scholarly journals Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part I: Formulation and the Case of the Tropical Oceans

2011 ◽  
Vol 68 (10) ◽  
pp. 2169-2190 ◽  
Author(s):  
Yevgeniy Frenkel ◽  
Boualem Khouider ◽  
Andrew J. Majda
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Deep Convection ◽  
Potential Temperature ◽  
Bowen Ratio ◽  
Tropical Convection ◽  
Heat Fluxes ◽  
Tropical Precipitation ◽  
Shallow Cumulus ◽  
Early Afternoon

Abstract The variation of tropical precipitation due to the diurnal cycle of solar heating is examined here in the context of two simple models for tropical convection. The models utilize three cloud types—congestus, deep, and stratiform—that are believed to characterize organized tropical convection and are based on the two first baroclinic modes of vertical structure plus a boundary layer mode. The two models differ mainly in the way they treat the boundary layer dynamics. The first one is purely thermodynamical and is reduced to a single equation for the equivalent potential temperature θe connecting the boundary layer to the upper troposphere through downdrafts and to the surface through evaporation while the second uses full bulk boundary layer (FBBL) dynamics with a careful separation between sensible and latent heat fluxes and parameterization of nonprecipitating shallow cumulus. It turns out that in the case of the precipitation over the ocean where the Bowen ratio is small, both models yield a qualitatively similar solution, characterized by an overnight initiation and early morning peak in precipitation consistent with observations. The modeled diurnal cycle of precipitation over the ocean is divided into four cyclic phases: 1) a CAPE (re)generation phase characterized by the enhancement of the boundary layer θe and moisture fluxes during midday and early afternoon that is followed by 2) a (re)moistening phase dominated by congestus heating during the late afternoon and moistening from downdrafts (due to detrainment of shallow cumulus, specifically in the FBBL model) and radiative cooling that lasts until midnight. 3) Deep convection is initiated around midnight when the midtroposphere is sufficiently moist and cool and (re)establishes the precipitation level near its radiative convective equilibrium (1 K day−1) and then 4) peaks with sunrise at 0600 LST to yield a precipitation maximum of roughly 2 K day−1 at around 0900 LST that dries the troposphere and consumes CAPE and closes the cycle.

scholarly journals Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part II: The Continental Regime

2011 ◽  
Vol 68 (10) ◽  
pp. 2192-2207 ◽  
Author(s):  
Yevgeniy Frenkel ◽  
Boualem Khouider ◽  
Andrew J. Majda
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Bowen Ratio ◽  
Tropical Convection ◽  
Surface Fluxes ◽  
Sensible Heat ◽  
Middle Troposphere ◽  
Tropical Precipitation ◽  
Strong Inversion ◽  
Active Mixing

Abstract The variation of precipitation over land due to the diurnal cycle of solar heating is examined here in the context of a simple multicloud model for tropical convection with bulk atmospheric boundary layer (ABL) dynamics. The model utilizes three cloud types (congestus, deep, and stratiform) that are believed to characterize organized tropical convection based on the first two baroclinic modes of vertical structure in the free troposphere, coupled to the ABL through full bulk boundary layer (FBBL) dynamics, that allow a careful separation between sensible and latent heat surface fluxes. In a land parameter regime, characterized by a strong inversion profile, a large Bowen ratio of 0.4, and active mixing of sensible heat due to cumulus entrainment and downdraft fluxes at the top of the ABL, the model supports a stable 1-day periodic solution that is characterized by a pronounced (7 K day−1) afternoon peak in precipitation consistent with observations of tropical precipitation over continental regions. The current study suggests a division of the diurnal cycle of precipitation over land into a cycle of five phases: 1) an overnight phase of a radiative–convective equilibrium (RCE) state between 2000 and 0600 LST; 2) an early morning CAPE buildup accompanied by a sudden rise in precipitation that quickly dries the middle troposphere occurs between 0600 and roughly 1000 LST; 3) a moistening phase between roughly 1000 and 1600 LST; 4) a phase of maximum precipitation between 1600 and 1800 LST that dries the middle troposphere and quickly consumes CAPE; and 5) a rapid remoistening phase that restores the moisture level to sustain the overnight RCE precipitation and connects to phase 1 in the cycle. Sensitivity tests in the model confirm that the late afternoon precipitation maximum over land depends crucially on a strong inversion, the large Bowen ratio, and the active mixing of sensible heat due to cumulus entrainment and downdraft fluxes at the top of the ABL.

