Hans A. Panofsky’s Integral Similarity Function—At Fifty
Abstract: Fifty years ago, Hans A. Panofsky published a paper entitled Determination of stress from wind and temperature measurements. In his famous paper, he presented a new profile function for the mean horizontal wind speed under the condition of diabatic stratification that includes his integral similarity function. With his integral similarity function, he opened the door for Monin-Obukhov scaling in a wide range of micrometeorological and microclimatological applications. In a historic survey ranging from the sixt… Show more
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Cited by 8 publications
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Insights into the size-resolved dust emission from field measurements in the Moroccan Sahara
Abstract. Mineral dust effects upon climate are strongly affected by its particle size distribution (PSD). In particular, the emitted dust PSD partly controls the dust lifetime and its global distribution. Despite the extensive research performed on this topic over the last decades, there are still substantial gaps in our understanding of the emitted PSD along with its potential variability and associated causes. In this study, we provide insights into the saltation and size-resolved dust emission process based on measurements obtained during a comprehensive wind erosion and dust emission field campaign that took place in the Moroccan Sahara in September 2019 in the context of the FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe (FRAGMENT) project. The measurement site located in a remote ephemeral lake, consisting of a smooth hard-crusted paved sediment surface surrounded by small sand dunes, is characterized by strong and frequent saltation and dust emission conditions, and relatively low sandblasting efficiencies. Our study, which thoroughly analyses the number and mass PSDs of both the concentration and diffusive flux (the latter typically assumed to be equivalent to the emitted dust PSD), detects statistically significant dependencies upon friction velocity (u*), wind direction and type of event (regular events vs haboob events). We discuss the potential underlying causes of such variability, including the effect of dry deposition, an enhanced fragmentation of aggregates, and the impact of the haboob gust front. We clearly identify and quantify the major role played by dry deposition in shaping the diffusive flux PSD variations, modulated by the wind direction-dependent fetch length of our measurement location and u*. Our estimates show the importance of dry deposition relative to emission, representing up to ∼40 % for super-coarse particles (> 10 μm) and up to ∼20 % for particles as small as ∼5 μm in diameter. While we attribute the enhancement (reduction) in submicron (supermicron) particles with u* to the effect of dry deposition, an enhanced fragmentation of aggregates with u* could still play a complementary yet arguably smaller role. We additionally find clear differences in the PSDs associated to haboob events in comparison with the regular events, i.e., a higher (lower) proportion of supermicron (submicron) particles for equivalent or higher u* values, and more vigorous dry deposition and variability in the coarse and super-coarse dust mass fractions. We hypothesize that these differences are due to 1) a smaller horizontal (spatial) extent of the haboob events (which is equivalent to the effect of a smaller fetch), 2) the effect of the moving haboob gust front, where u* and dust emission are maximized, along with its changing proximity to the measurement site (which is equivalent to a variable fetch), and/or 3) the increased resistance of soil aggregates to fragmentation associated to the observed increases in relative humidity along the haboob outflow. We finally compare the obtained PSDs with both the PSDs predicted by the original and a recently updated version Brittle Fragmentation Theory (BFT), the latter accounting for super-coarse dust emission. For the comparison with the updated BFT we transform our optical diameters into geometric diameter PSDs, assuming dust particles are tri-axial ellipsoids with an index of refraction consistent with measured optical properties during the campaign. We obtain a substantially lower (higher) proportion of submicron (supermicron) particles in the diffusive flux PSDs in comparison with the original BFT PSDs. Also, our PSDs show a higher proportion of particles above ∼2 μm and a higher mass fraction of super-coarse particles, despite large effect of dry deposition upon this fraction. All in all, our results indicate that dry deposition needs to be adequately considered to estimate the emitted PSD, even in studies limited to the fine and coarse size ranges (< 10 μm), and particularly in measurement locations with long fetches.
