scholarly journals Boundary Layer and Cloud Structure Controls on Tropical Low Cloud Cover Using A-Train Satellite Data and ECMWF Analyses

2011 ◽  
Vol 24 (1) ◽  
pp. 194-215 ◽  
Author(s):  
Terence L. Kubar ◽  
Duane E. Waliser ◽  
J-L. Li
Keyword(s):  
Boundary Layer ◽  
Cross Section ◽  
Large Scale ◽  
Layer Structure ◽  
Deep Convection ◽  
Static Stability ◽  
Boundary Layer Stability ◽  
Middle Point ◽  
Cloud Data ◽  
Low Cloud

Abstract The Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), CloudSat radar, and the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud data on the A-Train constellation complemented with the European Centre for Medium-Range Forecasts (ECMWF) analyses are used to investigate the cloud and boundary layer structure across a 10° wide cross section starting at 5°S near the international date line and extending to 35°N near the California coast from March 2008 to February 2009. The mean large-scale inversion height and low-level cloud tops, which correspond very closely to each other, are very shallow (∼500 m) over cold SSTs and high static stability near California and deepen southwestward (to a maximum of ∼1.5–2.0 km) along the cross section as SSTs rise. Deep convection near the ITCZ occurs at a surface temperature close to 298 K. While the boundary layer relative humidity (RH) is nearly constant where a boundary layer is well defined, it drops sharply near cloud top in stratocumulus regions, corresponding with strong thermal inversions and water vapor decrease, such that the maximum (−∂RH/∂z) marks the boundary layer cloud top very well. The magnitude correlates well with low cloud frequency during March–May (MAM), June–August (JJA), and September–November (SON) (r 2 = 0.85, 0.88, and 0.86, respectively). Also, CALIPSO and MODIS isolated low cloud frequency generally agree quite well, but CloudSat senses only slightly more than one-third of the low clouds as observed by the other sensors, as many clouds are shallower than 1 km and thus cannot be discerned with CloudSat due to contamination from the strong signal from surface clutter. Mean tropospheric ω between 300 and 700 hPa is examined from the ECMWF Year of Tropical Convection (YOTC) analysis dataset, and during JJA and SON, strong rising motion in the middle troposphere is confined to a range of 2-m surface temperatures between 297 and 300 K, consistent with previous studies that show a narrow range of SSTs over which deep ascent occurs. During December–February (DJF), large-scale ascending motion extends to colder SSTs and high boundary layer stability. A slightly different boundary layer stability metric is derived, the difference of moist static energy (MSE) at the middle point of the inversion (or at 700 hPa if no inversion exists) and the surface, referred to as ΔMSE. The utility of ΔMSE is its prediction of isolated uniform low cloud frequency, with very high r 2 values of 0.93 and 0.88, respectively, for the MODIS and joint lidar plus radar product during JJA but significantly lower values during DJF (0.46 and 0.40), with much scatter. To quantify the importance of free tropospheric dynamics in modulating the ΔMSE–low cloud relationships, the frequency as a function of ΔMSE of rising motion profiles (ω < −0.05 Pa s−1) is added to the observed low cloud frequency for a maximum hypothetical low cloud frequency. Doing this greatly reduces the interseasonal differences and holds promise for using ΔMSE for parameterization schemes and examining low cloud feedbacks.

Comparison Studies of Cloud- and Convection-Related Processes Simulated by the Canadian Regional Climate Model over the Pacific Ocean

2008 ◽  
Vol 136 (11) ◽  
pp. 4168-4187 ◽  
Author(s):  
Yanjun Jiao ◽  
Colin Jones
Keyword(s):  
Boundary Layer ◽  
Cross Section ◽  
Cloud Cover ◽  
Large Scale ◽  
Climate Model ◽  
Deep Convection ◽  
Regional Climate ◽  
Eddy Diffusivity ◽  
Shallow Convection ◽  
The Tropical Pacific