scholarly journals Reexamining the Gradient and Countergradient Representation of the Local and Nonlocal Heat Fluxes in the Convective Boundary Layer

2018 ◽  
Vol 75 (7) ◽  
pp. 2317-2336 ◽  
Author(s):  
Bowen Zhou ◽  
Shiwei Sun ◽  
Kai Yao ◽  
Kefeng Zhu
Keyword(s):  
Boundary Layer ◽  
Temperature Gradient ◽  
Mixed Layer ◽  
Convective Boundary Layer ◽  
Correction Term ◽  
Potential Temperature ◽  
Heat Fluxes ◽  
Gradient Term ◽  
Convective Boundary ◽  
Upper Mixed Layer

Abstract Turbulent mixing in the daytime convective boundary layer (CBL) is carried out by organized nonlocal updrafts and smaller local eddies. In the upper mixed layer of the CBL, heat fluxes associated with nonlocal updrafts are directed up the local potential temperature gradient. To reproduce such countergradient behavior in parameterizations, a class of planetary boundary layer schemes adopts a countergradient correction term in addition to the classic downgradient eddy-diffusion term. Such schemes are popular because of their simple formulation and effective performance. This study reexamines those schemes to investigate the physical representations of the gradient and countergradient (GCG) terms, and to rebut the often-implied association of the GCG terms with heat fluxes due to local and nonlocal (LNL) eddies. To do so, large-eddy simulations (LESs) of six idealized CBL cases are performed. The GCG fluxes are computed a priori with horizontally averaged LES data, while the LNL fluxes are diagnosed through conditional sampling and Fourier decomposition of the LES flow field. It is found that in the upper mixed layer, the gradient term predicts downward fluxes in the presence of positive mean potential temperature gradient but is compensated by the upward countergradient correction flux, which is larger than the total heat flux. However, neither downward local fluxes nor larger-than-total nonlocal fluxes are diagnosed from LES. The difference reflects reduced turbulence efficiency for GCG fluxes and, in terms of physics, conceptual deficiencies in the GCG representation of CBL heat fluxes.

scholarly journals Occurrence and growth of sub-50 nm aerosol particles in the Amazonian boundary layer

2021 ◽  
Author(s):  
Marco A. Franco ◽  
Florian Ditas ◽  
Leslie Ann Kremper ◽  
Luiz A. T. Machado ◽  
Meinrat O. Andreae ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Meteorological Data ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Potential Temperature ◽  
Meteorological Conditions ◽  
Subsequent Growth ◽  
Wet Season ◽  
Particle Injection ◽  
Continuous Growth

Abstract. New particle formation (NPF), referring to the nucleation of molecular clusters and their subsequent growth into the cloud condensation nuclei (CCN) size range, is a globally significant and climate-relevant source of atmospheric aerosols. Classical NPF exhibiting continuous growth from a few nanometers to the Aitken mode around 60–70 nm is widely observed in the planetary boundary layer (PBL) around the world, but not in central Amazonia. Here, classical NPF events are rarely observed in the PBL, but instead, NPF begins in the upper troposphere (UT), followed by downdraft injection of sub-50 nm (CN< 50) particles into the PBL and their subsequent growth. Central aspects of our understanding of these processes in the Amazon have remained enigmatic, however. Based on more than six years of aerosol and meteorological data from the Amazon Tall Tower Observatory (ATTO, Feb 2014 to Sep 2020), we analyzed the diurnal and seasonal patterns as well as meteorological conditions during 254 of such Amazonian growth events on 217 event days, which show a sudden occurrence of particles between 10 and 50 nm in the PBL, followed by their growth to CCN sizes. The occurrence of events was significantly higher during the wet season, with 88 % of all events from January to June, than during the dry season, with 12 % from July to December, probably due to differences in the condensation sink (CS), atmospheric aerosol load, and meteorological conditions. Across all events, a median growth rate (GR) of 5.2 nm h−1 and a median CS of 0.0011 s−1 were observed. The growth events were more frequent during the daytime (74 %) and showed higher GR (5.9 nm h−1) compared to nighttime events (4.0 nm h−1), emphasizing the role of photochemistry and PBL evolution in particle growth. About 70 % of the events showed a negative anomaly of the equivalent potential temperature (∆θ'e) – as a marker for downdrafts – and a low satellite brightness temperature (Tir) – as a marker for deep convective clouds – in good agreement with particle injection from the UT in the course of strong convective activity. About 30 % of the events, however, occurred in the absence of deep convection, partly under clear sky conditions, and with a positive ∆θ'e anomaly. Therefore, these events do not appear to be related to downdraft injection and suggest the existence of other currently unknown sources of the sub-50 nm particles.