Insights into the size-resolved dust emission from field measurements in the Moroccan Sahara
Abstract. Mineral dust effects upon climate are strongly affected by its particle size distribution (PSD). In particular, the emitted dust PSD partly controls the dust lifetime and its global distribution. Despite the extensive research performed on this topic over the last decades, there are still substantial gaps in our understanding of the emitted PSD along with its potential variability and associated causes. In this study, we provide insights into the saltation and size-resolved dust emission process based on measurements obtained during a comprehensive wind erosion and dust emission field campaign that took place in the Moroccan Sahara in September 2019 in the context of the FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe (FRAGMENT) project. The measurement site located in a remote ephemeral lake, consisting of a smooth hard-crusted paved sediment surface surrounded by small sand dunes, is characterized by strong and frequent saltation and dust emission conditions, and relatively low sandblasting efficiencies. Our study, which thoroughly analyses the number and mass PSDs of both the concentration and diffusive flux (the latter typically assumed to be equivalent to the emitted dust PSD), detects statistically significant dependencies upon friction velocity (u*), wind direction and type of event (regular events vs haboob events). We discuss the potential underlying causes of such variability, including the effect of dry deposition, an enhanced fragmentation of aggregates, and the impact of the haboob gust front. We clearly identify and quantify the major role played by dry deposition in shaping the diffusive flux PSD variations, modulated by the wind direction-dependent fetch length of our measurement location and u*. Our estimates show the importance of dry deposition relative to emission, representing up to ∼40 % for super-coarse particles (> 10 μm) and up to ∼20 % for particles as small as ∼5 μm in diameter. While we attribute the enhancement (reduction) in submicron (supermicron) particles with u* to the effect of dry deposition, an enhanced fragmentation of aggregates with u* could still play a complementary yet arguably smaller role. We additionally find clear differences in the PSDs associated to haboob events in comparison with the regular events, i.e., a higher (lower) proportion of supermicron (submicron) particles for equivalent or higher u* values, and more vigorous dry deposition and variability in the coarse and super-coarse dust mass fractions. We hypothesize that these differences are due to 1) a smaller horizontal (spatial) extent of the haboob events (which is equivalent to the effect of a smaller fetch), 2) the effect of the moving haboob gust front, where u* and dust emission are maximized, along with its changing proximity to the measurement site (which is equivalent to a variable fetch), and/or 3) the increased resistance of soil aggregates to fragmentation associated to the observed increases in relative humidity along the haboob outflow. We finally compare the obtained PSDs with both the PSDs predicted by the original and a recently updated version Brittle Fragmentation Theory (BFT), the latter accounting for super-coarse dust emission. For the comparison with the updated BFT we transform our optical diameters into geometric diameter PSDs, assuming dust particles are tri-axial ellipsoids with an index of refraction consistent with measured optical properties during the campaign. We obtain a substantially lower (higher) proportion of submicron (supermicron) particles in the diffusive flux PSDs in comparison with the original BFT PSDs. Also, our PSDs show a higher proportion of particles above ∼2 μm and a higher mass fraction of super-coarse particles, despite large effect of dry deposition upon this fraction. All in all, our results indicate that dry deposition needs to be adequately considered to estimate the emitted PSD, even in studies limited to the fine and coarse size ranges (< 10 μm), and particularly in measurement locations with long fetches.