Abstract This paper presents results from the Canadian Regional Climate Model (CRCM) contribution to the Global Energy and Water Cycle Experiment (GEWEX) Pacific Cross-section Intercomparison Project. This experiment constitutes a simulation of stratocumulus, trade cumulus, and deep convective transitions along a cross section in the tropical Pacific. The simulated seasonal mean cloud and convection are compared between an original version of CRCM (CRCM4) and a modified version (CRCMM) with refined parameterizations. Results are further compared against available observations and reanalysis data. The specific parameterization refinements touch upon the triggering and closure of shallow convection, the cloud and updraft characteristics of deep convection, the parameterization of large-scale cloud fraction, the calculation of the eddy diffusivity in the boundary layer, and the evaporation of falling large-scale precipitation. CRCMM shows substantial improvement in many aspects of the simulated seasonal mean cloud, convection, and precipitation over the tropical Pacific, CRCMM-simulated total column water vapor, total cloud cover, and precipitation are in better agreement with observations than in the original CRCM4 model. The maximum frequency of the shallow convection shifts from the ITCZ region in CRCM4 to the subtropics in CRCMM; accordingly, excessive cloud in the shallow cumulus region in CRCM4 is greatly diminished. Finally, CRCMM better simulates the vertical structure of relative humidity, cloud cover, and vertical velocity, at least when compared to the 40-yr ECMWF Re-Analysis. Analyses of sensitivity experiments assessing specific effects of individual parameterization changes indicate that the modification to the eddy diffusivity in the boundary layer and changes to deep convection contribute most significantly to the overall model improvements.

scholarly journals Identifying meteorological influences on marine low cloud mesoscale morphology using deep learning classifications

2020 ◽  
Author(s):  
Johannes Mohrmann ◽  
Robert Wood ◽  
Tianle Yuan ◽  
Hua Song ◽  
Ryan Eastman ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Layer Structure ◽  
Surface Wind ◽  
Cloud Properties ◽  
Southeastern Pacific ◽  
Distribution Boundary ◽  
Mesoscale Convective ◽  
Low Cloud ◽  
Meteorological Influences

Abstract. Marine low cloud mesoscale morphology in the southeastern Pacific Ocean is analyzed using a large dataset of machine-learning generated classifications spanning three years. Meteorological variables and cloud properties are composited by mesoscale cloud type, showing distinct meteorological regimes of marine low cloud organization from the tropics to the midlatitudes. The presentation of mesoscale cellular convection, with respect to geographic distribution, boundary layer structure, and large-scale environmental conditions, agrees with prior knowledge. Two tropical and subtropical cumuliform boundary layer regimes, suppressed cumulus and clustered cumulus, are studied in detail. The patterns in precipitation, circulation, column water vapor, and cloudiness are consistent with the representation of marine shallow mesoscale convective self-aggregation by large eddy simulations of the boundary layer. Although they occur under similar large-scale conditions, the suppressed and clustered low cloud types are found to be well-separated by variables associated with low-level mesoscale circulation, with surface wind divergence being the clearest discriminator between them, whether reanalysis or satellite observations are used. Clustered regimes are associated with surface convergence and suppressed regimes are associated with surface divergence.

scholarly journals Identifying meteorological influences on marine low-cloud mesoscale morphology using satellite classifications

2021 ◽  
Vol 21 (12) ◽  
pp. 9629-9642
Author(s):  
Johannes Mohrmann ◽  
Robert Wood ◽  
Tianle Yuan ◽  
Hua Song ◽  
Ryan Eastman ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Layer Structure ◽  
Surface Wind ◽  
Cloud Properties ◽  
Southeastern Pacific ◽  
Distribution Boundary ◽  
Mesoscale Convective ◽  
Low Cloud ◽  
Meteorological Influences

Abstract. Marine low-cloud mesoscale morphology in the southeastern Pacific Ocean is analyzed using a large dataset of classifications spanning 3 years generated by machine learning methods. Meteorological variables and cloud properties are composited by the mesoscale cloud type of the classification, showing distinct meteorological regimes of marine low-cloud organization from the tropics to the midlatitudes. The presentation of mesoscale cellular convection, with respect to geographic distribution, boundary layer structure, and large-scale environmental conditions, agrees with prior knowledge. Two tropical and subtropical cumuliform boundary layer regimes, suppressed cumulus and clustered cumulus, are studied in detail. The patterns in precipitation, circulation, column water vapor, and cloudiness are consistent with the representation of marine shallow mesoscale convective self-aggregation by large eddy simulations of the boundary layer. Although they occur under similar large-scale conditions, the suppressed and clustered low-cloud types are found to be well separated by variables associated with low-level mesoscale circulation, with surface wind divergence being the clearest discriminator between them, regardless of whether reanalysis or satellite observations are used. Clustered regimes are associated with surface convergence, while suppressed regimes are associated with surface divergence.

scholarly journals Effects of Parameterized Boundary Layer Structure on Hurricane Rapid Intensification in Shear

