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David R. Fitzjarrald

Abstract

This paper presents a one-dimensional model of katabatic winds in ambient flow and examines types of possible solutions. Results presented in dimensionless form indicate that 1) cooling along a slope with upslope ambient flow can lead to tranquil (flow speed less than gravity wave speed) solutions characterized by appreciable entrainment, and 2) the onset time of katabatic winds is particularly sensitive to the retarding effects of opposing flow and reduced cooling rates. Model results present a plausible explanation for observed delays in the onset time of downslope flow of up to 8 h after sunset along certain humid tropical slopes.

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David R. Fitzjarrald

Abstract

Detailed boundary-layer measurements are presented to show how slope winds affect the regional climate in central Veracruz state, Mexico. Observations include the growth of an anabatic convective boundary layer in summer, production of katabatic storm outflows moving contrary to prevailing easterlies, and detailed sequences of the wind and temperature in katabatic wind. The onset time for downslope motion is just before dawn after a clear night in summer, although observed within three hours of clear-sky conditions in winter. The delay in onset is most likely due to opposing upslope flow in summer. The vertical structure of the katabatic wind resembles that of entraining gravity flows. We make a distinction between an inner katabatic layer, near the level of wind maximum, in which the buoyancy force is large, and an entraining outer layer above, that is primarily forced by the inner layer.

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Leif Kristensen and David R. Fitzjarrald

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We present a theoretical analysis of the effect of line averaging by a sonic anemometer on scalar fluxes and an observational study of this phenomenon in the atmospheric surface layer. The theoretical analysis rests on an axisymmetric model for the cross-spectral tensor of vertical velocity and scalar fluctuations, limiting the validity of the derived transfer function to a constant-flux layer. Observations of temperature flux in the unstable surface layer confirm the theoretical prediction that line averaging does not significantly affect the flux estimate down to heights only several times the sonic path length. However, the observations exhibit large scatter at small height, indicating that problems with the representativity of the measurement and not with line averaging may become a limiting factor. Based on the analysis of the Kansas data and the characteristics of the transfer function, we infer that temperature flux measurements will, in general, be affected by some line averaging under stable conditions at all heights in the surface layer. In these cases, the theoretical transfer function may be used to correct the measured heat fluxes.

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David R. Fitzjarrald and Michael Garstang

Abstract

Results of a one-dimensional mixed-layer growth model are compared with thermodynamic observations made during the GARP Atlantic Tropical Experiment (GATE). Observed drying at low levels accompanying warming in the wake of a storm is hypothesized to be the result of rapid mixed-layer growth as well as of subsidence in the environment outside the storm. Sensitivity of the model solutions to changes in subsidence and stability above the mixed layer is shown. Model solutions show protracted recovery periods only when the wind speed near the surface is moderate (∼3 m s−1). Recovery of the mixed layer in the wake of the 12 September 1974 GATE squall line is simulated by the model.

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Luiz E. Medeiros and David R. Fitzjarrald

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The authors examine how terrain texture and topography influence nocturnal mixing rates. Local topographic curvature and site sheltering exhibit systematic influences on nocturnal heat and momentum fluxes and the near-surface potential temperature distribution. This influence is particularly evident in hilly terrain with patchy forested areas, typical of eastern North America and many other regions. Exposure to local obstacles, quantified using Fujita’s “transmission factor,” has its maximum influence on mixing during strong winds (>5 m s−1), whereas the effects of local terrain curvature dominate under weaker winds. Such complementary dominance conditions currently limit direct comparison of the two effects. Even with a limited network of 10 stations, it is clear that preferred regions for mixing can be identified in advance given knowledge of land cover and topography. When designing a network of surface stations to be deployed in heterogeneous terrain, one should consider site curvature, slope, and exposure in addition to spatial coverage.

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Luiz E. Medeiros and David R. Fitzjarrald

Abstract

Average heat and momentum fluxes observed by a network of surface stations during the Hudson Valley Ambient Meteorology Study (HVAMS) were found as functions of a spatially representative bulk Richardson number Ribr. Preferential sites were identified for the occurrence of strong turbulence under mesoscale stability conditions common to all stations. Locally sensed turbulence intermittency depends on the mesoscale flow stability. Nearly continuous turbulence with few long-lived intermittent events occurs when Ribr < Ricr, the critical gradient Richardson number. Less-continuous mixing associated with a larger number of events occurs when Ricr < Ribr < 5, with the weakest turbulence and fewer events observed for Ribr ≫ Ricr. It was found that the need to allow for extra mixing above the conventional critical bulk Richardson number in numerical weather prediction models is primarily a consequence of spatial averaging in a heterogeneous landscape and is secondarily the result of turbulence above Ricr at locations with “nonideal fetch.”