Based on wind-speed records of Alaska's 19 first-order weather stations, we analyzed the near-surface wind-speed stilling for January 1, 1984 to December 31, 2016. With exception of Big Delta that indicates an increase of 0.0157 m•s −1 •a −1 , on average, all other first-order weather stations show declining trends in the near-surface wind speeds. In most cases, the average trends are less then −0.0300 m•s −1 •a −1 . The strongest average trend of −0.0500 m•s −1 •a −1 occurred at Homer, followed by −0.0492 m•s −1 •a −1 at Bettles, and −0.0453 m•s −1 •a −1 at Yakutat, while the declining trend at Barrow is marginal. The impact of the near-surface wind-speed stilling on the wind-power potential expressed by the wind-power density was predicted and compared with the wind-power classification of the National Renewable Energy Laboratory and the Alaska Energy Authority. This wind-power potential is, however, of subordinate importance because wind turbines only extract a fraction of the kinetic energy from the wind field characterized by the power efficiency. Since wind turbine technology has notably improved during the past 35 years, we hypothetically used seven currently available wind turbines of different rated power and three different shear exponents to assess the wind-power sustainability under changing wind regimes. The shear exponents 1/10, 1/7, and 1/5 served to examine the range of wind power for various conditions of thermal stratification.
(English) Atmospheric mineral dust is composed of a variety of mineral particles exhibiting distinct composition, shape, and size. The wide range of diameters exhibited by dust particles, spanning more than three orders of magnitude, partly determines their effects within the Earth System. This PhD thesis focuses on dust emission, and especially on the emitted dust particle size distribution (PSD) and its variability. It has been conducted within the context of the ERC project “FRontiers in dus t minerAloGical coMpos ition and its Effects upoN climaTe” (FRAGMENT), which aims to understand global dus t compos ition and its climate effects. Dust is generated through saltation bombardment and aggregate disintegration, and less efectively by aerodynamic entrainment. Constraining its PSD at emission is crucial as it strongly affects the impacts, lifetime, and global distribution of dust. However, our understanding of the emitted dust PSD, including its potential variability, its underlying causes, and the fraction of particles with diameter >10μm, remains limited. This thesis provides new insights into the emitted dust PSD and its variability based on meteorological, saltation and size - resolved dust concentration measurements conducted during the FRAGMENT campaign in the Moroccan Sahara in September 2019. Saltation and dust emission occurred regularly at this location, although sandblasting efficiency was lower compared to previous studies, probably due to the paved sediment surface. During the campaign two types of dust events were identified: regular events associated with diurnal wind cycles caused by solar heating, and haboobs, intense dust storms formed by convective downdrafts. The obtained dust concentration and diffusive flux PSDs show statistically significant dependencies upon friction velocity (u*), wind direction, and type of event. Notably, diffusive flux PSDs shift towards finer particles with increasing u*. In this thesis, this feature has been attributed to a large extent to the effect of dry d eposition, which is modulated by the wind-direction-dependent fetch length, and u*. However, an enhanced fragmentation of aggregates with u* may also play a smaller role. The dry deposition flux was estimated using a resistance model constrained with field observations, which was then used to estimate the emitted dust flux. It has been shown that the deposition flux could represent up to ˜ 90% of the emitted dust flux for particles >10μm in diameter and up to ˜ 65% for particles as small as ˜ 5μm. These results imply that the emitted dust PSD is coarser and less variable than the diffusive flux PSD. As far as I know, this is the first time that the effect of dry deposition upon the diffusive fluxes is identified and quantified experimentally, supporting recent results based on numerica l modelling. This finding has implications for the evaluation of dust emission schemes and their implementation in transport models as the typical assumption that the diffusive and the emitted dust