2019 ◽  
Vol 147 (3) ◽  
pp. 853-871 ◽  
Author(s):  
Jun A. Zhang ◽  
Robert F. Rogers
Keyword(s):  
Boundary Layer ◽  
Angular Momentum ◽  
Layer Structure ◽  
Deep Convection ◽  
Eddy Diffusivity ◽  
Static Stability ◽  
Intensity Change ◽  
Rapid Intensification ◽  
Boundary Layer Structure ◽  
Budget Analysis

Abstract This study investigates the role of the parameterized boundary layer structure in hurricane intensity change using two retrospective HWRF forecasts of Hurricane Earl (2010) in which the vertical eddy diffusivity Km was modified during physics upgrades. Earl undergoes rapid intensification (RI) in the low-Km forecast as observed in nature, while it weakens briefly before resuming a slow intensification at the RI onset in the high-Km forecast. Angular momentum budget analysis suggests that Km modulates the convergence of angular momentum in the boundary layer, which is a key component of the hurricane spinup dynamics. Reducing Km in the boundary layer causes enhancement of both the inflow and convergence, which in turn leads to stronger and more symmetric deep convection in the low-Km forecast than in the high-Km forecast. The deeper and stronger hurricane vortex with lower static stability in the low-Km forecast is more resilient to shear than that in the high-Km forecast. With a smaller vortex tilt in the low-Km forecast, downdrafts associated with the vortex tilt are reduced, bringing less low-entropy air from the midlevels to the boundary layer, resulting in a less stable boundary layer. Future physics upgrades in operational hurricane models should consider this chain of multiscale interactions to assess their impact on model RI forecasts.

scholarly journals Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud fraction over the southeast Atlantic

2021 ◽  
Author(s):  
Jianhao Zhang ◽  
Paquita Zuidema
Keyword(s):  
Boundary Layer ◽  
Seasonal Cycle ◽  
Large Scale ◽  
Cloud Fraction ◽  
Small Scale ◽  
Aerosol Cloud ◽  
Ascension Island ◽  
Tropospheric Aerosol ◽  
Low Cloud ◽  
Aerosol Cloud Interactions

Abstract. Many studies examining shortwave-absorbing aerosol-cloud interactions over the southeast Atlantic apply a seasonal averaging. This disregards a meteorology that raises the mean altitude of the smoke layer from July to October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass-burning aerosol loading at Ascension Island from July to October, based on measurements from Ascension Island (8º S, 14.5º W), satellite retrievals and reanalysis. In July and August, variability in the smoke loading predominantly occurs in the boundary layer. During both months, the low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. The meteorology varies little, suggesting aerosol-cloud interactions consistent with a boundary-layer semi-direct effect can explain the cloudiness changes. September marks a transition month during which mid-latitude disturbances can intrude into the Atlantic subtropics, constraining the land-based anticyclonic circulation transporting free-tropospheric aerosol to closer to the coast. Stronger boundary layer winds help deepen, dry, and cool the boundary layer near the main stratocumulus deck compared to that on days with high smoke loadings, with stratocumulus reducing everywhere but at the northern deck edge. Longwave cooling rates generated by a sharp water vapor gradient at the aerosol layer top facilitates small-scale vertical mixing, and could help to maintain a better-mixed September free troposphere. The October meteorology is more singularly dependent on the strength of the free-tropospheric winds advecting aerosol offshore. Free-tropospheric aerosol is less, and moisture variability more, compared to September. Low-level clouds increase and are more stratiform, when the smoke loadings are higher. The increased free-tropospheric moisture can help sustain the clouds through reducing evaporative drying during cloud-top entrainment. Enhanced subsidence above the coastal upwelling region increasing cloud droplet number concentrations may further prolong cloud lifetime through microphysical interactions. Reduced subsidence underneath stronger free-tropospheric winds at Ascension supports slightly higher cloud tops during smokier conditions. Overall the monthly changes in the large-scale aerosol and moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology, raising a climate importance as cloudiness changes dominate changes in the top-of-atmosphere radiation budget.

How is the marine atmospheric boundary layer turbulence organized in the trades ?

