiTheir influences differ significantly in the Mountain States for the warm season and along the East/Gulf Coasts and CA for the cold season

Based on the results from 11 flux sites during the third Tibetan Plateau (TP) Experiment (TIPEX III), land surface parameters and the turbulence characteristics of the atmospheric surface layer over the TP and surrounding region are analyzed. Monin‐Obukhov similarity theory has been used to calculate the aerodynamic roughness length z0mand the excess resistance to heat transfer kB− 1= ln(z0m/z0h), and the factors that cause variations of z0mand kB− 1are investigated. The main drivers for the diurnal variations of surface albedo (α) at different sites are solar elevation, solar radiation, and soil moisture. The eddy correlation method is utilized to inversely calculate bulk transfer coefficients for momentum (CD) and heat (CH) at different sites. The relationships between CDand CHand the wind speed at 10 m follow a power law for unstable stratification. For stable stratification, both CDand CHincrease with increasing wind speed when wind speed is less than 5 m/s. Diurnal variations of turbulent fluxes are compared at different sites, and the relationships between turbulent fluxes and other variables are analyzed. Wind speed variance normalized by the friction velocity (σu/u*, σv/u*, σw/u*) for neutral stratification (Cu1, Cv1, Cw1), and temperature and humidity variance normalized by a temperature and humidity scale (σT/T*, σq/q*) under free convection (z/L< −0.1) (CT, Cq) are fitted with similarity relations. The differences in similarity constants (Cu1, Cv1, Cw1, CT, Cq) at different sites are discussed. For stable stratification, cases are divided into weakly stable conditions and intermittent turbulence, and the critical values for these two states are determined. Shear and buoyancy terms in the turbulence kinetic energy equation for different stratifications are analyzed. A multiscale network of land‐air flux comprehensive observation sites has been implemented for the first time on the TP in TIPEX IIIHere we place great emphasis on land surface parameters and turbulence characteristics on the TP and surrounding region

Turbulent erosion has been proposed as a major mechanism for removing wintertime cold-air pools (CAPs) from basins and valleys. The time scales involved in this erosion process, which are of great interest for winter weather forecasting, have not been studied systematically in the past. In this short contribution, a semi-analytical model is developed to estimate the time required for the dissipation of cold air pools from above by downward micro-scale turbulent erosion for different wind speeds aloft, different static stabilities inside the cold pool, and for different basin cross sections. The calculations show that micro-scale turbulent erosion is a rather slow process and that the erosion rate decreases rapidly with time as static stability increases in the capping inversion at the top of the cold pool. The rate of erosion is determined mainly by wind speed above the CAP and the temperature inversion strength inside the CAP; it is less sensitive to the shape of the topography. Shallow CAPs of a few tens of meters in depth with a weak inversion may be removed in a matter of hours if winds aloft are sufficiently strong to initiate and maintain turbulent mixing. It is unlikely, however, that deeper CAPs with a moderate to strong inversion can be destroyed by micro-scale turbulent erosion unless combined with other regional and synoptic-scale processes that produce larger-scale turbulent mixing. Erosion durch Turbulenz ist schon lange als wesentlicher Prozess bei der Zerstörung von winterlichen Kaltluftseen in Becken und TÄlern betrachtet worden. Der Zeitraum dieses Erosionsprozesses, der besonders wichtig für Vorhersagen im Winter ist, ist bis jetzt jedoch noch nicht systematisch untersucht worden. In diesem Beitrag wird ein semi-analytisches Modell beschrieben, das die benötigte Zeit für die Auflösung eines Kaltluftsees durch mikroskalige turbulente Erosion von oben abschÄtzt. Dieses Modell berücksichtigt die Windgeschwindigkeit oberhalb und die StabilitÄt innerhalb des Kaltluftsees sowie den Querschnitt des Beckens bzw. Tales. Die Berechnungen zeigen, dass mikroskalige turbulente Erosion ein langsamer Prozess ist und dass sich die Erosionsrate im Laufe der Zeit stark verlangsamt, wenn die statische StabilitÄt in der abgrenzenden Inversion oberhalb des Kaltluftsees zunimmt. Die Erosionsrate wird vor allem von der Windgeschwindigkeit oberhalb des Kaltluftsees und vom Temperaturgradienten innerhalb des Sees bestimmt. Der Querschnitt des Beckens bzw. Tales spielt dabei eine weniger wichtige Rolle. Ein flacher Kaltluftsee von einigen Metern (< 50 m) mit einem schwachen Temperaturgradienten kann durch mikroskalige turbulente Erosion in einigen Stunden zerstört werden, wenn die Windgeschwindigkeit oberhalb des Sees ausreichend ist, um turbulente Durchmischung zu initiieren bzw. aufrechtzuhalten. Es ist unwahrscheinlich, dass ein tiefer Kaltluftsee mit einem mÄßigen bis starken Temperaturgradienten allein durch mikroskalige turbulente Erosion zerstört werden kann, außer dass regionale oder synoptische Prozesse vorhanden sind, die größere Wirbel verursachen.

A short review of solar semidiurnal atmospheric tides is presented. Semidiurnal atmospheric tides have been documented in the troposphere primarily through analyses of long time series of surface pressure measurements, although the winds produced by these tides have, by now, been well documented in the middle and upper atmosphere. Recent research using UHF and very-high-frequency radar wind profilers has now identified tidal wind perturbations in tropospheric data. This review focuses on the tidal wind characteristics and the distinctive signature of this wind system in radar profiler data analyses.