Surface Bowen Ratio Research Articles

Daytime convective development over land: The role of surface forcing

AbstractWater availability at the Earth surface determines the portioning of the surface heat flux into its sensible and latent components, that is, the surface heat flux Bowen ratio. The two components affect differently the surface buoyancy flux and thus the development and growth of the convective boundary layer. As a result, the Bowen ratio has a strong impact on the daytime development of dry and moist convection over land. We use two canonical modeling test cases, one for the shallow convection and one for the shallow‐to‐deep convection transition, to document the impact of the surface Bowen ratio on daytime convection development. A simple approach is used where results from simulations featuring the original setup are contrasted with simulations where the surface‐sensible heat flux takes on values of the latent heat flux and vice versa. Such a change illustrates the key impact of the surface water availability without changing the total surface heat flux. Because of the larger surface buoyancy flux, simulations with the reversed surface heat fluxes feature faster deepening of the convective boundary layer and wider clouds once moist convection develops. Mean cloud‐base width of cumulus clouds increases with the boundary‐layer depth. For shallow‐to‐deep convection transition, comparison of simulations with the observed large‐scale flow that feature substantial lower‐tropospheric shear with simulations with no mean horizontal flow suggests that the organization of convective circulations within the boundary layer strongly affects initial size and organization of shallow convection, as well as the subsequent width increase once convection deepens and eventually transitions to deep precipitating convection. A simple explanation is provided of why a deeper well‐mixed convective subcloud layer results in wider clouds. The key is the larger width of boundary‐layer coherent updraft structures when the mixed layer is deeper.

Atmospheric turbulence and land - atmosphere energy transfer characteristics in the surface layer of the Northern slope of Mt. Qomolangma area : abstract

Based on the turbulent data collected at Quzong site, on the northern slope of Mt. Qomolangma, from April 2005 to March 2006, macro-scale turbulent statistical characteristics and land-atmosphere energy transfer before and after the onset of southwest monsoon were acquired by the eddy correlation method. It was found that Monin-Obukhov similarity theory is applicable for Mt. Qomolangma area. The relationship between normalized wind speed standard deviation and atmospheric stability, variances of normalized temperature and humidity standard deviation with atmospheric stability were simulated in the study. It was also found that energy balance components (net radiation flux, sensible heat flux, latent heat flux and soil heat flux) and surface heating filed had evident diurnal and seasonal changes. Especially under the influence of southwest monsoon, the sensible heat flux and latent heat flux in Quzong area have evident opposite changing trends. The variation characteristics of other surface parameters (surface reflectance and Bowen ratio) is very clear before and after the breakout of southwest monsoon.

Theoretical Understanding of the Linear Relationship between Convective Updrafts and Cloud-Base Height for Shallow Cumulus Clouds. Part II: Continental Conditions

Abstracts This is the Part II of a two-part study that seeks a theoretical understanding of an empirical relationship for shallow cumulus clouds: subcloud updraft velocity covaries linearly with the cloud-base height. This work focuses on continental cumulus clouds that are more strongly forced by surface fluxes and more deviated from equilibrium than those over oceans (Part I). We use a simple analytical model for shallow cumulus that is well tested against a high-resolution (25 m in the horizontal) large-eddy simulation model. Consistent with a conventional idea, we find that surface Bowen ratio is the key variable that regulates the covariability of both parameters: under the same solar insolation, a drier surface allows for stronger buoyancy flux, triggering stronger convection that deepens the subcloud layer. We find that the slope of the Bowen-ratio-regulated relationship between the two parameters (defined as λ) is dependent on both the local time and the stability of the lower free atmosphere. The value of λ decreases with time exponentially from sunrise to early afternoon and linearly from early afternoon to sunset. The value of λ is larger in a more stable atmosphere. In addition, continental λ in the early afternoon more than doubles the oceanic λ. Validation of the theoretical results against ground observations over the Southern Great Plains shows a reasonable agreement. Physical mechanisms underlying the findings are explained from the perspective of different time scales at which updrafts and cloud-base height respond to a surface flux forcing.