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Matthew J. Czikowsky and David R. Fitzjarrald

Abstract

Hydrologic records provide some of the most widespread, long-term data available that can be expected to contain signals of climate variation over time. Yet such data have not been used to identify the widespread effects of spring onset in the forested eastern United States, where streamflow is strongly influenced by vegetation. Three independent runoff characteristics are affected by the enhanced evapotranspiration (ET) that occurs with annual leaf emergence: (i) PR, the difference between precipitation and runoff, returns to dormant season values and increases due to ET; (ii) the streamflow recession time constant (a measure of the time required after rainfall for streams to return to their baseflow levels) shortens; and (iii) the diurnal streamflow amplitude increases. The smallest watersheds in our dataset (area <200 km2) exhibit the diurnal streamflow signal most often and with the greatest amplitude, in accord with accepted relationships between channel length and watershed area. The dynamics of ET effects on runoff characteristics, illustrated using a simple model, suggest that the recession time constant and diurnal amplitude depend on bulk characteristics of a watershed. Using the PR, streamflow recession, and diurnal streamflow signal methods, a spring onset date is obtained from the historical hydrologic datasets. The PR and streamflow recession spring onset dates proceed more slowly than the diurnal amplitude spring onset date and those obtained by independent methods.

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Otávio C. Acevedo and David R. Fitzjarrald

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On clear nights with appreciable radiative cooling, rates of change of mean quantities observed in the first 1 or 2 h after sunset are many times larger than they are subsequently until sunrise. These variations include large temperature drops, specific humidity increases, and abrupt wind speed decay. The early evening transition (EET) is dominated by vertical surface flux convergence as the turbulent mixing layer becomes confined to a shallow stable layer near the surface. Effects of surface heterogeneities are enhanced by the subsequent small eddy size, so that spatial variability of mean variables peaks during the EET. Hilltops do not experience such large variations at EET, as turbulence persists longer at those locations. Wind speed decays faster at obstructed sites, which show earlier transitions. The Richardson number increases exponentially at the EET, and the rate of exponential increase is proportional to the rate of wind decay. Qualitative aspects of the EET at a single point are resolved by a two-layer model, similar to those currently used in parameterizations of the surface exchange in mesoscale simulations. However, in order to simulate the different behaviors observed in an area as big as one grid cell in a mesoscale model, very different values of the geostrophic wind need to be imposed. A large eddy model is used to verify that the specific humidity jump and maximum cooling rate during the EET are primarily the consequence of enhanced vertical flux divergence, though in later stages of the EET, advective effects become more important.

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David R. Fitzjarrald and G. Garland Lala

Abstract

Observations of 14 cases of radiation fog in the Hudson River valley in New York State are presented. Our emphasis is to connect the fog prediction problem to mechanisms in the nocturnal boundary layer that influence heat and moisture balances. Surface layer and boundary layer fogs are distinguished by the difference in dominant terms in the saturation specific humidity deficit budget. Fogs that persist longer than approximately 30 minutes are most frequently thicker than 50 m. The ultimate depth to which the fog grows is shown to be determined by initial conditions at sunset and by subsequent evolution of winds in the nocturnal boundary layer, as well as by surface transports and radiative cooling. Estimates of the surface and boundary layer heat budget are presented. Two new phenomena are identified: 1) A jump in specific humidity occurring during the early evening transition that shortens the time required to reach surface layer saturation; and 2) along-valley jetlike winds with maxima near 100 m altitude are shown to be frequent and their occurrence is associated with a threshold value of the along-valley surface pressure gradient. Such jets appear to have an important influence on deep fog, increasing or decreasing its likelihood depending on the sign of heat and moisture advection they associate with.

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Jeffrey M. Freedman and David R. Fitzjarrald

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The northeastern United States is subject to relatively frequent passages of frontal systems during the growing season. After a frontal passage, the newly arrived air mass is gradually modified by the underlying, mostly vegetated landscape. For the 1995–98 growing seasons, 25 frontal sequences with at least 4 days between frontal passages were identified; 16 had sufficient data continuity for rigorous analysis. A composite of sequences featuring the daily appearance of boundary layer cumulus clouds (BLcu) indicates a diminished role for entrainment and other external forcings because of the daily occurrence of a rapid growth phase in the mixed-layer (ML) diurnal evolution subsequent to day 1. Between frontal passages, net heat and moisture flux convergence in the ML is near zero, but during the warming and moistening phase, the surface flux terms, through a net radiation–BLcu feedback, are the principal controls on the tendencies of the ML temperature θ and specific humidity q. The combination of the θ and q tendencies leads to a nearly constant lifting condensation level, relative humidity, and BLcu cloud fraction during the latter part of the sequences. The presence of BLcu enhances water use efficiency and net afternoon carbon uptake throughout the sequence, with day 4 featuring optimal conditions. A multiday box model was used to perform sensitivity studies on subsidence, the lapse rate γθυ above the ML, cloud mass flux, and the regional surface Bowen ratio β reg. The effects of subsidence and γθυ on ML processes are most conspicuous on day 1; during subsequent days, the rapid growth phase dominates the ML growth equation and reduces the impact of these external terms. Increasing β reg to 3.5 reduces BLcu fraction to less than 20% and produces little net moistening of the ML, whereas reducing β reg by 30% increases sequence BLcu coverage by 30%–80%. In sum, the presence of a net radiation–BLcu feedback allows for the establishment of an equilibrium in the ML heat and moisture tendencies and ensures the appearance of BLcu on each day of the sequence, thus sustaining favorable conditions for forest–atmosphere exchange (i.e., carbon uptake).

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