PSDs are equivalent could be invalidated. Another remarkable feature is the difference in PSDs between haboob and regular events. Haboob PSDs show lower proportions of sub-micrometre particles for equivalent or higher u* intervals, along with more dry deposition and variability in dust mass fractions with diameters >3μm. The mechanisms proposed to explain this variability include a smaller and variable effective fetch during the haboob events, and/or an increased resistance of soil aggregates to fragmentation associated with the observed increase in relative humidity along the haboob outflow. Finally, compared to the invariant emitted dust flux PSD predicted by Brittle Fragmentation Theory, our dust flux shows a substantially higher proportion of super-micrometre particles. Overall, these results highlight the need to adequately consider dry deposition when estimating the emitted PSD from concentration measurements, even in studies limited to size ranges <10 (Català) La pols atmosfèrica està composta per partícules minerals de diversa composició, forma i mida. L’àmplia gamma de diàmetres de les partícules de pols, determina en part els seus efectes en el Sistema Terra. Aquesta tesi es centra en l'emissió de pols i, més concretament, en la distribució de tamanys de les partícules emes es (“PSD”) i la seva variabilitat. S'ha realitzat dins del projecte ERC FRAGMENT, que busca comprendre millor la composici ó global de la pols i els seus efectes en el clima. La pols es genera a través del bombardeig per saltació, la desintegració d'agregats i, en menor mesura, mitjançant l'arrossegament aerodinàmic. Determinar la PSD de la pols en emissió és crucial, ja que afecta els seus impactes, temps de vida i distribució global. No obstant això, la nostra comprensió de la PSD de la pols emesa, incloent-hi la seva possible variabilitat, les causes subjacents i la fracció de pols amb diàmetre >10μm és limitada. Aquesta tesi ofereix noves perspectives sobre la PSD de la pols emesa i la seva variabilitat a partir de les observacions meteorològiques i mesures de concentració de pols i de saltació d'una campanya de FRAGMENT a Marroc el 2019. La saltació i l'emissió de pols van ser freqüents al lloc triat, tot i que l'eficiència de saltació va ser inferior a altres estudis, probablement a causa del sediment compactat de la superfície. Durant la campanya es van identificar dos tipus d'esdeveniments de pols: els regulars, associats al cicle diari del vent causat per l'escalfament solar, i els haboobs, formats per corrents descendents d'aire en esdeveniments convectius. Les PSDs de la concentració i el flux difusiú mostren dependències estadísticament significatives en funció de la velocitat de fricció (u*), la direcció del vent, i el tipus d'esdeveniment. En particular, s'observa un desplaçament cap a PSDs més fines en el flux difusiu a mesura que augmenta la u*, atribuït principalment a l'efecte de la deposició seca, modulada per l'extensió de la font (“fetch”) i la u*. No obstant això, una major fragmentació d'agregats a mesura que augmenta la u* podria també jugar un paper secundari. El flux de deposició seca es va estimar mitjançant un model de res is tències calibrat amb observacions i, a partir d'aquest, s'ha estimat també el flux de pols emesa. S'ha demostrat que la deposició podria representar fins a un ˜ 90% del flux emès per a partícules amb diàmetres >10μm i fins a ˜ 65% per a partícules ˜ 5μm. Això implica que la PSD de la pols emesa és més gruixuda i menys variable que la PSD del flux difusiu. Aquesta és la primera vegada que s'identifica i quantifica experimentalment l'efecte de la deposició seca en els fluxos difusius, recolzant resultats recents obtinguts amb modelització numèrica. Aquest descobriment té implicacions en l'avaluació dels esquemes d'emissió de pols i la seva implementació en els models de transport de pols, ja que l'assumpció típica que les PSD del flux difusiú i emès són equivalents podria no ser vàlida. Una altra característica destacable són les diferències de PSD entre els esdeveniments regulars i els haboobs. Els haboobs mostren una menor proporció de partícules submicròniques per a intervals de u* iguals o majors, i una major deposició seca i variabilitat en les fraccions de massa >3μm. Els mecanismes proposats per explicar aquesta variabilitat inclouen un “fetch” efectiu més petit i variable durant els haboobs, i/o una major resistència dels agregats del sòl a la fragmentació, associada a l'augment observat