Modification of surface energy balance during springtime: The relative importance of biophysical and meteorological changes

In ecosystems characterized by strong seasonality in leaf area, the emergence of leaves during springtime modifies land surface energy balance by altering surface biophysical properties during a period when atmospheric conditions are also changing. However, the relative importance and interactions among surface biophysical and atmospheric variables in modifying the surface energy balance are not well understood. In this study, we use a physically-based attribution method to quantify the relative importance of covarying surface biophysical and atmospheric variables in modifying the surface energy balance during springtime. Results show that the widely observed decrease in the Bowen ratio that occurs with leaf emergence is not solely attributable to the sharp decrease in surface resistance caused by increasing leaf area. Rather, decreases in the Bowen ratio reflect the combined effects of changes in surface properties and atmospheric conditions. Specifically, decreasing surface resistance and increasing air temperature both act to reduce the Bowen ratio, while concurrent increases in specific humidity provide a negative feedback that constrains evaporative fluxes. In parallel, aerodynamic resistance tends to increase after leaf emergence largely because wind speed tends to decrease during springtime. These findings provide a refined characterization of surface energy balance dynamics during springtime when both surface and atmospheric conditions are changing rapidly and reveal previously understudied properties of the near-surface atmosphere that influence surface Bowen ratio and aerodynamic resistance.

Comparing ground‐based observations and a large‐eddy simulation of shallow cumuli by isolating the main controlling factors of the mass flux distribution

AbstractThe distribution of mass flux at the cloud base has long been thought to be independent of large‐scale forcing. However, recent idealized modelling studies have revealed its dependence on some large‐scale conditions. Such dependence makes it possible to isolate the observed large‐scale conditions, which are similar to those in large‐eddy simulations (LES), in order to compare the observed and modelled mass flux distributions. In this study, we derive for the first time the distribution of the cloud‐base mass flux among individual shallow cumuli from ground‐based observations at the Barbados Cloud Observatory (BCO) and compare it with the Rain In Cumulus over the Ocean (RICO) LES case study. The procedure of cloud sampling in LES mimics the pointwise measurement procedure at the BCO to provide a mass flux metric that is directly comparable with observations. We find a difference between the mass flux distribution observed during the year 2017 at the BCO and the distribution modelled by LES that is comparable to the seasonal changes in the observed distribution. This difference between the observed and modelled distributions is diminished and an extremely good match is found by subsampling the measurements under a similar horizontal wind distribution and area‐averaged surface Bowen ratio to those modelled in LES. This provides confidence in our observational method and shows that LES produces realistic clouds that are comparable to those observed in nature under the same large‐scale conditions. We also confirm that the stronger horizontal winds and higher Bowen ratios in our case study shift the distributions to higher mass flux values, which is coincident with clouds of larger horizontal areas and not with stronger updrafts.

A semi-empirical model of the energy balance closure in the surface layer.

It has been hypothesized that the energy balance closure problem of single-tower eddy-covariance measurements is linked to large-scale turbulent transport. In order to shed light on this problem, we investigate the functional dependence of the normalized residual for the potential temperature and humidity conservation equations, i.e. the imbalance ratio for the fluxes of latent and sensible heat. We set up a suite of simulations consisting of cases with different stability and surface Bowen ratio. We employ a nesting approach in the lower part of the atmospheric boundary-layer to achieve higher spatial resolution near the surface. Our simulations reproduce earlier simulation results for the mixed layer and also mimic the saw-blade pattern of real flux measurements. Focusing on homogeneous terrain, we derive a parameterization for the spatially averaged flux imbalance ratios of latent and sensible heat in the surface layer. We also investigate how the remaining imbalance for a given point measurement is related to the local turbulence, by deriving a statistical model based on turbulence characteristics that are related to large-scale turbulence. The average imbalance ratio scales well with friction velocity, especially for sensible heat. For the latent heat flux, our results show that the Bowen ratio also influences the underestimation. Furthermore, in the surface layer the residual has a linear dependence on the absolute height divided by the boundary-layer height. Our parameterization allows us to deduce an expression for the residual in the energy budget for a particular measurement half hour, based on the measurement height and stability.