2021 ◽  
Author(s):  
Pierre-Etienne Brilouet ◽  
Marie Lothon ◽  
Sandrine Bony
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Vertical Velocity ◽  
Large Scale ◽  
Layer Structure ◽  
Potential Temperature ◽  
Surface Wind ◽  
Cloud Amount ◽  
Velocity Spectrum ◽  
Marine Atmospheric Boundary Layer

<p>Tradewind clouds can exhibit a wide diversity of mesoscale organizations, and the turbulence of marine atmospheric boundary layer (MABL) can exhibit coherent structures and mesoscale circulations. One of the objectives of the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) field experiment was to better understand the tight interplay between the mesoscale organization of clouds, boundary-layer processes, and the large-scale environment.</p><p>During the experiment, that took place East of Barbados over the Western Tropical Atlantic Ocean in Jan-Feb 2020, the French ATR-42 research aircraft was devoted to the characterization of the cloud amount and of the subcoud layer structure. <span>During its 17 research flights, </span><span>it</span> <span>sampled a </span><span>large diversity of large scale conditions and </span><span>cloud patterns</span><span>. </span>Multiple sensors onboard t<span>he aircraft measure</span><span>d</span> <span>high-frequency </span><span>fluctuations of potential temperature, water vapour mixing ratio and wind , allowing </span><span>for </span><span>an extensive characterization </span><span> of</span><span> the turbulence </span><span>within</span><span> the subcloud layer. </span> <span>A </span><span>quality-controled and calibrated turbulence data</span><span>set</span><span> was produced </span><span>on the basis of these measurements</span><span>, which is now </span><span> available on the EUREC4A AERIS data portal.</span></p><p><span>The </span><span>MABL </span><span>turbulent </span><span>structure i</span><span>s</span><span> studied </span><span>using this dataset, </span><span>through a spectral analysis </span><span>of the vertical velocity</span><span>. Vertical profiles of characteristic length scales reveal a non-isotropic structure with a stretching of the eddies along the mean wind. The organization strength of the turbulent field is also explored </span><span>by defining</span><span> a diagnostic based on the shape of the vertical velocity spectrum. </span><span>The </span><span>structure and the degree of organization of the </span><span>subcloud layer </span><span>are</span><span> characterized for </span><span> different type</span><span>s</span><span> of mesoscale </span><span>convective </span><span>pattern </span><span>and </span><span>as a function of</span><span> the large-scale environment, </span><span>including</span> <span>near-</span><span>surface wind </span><span>and</span> <span>lower-</span><span>tropospheric</span><span> stability conditions.</span></p><p> </p>

scholarly journals Synoptic-scale controls of fog and low-cloud variability in the Namib Desert

2020 ◽  
Vol 20 (6) ◽  
pp. 3415-3438 ◽  
Author(s):  
Hendrik Andersen ◽  
Jan Cermak ◽  
Julia Fuchs ◽  
Peter Knippertz ◽  
Marco Gaetani ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Cloud Cover ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Synoptic Scale ◽  
Namib Desert ◽  
Data Set ◽  
Air Masses ◽  
Low Cloud ◽  
Low Cloud Cover

Abstract. Fog is a defining characteristic of the climate of the Namib Desert, and its water and nutrient input are important for local ecosystems. In part due to sparse observation data, the local mechanisms that lead to fog occurrence in the Namib are not yet fully understood, and to date, potential synoptic-scale controls have not been investigated. In this study, a recently established 14-year data set of satellite observations of fog and low clouds in the central Namib is analyzed in conjunction with reanalysis data in order to identify synoptic-scale patterns associated with fog and low-cloud variability in the central Namib during two seasons with different spatial fog occurrence patterns. It is found that during both seasons, mean sea level pressure and geopotential height at 500 hPa differ markedly between fog/low-cloud and clear days, with patterns indicating the presence of synoptic-scale disturbances on fog and low-cloud days. These regularly occurring disturbances increase the probability of fog and low-cloud occurrence in the central Namib in two main ways: (1) an anomalously dry free troposphere in the coastal region of the Namib leads to stronger longwave cooling of the marine boundary layer, increasing low-cloud cover, especially over the ocean where the anomaly is strongest; (2) local wind systems are modulated, leading to an onshore anomaly of marine boundary-layer air masses. This is consistent with air mass back trajectories and a principal component analysis of spatial wind patterns that point to advected marine boundary-layer air masses on fog and low-cloud days, whereas subsiding continental air masses dominate on clear days. Large-scale free-tropospheric moisture transport into southern Africa seems to be a key factor modulating the onshore advection of marine boundary-layer air masses during April, May, and June, as the associated increase in greenhouse gas warming and thus surface heating are observed to contribute to a continental heat low anomaly. A statistical model is trained to discriminate between fog/low-cloud and clear days based on information on large-scale dynamics. The model accurately predicts fog and low-cloud days, illustrating the importance of large-scale pressure modulation and advective processes. It can be concluded that regional fog in the Namib is predominantly of an advective nature and that fog and low-cloud cover is effectively maintained by increased cloud-top radiative cooling. Seasonally different manifestations of synoptic-scale disturbances act to modify its day-to-day variability and the balance of mechanisms leading to its formation and maintenance. The results are the basis for a new conceptual model of the synoptic-scale mechanisms that control fog and low-cloud variability in the Namib Desert and will guide future studies of coastal fog regimes.