en la humitat relativa al llarg del flux de sortida del haboob. En comparació amb la PSD del flux de pols emès que proposa la teoria de fragmentació de materials fràgils, els nostres resultats del flux de pols mostren una proporció substancialment més gran de partícules supermicròniques. Aquests resultats indiquen que la depos ició s eca ha de s er cons iderada adequadament per a es timar la PSD emes a, fins i tot en estudis limitats a rangs de tamany <10μm. (Español) El polvo atmosférico se compone de partículas minerales con una composición, forma y tamaño distintos. El amplio rango de diámetros de las partículas de polvo determina en parte sus efectos en el Sistema Tierra. Esta tesis se centra en la emisión de polvo, especialmente en la distribución de tamaños de las partículas emitidas ("PSD") y en su variabilidad. Se ha llevado a cabo en el marco del proyecto ERC FRAGMENT, que busca comprender mejor la composici ón global del polvo y sus efectos en el clima. El polvo se genera a través del bombardeo por saltación, la desintegración de agregados y, en menor medida, el arrastre aerodinámico. Determinar la PSD del polvo en emisión es crucial, ya que afecta a sus impactos, tiempo de vida y distribución global. Sin embargo, nuestra comprensi ón de la PSD del polvo emitido, incluyendo su posible variabilidad, las causas subyacentes y la fracci ón de polvo con diámetro >10μm es limitada. Esta tesis ofrece nuevas perspectivas sobre la PSD del polvo emitido y su variabilidad a partir de las observaciones meteorológicas y medidas de concentración de polvo y saltación de una campaña de FRAGMENT en Marruecos en 2019. La saltación y emisión de polvo fueron frecuentes en el sitio elegido, aunque la eficiencia de saltaci ón fue menor que en otros estudios, probablemente por el sedimento compactado que cubría la superficie. Durante la campaña hubo dos tipos de eventos de polvo: regulares, asociados al ciclo diario del viento causado por el calentamiento solar, y haboobs, formados por las corrientes descendentes de aire en eventos convectivos. Las PSDs de la concentraci ón y el flujo difusivo muestran dependencias estadisticamente significativas con la velocidad de fricci ón (u*), la dirección del viento, y el tipo de evento. En particular, se observa un desplazamiento hacia PSDs más finas en el flujo difusivo a medida que aumenta la u*, que se ha atribuido en gran medida al efecto de la deposición seca, modulada por la extensión de la fuente de polvo ("fetch"), y la u*. No obstante, una mayor fragmentación de agregados a medida que aumenta la u* podría desempeñar también un papel secundario. El flujo de deposición seca se estimó mediante un modelo calibrado con observaciones, a partir del cual se estimó también el flujo de polvo emitido. Se ha visto que la deposici ón podría representar hasta un ˜ 90% del flujo emitido para partículas con diámetros >10μm y hasta ˜ 65% para partículas ˜ 5μm. Esto implica que la PSD del polvo emitido es más gruesa y menos variable que la PSD del flujo difusivo. Ésta es la primera vez que se identifica y cuantifica experimentalmente el efecto de la deposición seca en los flujos difusivos, respaldando resultados recientes obtenidos con modelizaci ón numérica. Este hallazgo tiene implicaciones en la evaluaci ón de los esquemas de emisión de polvo y su implementación en los modelos de transporte ya que la típica suposición de que las PSDs del flujo difusivo y emitido son equivalentes podría no ser válida. Otro hallazgo destacado son las diferencias entre las PSDs de los eventos regulares y los haboobs. Los haboobs muestran una menor proporción de partículas submicrónicas para intervalos de u* iguales o mayores, y una mayor deposici ón y variabilidad en la fracción de masa >3μm. Los mecanismos propues tos para explicar es ta variabilidad incluyen un “fetch” efectivo más pequeño y variable durante los eventos de haboob, y/o una mayor resistencia de los agregados del suelo a la fragmentación, asociada a una mayor humedad relativa a lo largo del flujo de salida del haboob. En comparación con la PSD del flujo de polvo propuesta por la teoría de fragmentación de materiales frágiles, nuestros resultados del flujo de polvo muestran una proporci ón sustancialmente mayor de partículas supermicrónicas. Estos hallazgos indican que la deposición seca debe ser considerada adecuadamente al estimar la PSD emitida, incluso en estudios limitados a rangos de tamaño <10μm.
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