Analytical solution to a thermodynamic model for the sensitivity of afternoon deep convective initiation to the surface Bowen ratio

The tendency of convective rainfall to initiate over a wetter or drier land surface is a critical feedback process in the climate system, influencing the hydrological cycle on a variety of spatial scales, especially in parts of the world where water is limited. A simple algebraic solution is derived from fundamental physical equations, to predict the sign of this convective rainfall feedback with the surface. The tendency for convection to occur is evaluated by the rate at which the convective boundary‐layer top approaches the level of free convection. Well‐known integral models predict the rate of ascent of the boundary‐layer top, which tends to be faster over a dry surface. The associated changes in equivalent potential temperature in the boundary layer determine the rate at which the level of free convection descends, typically faster over a wet surface, as a function of the ambient profile, the thermodynamic forcing and the surface Bowen ratio. The resulting system is controlled by three parameters. Two nondimensional parameters determine whether there is wet or dry “advantage”; the Bowen ratio at the boundary‐layer top and a “convective instability parameter,” defined as the ratio of the vertical gradient of saturated equivalent potential temperature at the level of free convection to the profile stability just above the boundary layer. A dimensional function, dependent on the surface fluxes, the boundary‐layer depth, and the profile stability, provides the magnitude of the response. In comparison with previous work, the solution is both rigorously derived from physical principles and encapsulated in a simple algebraic form. A first evaluation of the theoretical framework has been made using data from a convection‐permitting numerical model simulation over India, and this indicates that the equations successfully determine the conditions under which convection is triggered over dry surfaces.

What caused the spring intensification and winter demise of the 2011 drought over Texas?

The 2011 Texas drought, the worst 1-year drought on record, was characterized by spring intensification of rainfall deficit and surface dryness. Such spring intensification was led by an unusually strong increase of convective inhibition (CIN), which suppressed convection at the time critical for the onset of the April–June rainfall season. The CIN increase appeared to be caused by strong sub-seasonal anomalously westerly winds at 850 hPa (U850) in April, in addition to surface dryness due to cumulative rainfall deficit since fall of 2010. The anomalous U850 advected warm dry air from the Mexican Plateau to Texas, enhancing cap inversion, and exacerbating static stability initially elevated by an anomalously high surface Bowen ratio due to rainfall deficits from winter through spring over Texas. Strengthened westerly U850 in April, in addition to the persistent rainfall deficits from winter through spring, are common characteristics in other strong drought events experienced over Texas. Atmospheric Model Intercomparison Project-type simulations with prescribed La Nina SSTAs in the tropical Pacific do not show a strengthening of westerly U850 in April, suggesting that internal atmospheric variability at intraseasonal scale, instead of La Nina, may initiate the spring drought intensification over Texas. Soil moisture deficits in late spring are significantly correlated with positive 500 hPa geopotential height anomalies over the south central U.S. 2–3 weeks later, suggesting that intensified surface dryness in late-spring could reinforce the drought-inducing anomalous mid-tropospheric high. The drought diminished in the winter of 2011/2012 despite a second La Nina event. Our analysis suggests an important role for strong westerly wind anomalies, the resultant increase of CIN in spring, and subsequent positive feedback between dry surface anomalies and the anomalous large-scale circulation pattern in drought intensification. Clarification of the mechanisms behind the strong increase of CIN and land–atmosphere feedbacks may provide a key for improving our understanding of drought predictability in spring and summer, and a scientific basis for the early warning of strong summer drought. The demise of the 2011 drought appears to have resulted from internal atmospheric circulation variability, thus intrinsically unpredictable.

Higher surface Bowen ratios ineffective at increasing updraft intensity

AbstractThe sensitivity of various metrics of convective intensity to changes in boundary layer depth via changes in the surface Bowen ratio is explored with radiative‐convective equilibrium (RCE) and initial condition simulations in the System for Atmospheric Modeling, a cloud‐resolving model. In the RCE simulations, high‐percentile updrafts showed little change in response to changes in the surface Bowen ratio. Initial condition simulations showed low surface Bowen ratios having stronger updrafts than high surface Bowen ratios. A parcel model was used to explore whether RCE results could be explained with an entrainment parameter independent of boundary layer depth. It was found that for every set of simulations in RCE, entrainment rates independent of boundary layer depth could explain the lack of change in updraft velocities with boundary layer depth. Given the indifference of high‐percentile updraft velocities in our simulations to changes in the surface Bowen ratio, we conclude that convective intensity as measured by this quantity in the cloud‐resolving model is not sensitive to this forcing.