scholarly journals Transport by deep convection in basin-scale geostrophic circulation: turbulence-resolving simulations

2019 ◽  
Vol 865 ◽  
pp. 681-719
Author(s):  
Catherine A. Vreugdenhil ◽  
Bishakhdatta Gayen ◽  
Ross W. Griffiths
Keyword(s):  
Boundary Layer ◽  
Thermal Boundary Layer ◽  
Large Scale ◽  
Deep Convection ◽  
Ocean Basin ◽  
Open Ocean ◽  
Small Scale ◽  
Large Scale Circulation ◽  
Buoyancy Forcing ◽  
Basin Scale

Direct numerical simulations are used to investigate the nature of fully resolved small-scale convection and its role in large-scale circulation in a rotating $f$-plane rectangular basin with imposed surface temperature difference. The large-scale circulation has a horizontal geostrophic component and a deep vertical overturning. This paper focuses on convective circulation with no wind stress, and buoyancy forcing sufficiently strong to ensure turbulent convection within the thermal boundary layer (horizontal Rayleigh numbers $Ra\approx 10^{12}{-}10^{13}$). The dynamics are found to depend on the value of a convective Rossby number, $Ro_{\unicode[STIX]{x0394}T}$, which represents the strength of buoyancy forcing relative to Coriolis forces. Vertical convection shifts from a mean endwall plume under weak rotation ($Ro_{\unicode[STIX]{x0394}T}>10^{-1}$) to ‘open ocean’ chimney convection plus mean vertical plumes at the side boundaries under strong rotation ($Ro_{\unicode[STIX]{x0394}T}<10^{-1}$). The overall heat throughput, horizontal gyre transport and zonally integrated overturning transport are then consistent with scaling predictions for flow constrained by thermal wind balance in the thermal boundary layer coupled to vertical advection–diffusion balance in the boundary layer. For small Rossby numbers relevant to circulation in an ocean basin, vertical heat transport from the surface layer into the deep interior occurs mostly in ‘open ocean’ chimney convection while most vertical mass transport is against the side boundaries. Both heat throughput and the mean circulation (in geostrophic gyres, boundary currents and overturning) are reduced by geostrophic constraints.

scholarly journals The Dryline on 22 May 2002 during IHOP_2002: Convective-Scale Measurements at the Profiling Site

2006 ◽  
Vol 134 (1) ◽  
pp. 294-310 ◽  
Author(s):  
Belay Demoz ◽  
Cyrille Flamant ◽  
Tammy Weckwerth ◽  
David Whiteman ◽  
Keith Evans ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Continuous Wave ◽  
Doppler Radar ◽  
Layer Structure ◽  
Deep Convection ◽  
Radiosonde Data ◽  
Small Scale ◽  
Convective Initiation ◽  
Near Surface ◽  
Fine Line

Abstract A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer. The eastern dryline advanced and then retreated over the Homestead profiling site in the Oklahoma panhandle, providing conditions ripe for a detailed observation of the small-scale variability within the boundary layer and the dryline. In situ aircraft data, dropsonde and radiosonde data, along with NCAR S-band dual-polarization Doppler radar (S-Pol) measurements, are also used to provide the larger-scale picture of the double-dryline environment. Moisture and temperature jumps of about 3 g kg−1 and 1–2 K, respectively, were observed across the eastern radar fine line (dryline), more than the moisture jumps (1–2 g kg−1) observed across the western radar fine line (secondary dryline). Most updraft plumes observed were located on the moist side of the eastern dryline with vertical velocities exceeding 3 m s−1 and variable horizontal widths of 2–5 km, although some were as wide as 7–8 km. These updrafts were up to 1.5 g kg−1 moister than the surrounding environment. Although models suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms over the intensive observation region (IOR). Possible reasons for this lack of convection are discussed. Strong capping inversion and moisture detrainment between the lifting condensation level and the level of free convection related to an overriding drier air, together with the relatively small near-surface moisture values (less than 10 g kg−1), were detrimental to CI in this case.

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