Two perspectives on the coupled carbon, water and energy exchange in the planetary boundary layer

Abstract. Understanding the interactions between the land surface and the atmosphere is key to modelling boundary-layer meteorology and cloud formation, as well as carbon cycling and crop yield. In this study we explore these interactions in the exchange of water, heat and CO2 in a cropland–atmosphere system at the diurnal and local scale. To that end, we couple an atmospheric mixed-layer model (MXL) to two land-surface schemes developed from two different perspectives: while one land-surface scheme (A-gs) simulates vegetation from an atmospheric point of view, the other (GECROS) simulates vegetation from a carbon-storage point of view. We calculate surface fluxes of heat, moisture and carbon, as well as the resulting atmospheric state and boundary-layer dynamics, over a maize field in the Netherlands, on a day for which we have a rich set of observations available. Particular emphasis is placed on understanding the role of upper-atmosphere conditions like subsidence in comparison to the role of surface forcings like soil moisture. We show that the atmospheric-oriented model (MXL-A-gs) outperforms the carbon storage-oriented model (MXL-GECROS) on this diurnal scale. We find this performance is partly due to the difference of scales at which the models were made to run. Most importantly, this performance strongly depends on the sensitivity of the modelled stomatal conductance to water stress, which is implemented differently in each model. This sensitivity also influences the magnitude of the surface fluxes of CO2, water and heat (surface control) and subsequently impacts the boundary-layer growth and entrainment fluxes (upper atmosphere control), which alter the atmospheric state. These findings suggest that observed CO2 mole fractions in the boundary layer can reflect strong influences of both the surface and upper-atmosphere conditions, and the interpretation of CO2 mole fraction variations depends on the assumed land-surface coupling. We illustrate this with a sensitivity analysis where high subsidence and soil moisture depletion, typical for periods of drought, have competing and opposite effects on the boundary-layer height h. The resulting net decrease in h induces a change of 12 ppm in the late-afternoon CO2 mole fraction. Also, the effect of such high subsidence and soil moisture depletion on the surface Bowen ratio are of the same magnitude. Thus, correctly including such two-way land-surface interactions on the diurnal scale can potentially improve our understanding and interpretation of observed variations in atmospheric CO2, as well as improve crop yield forecasts by better describing the water loss and carbon gain.

Impact of land use change on the diurnal cycle climate of the Canadian Prairies

AbstractThis paper uses hourly observations from 1953 to 2011 of temperature, relative humidity, and opaque cloud cover from 14 climate stations across the Canadian Prairies to analyze the impact of agricultural land use change on the diurnal cycle climate, represented by the mean temperature and relative humidity and their diurnal ranges. We show the difference between the years 1953–1991 and 1992–2011. The land use changes have been largest in Saskatchewan where 15–20% of the land area has been converted in the past four decades from summer fallow (where the land was left bare for 1 year) to annual cropping. During the growing season from 20 May to 28 August, relative humidity has increased by about 7%. During the first 2 months, 20 May to 19 July, maximum temperatures and the diurnal range of temperature have fallen by 1.2°C and 0.6°C, respectively, cloud cover has increased by about 4%, reducing surface net radiation by 6 W m−2, and precipitation has increased. We use the dry‐downs after precipitation to separate the impact of cloud cover and show the coupling between evapotranspiration and relative humidity. We estimate, using reanalysis data from ERA‐Interim, that increased transpiration from the larger area of cropland has reduced the surface Bowen ratio by 0.14–0.2. For the month on either side of the growing season, cloud cover has fallen slightly; maximum temperatures have increased, increasing the diurnal temperature range and the diurnal range of humidity.

Large‐eddy simulations of surface influences on planetary boundary layer development in southwest Western Australia

AbstractObservations from near‐simultaneous atmospheric soundings released over contrasting land surfaces in the southwest of Western Australia during December 2005 (austral summer) and August 2007 (late austral winter or early spring) have shown higher planetary boundary layer (PBL) heights over native vegetation as compared to agricultural land. The large‐eddy simulation technique is used to investigate the drivers behind these observed differences in PBL, and sensitivity tests are carried out with modified soil moisture and vegetation cover. It is shown that the differences in PBL for the December case are mainly driven by the change in vegetation cover, while a soil moisture gradient also played a role for the August case. The mixing diagram approach is used to further quantify the relative contributions of surface and entrainment fluxes on the growth of the PBL and it is shown that, while dry‐air entrainment plays an important role in PBL development, it is the higher surface Bowen ratio which drives the more vigorous PBL development over the native vegetation. It is also shown that the enhanced PBL development over the native vegetation leads to the preferential formation of shallow convective clouds for the August case. Copyright © 2012 Royal Meteorological Society

Roof–envelope ratio impact on green roof energy performance

This paper addresses the impact of roof-to-envelope ratio on overall energy savings of a green roof design over conventional roof designs. Simulations were performed using a modified version of the Environmental System Performance program simulator, developed at the University of Strathclyde. The modified design employed a model developed by Columbia University and the Goddard Institute of Space Science which models the evapotranspiritive effect of a green roof calculated using the Bowen ratio; that is, the ratio of sensible heat flux to the surrounding air to the latent heat flux resulting from evapourative energy losses. The resulting heat flux term is proportional to the external surface convection, but inversely proportional to the surface Bowen ratio, which is held constant and chosen to match experimental results obtained for a given roof design. The present study performed simulations for the month of July in a Toronto climate on square warehouse style one, two, and three-story buildings, with windows occupying 10% of the area of each wall. For the first set of simulations, the internal building load of each story was set to zero, and the roof–envelope ratio was increased by increasing the building width and length. For the final simulations, several roof–envelope ratios were chosen, and the internal load of each story was increased from 0 to 50,000 W. As the roof–envelope ratio increases, the cooling load of the upper floor for multi-story designs approaches the entire building cooling load. This indicates the importance of upper zone cooling in total building energy reductions. Furthermore, the total energy savings of a green-roofed building over a conventional roofed building were far more significant for single-story structures. A 250 × 250 m green-roof design with 50,000 W internal loading was found to have percentage energy savings of 73%, 29%, and 18%, for a one, two, and three-story design, respectively.

On the Growth of Layers of Nonprecipitating Cumulus Convection

A prototype problem of a nonprecipitating convective layer growing into a layer of uniform stratification and exponentially decreasing humidity is introduced to study the mechanism by which the cumulus-topped boundary layer grows. The problem naturally admits the surface buoyancy flux, outer layer stratification, and moisture scale as governing parameters. Large-eddy simulations show that many of the well-known properties of the cumulus-topped boundary layer (including a well-mixed subcloud layer, a cloud-base transition layer, a conditionally unstable cloud layer, and an inversion layer) emerge naturally in the simulations. The simulations also quantify the differences between nonprecipitating moist convection and its dry counterpart. Whereas dry penetrative convective layers grow proportionally to the square root of time (diffusively) the cumulus layers grow proportionally to time (ballistically). The associated downward transport of warm, dry air results in a significant decrease in the surface Bowen ratio. The linear-in-time growth of the cloud layer is shown to result from the transport and subsequent evaporation of liquid water into the inversion layer. This process acts as a sink of buoyancy, which acts to imbue the free troposphere with the properties of the cloud layer. A simple model, based on this mechanism, and formulated in terms of an effective dry buoyancy flux (which is constrained by the subcloud layer’s similarity to a dry convective layer), is shown to provide good predictions of the growth of the layer across a wide range of governing parameters.

Impact of agriculture, forest and cloud feedback on the surface energy budget in BOREAS

We explore the impact of agriculture, forest and cloud feedback on the surface energy budget using data obtained using a research aircraft, mesonet towers and model data. The forest has an order of magnitude larger roughness length, a lower albedo, a much smaller seasonal cycle in surface Bowen ratio (BR) and a weak mid-summer maximum of CO 2 uptake compared to agricultural areas, which have much smaller BR and much higher mid-summer CO 2 uptake, but a net CO 2 release and much reduced evaporation in spring and fall. Higher surface temperatures and the higher albedo over agricultural land reduce R net near local noon in the warm season by about 50 W m −2 in comparison with the adjacent boreal forest. The annual averaged R net, derived from 2 years of tower data, is 14 W m −2 less over grass sites than over forest sites. A reanalysis time-series for the BOREAS southern study area shows the coupling on daily timescales between the surface energy partition, the mean boundary layer depth, the cloud field and the long-wave and short-wave radiation fields. The albedo of the cloud field, the cloud short-wave forcing at the surface, varies over the range 0.1–0.8 with decreasing surface BR, and plays a major role in the surface energy budget. We estimate that this cloud feedback may increase albedo by 0.13 and reduce R net by 25 W m −2 in summer over agricultural land.

Surface diurnal cycle and boundary layer structure over Rondônia during the rainy season

An integrated data set with simultaneous observations at the surface, from tethered balloons within the boundary layer and from rawinsonde ascents, was collected during the wet season experiment of the Large‐Scale Biosphere‐Atmosphere (LBA) Experiment in Amazonia during January and February of 1999 in support of the ground validation for the Tropical Rainfall Measuring Mission (TRMM). We analyze the surface diurnal cycles of temperature, humidity, lifting condensation level, equivalent potential temperature, and the surface fluxes of sensible and latent heat, ground heat flux and net radiation, for easterly and westerly wind regimes in the lower troposphere. During the easterly wind regimes, the diurnal evolution of mixing ratio shows that the flux of water vapor through cloud base exceeds the large surface evaporation. There is a trend toward a wetter and cooler subcloud layer as the rainy season progresses. Daytime surface Bowen ratio for this pasture site is about 0.4, and falls slightly as the rainy season progresses. Typically in the afternoon, evaporatively driven downdrafts from convective rainbands transform the boundary layer. The fall of equivalent potential temperature in the boundary layer is similar for both regimes, but the boundary layer cooling by convective events during the westerly regimes is reduced, because the subcloud layer is shallower on average. Tethersonde ascents through the edges of gust fronts show that subcloud air is first cooled and moistened by rainfall evaporation before the arrival of downdraft air at the surface. These measurements provide a detailed observational basis for the validation and improvement of parameterizations for shallow and deep convection in numerical forecast models.

Postfrontal Airmass Modification

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 βreg. 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 βreg to 3.5 reduces BLcu fraction to less than 20% and produces little net moistening of the ML, whereas reducing βreg 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).

The ratio of eddy diffusivities for heat and water vapour under conditions of local advection

Both experimental (La Crau experiment) and modelling results (second-order closure) are used here to study the eddy diffusivities for sensible heat and water vapour under conditions of local advection. The main finding is that the eddy diffusivity for sensible heat is smaller than that of water vapour in the stable Internal Boundary Layer and larger in the unstable IBL. Since flux divergence cannot be neglected for this case, down to height-to-fetch ratios of at least 0.005, it is essential that the gradients and the fluxes are measured at the same height. These results also explain the apparently contrasting findings in earlier experiments concerning the behaviour of the eddy diffusivities. The overestimate due to flux divergence in the IBL of the surface Bowen ratio when estimated from vertical gradients measured just above the surface is partly compensated for by the variation of the ratio of eddy diffusivities with height for the case studied here.

Equilibrium evaporation beneath a growing convective boundary layer

Expressions for the equilibrium surface Bowen ratio (βs) and equilibrium evaporation are derived for a growing convective boundary layer (CBL) in terms of the Bowen ratio at the top of the mixed layerβi and the entrainment parameter AR. If AR is put equal to zero, the solution forβs becomes-that previously obtained for the zero entrainment or ‘closed box’ model. The Priestley-Taylor parameter α is also calculated and plotted in terms ofAR andβi. Realistic combinations of the atmospheric parameters give values of α in the range 1.1 to 1.4.

FIFE atmospheric boundary layer budget methods

The budget methods and the mixed layer model used to analyze the aircraft data from the First ISLSCP Field Experiment (FIFE) are outlined. The separation of the temporal and horizontal derivatives is discussed. Vector budgets for the mixed layer are presented on conserved variable diagrams. Theoretical solutions are given for the critical surface Bowen ratio, which produces no boundary layer moistening or equivalent potential temperature rise as a function of the Bowen ratio at the inversion. Improved measurement strategies are suggested.