scholarly journals Boundary Layer Dynamics in a Simple Model for Convectively Coupled Gravity Waves

2009 ◽  
Vol 66 (9) ◽  
pp. 2780-2795 ◽  
Author(s):  
Michael L. Waite ◽  
Boualem Khouider
Keyword(s):  
Boundary Layer ◽  
Gravity Waves ◽  
Deep Convection ◽  
Potential Temperature ◽  
Phase Speed ◽  
Free Troposphere ◽  
Synoptic Scale ◽  
Basic States ◽  
Boundary Layer Dynamics ◽  
Baroclinic Modes

Abstract A simplified model of intermediate complexity for convectively coupled gravity waves that incorporates the bulk dynamics of the atmospheric boundary layer is developed and analyzed. The model comprises equations for velocity, potential temperature, and moist entropy in the boundary layer as well as equations for the free tropospheric barotropic (vertically uniform) velocity and first two baroclinic modes of vertical structure. It is based on the multicloud model of Khouider and Majda coupled to the bulk boundary layer–shallow cumulus model of Stevens. The original multicloud model has a purely thermodynamic boundary layer and no barotropic velocity mode. Here, boundary layer horizontal velocity divergence is matched with barotropic convergence in the free troposphere and yields environmental downdrafts. Both environmental and convective downdrafts act to transport dry midtropospheric air into the boundary layer. Basic states in radiative–convective equilibrium are found and are shown to be consistent with observations of boundary layer and free troposphere climatology. The linear stability of these basic states, in the case without rotation, is then analyzed for a variety of tropospheric regimes. The inclusion of boundary layer dynamics—specifically, environmental downdrafts and entrainment of free tropospheric air—enhances the instability of both the synoptic-scale moist gravity waves and nonpropagating congestus modes in the multicloud model. The congestus mode has a preferred synoptic-scale wavelength, which is absent when a purely thermodynamic boundary layer is employed. The weak destabilization of a fast mesoscale wave, with a phase speed of 26 m s−1 and coupling to deep convection, is also discussed.

scholarly journals Is there a stratospheric fountain?

2007 ◽  
Vol 7 (3) ◽  
pp. 8933-8950 ◽  
Author(s):  
J.-P. Pommereau ◽  
G. Held
Keyword(s):  
Gravity Waves ◽  
Diurnal Cycle ◽  
Thermal Structure ◽  
Potential Temperature ◽  
Chemical Species ◽  
Maximum Amplitude ◽  
Weather Radar ◽  
Lapse Rate ◽  
Early Afternoon ◽  
The Impact

Abstract. The impact of convection on the thermal structure of the Tropical Tropopause Layer (TTL) was investigated from a series of four daily radiosonde ascents and weather S-band radar observations carried out during the HIBISCUS campaign in the South Atlantic Convergence Zone in Southeast Brazil in February 2004. The temperature profiles display a large impact of convective activity on the thermal structure of the TTL. Compared to non-active periods, convection is observed to result in a cooling of 4.5°C to 7.5°C at the Lapse Rate Tropopause at 16 km, propagating up to 19 km or 440 K potential temperature levels in the stratosphere in most intense convective cases. Consistent with the diurnal cycle of echo top heights seen by a S-band weather radar, a systematic temperature diurnal cycle is observed in the layer, displaying a rapid cooling of 3.5°C on average (–9°, –2°C extremes) during the development phase of convection in the early afternoon during the most active period. Since the cooling occurs during daytime within a timescale of 6-h, its maximum amplitude is at the altitude of the Cold Point Tropopause at 390 K and temperature fluctuations associated to gravity waves do not display significant diurnal change, the afternoon cooling of the TTL cannot be attributed to radiation, advection, gravity waves or adiabatic lofting. It implies a fast insertion of adiabatically cooled air parcels by overshooting turrets followed by mixing with the warmer environment. During most intense convective days, the overshoot is shown to penetrate the stratosphere up to 450 K potential temperature level. Such fast updraft offers an explanation for the presence of ice crystals, and enhanced water vapour layers observed up to 18–19 km (410–430 K) in the same area by the HIBISCUS balloons and the TROCCINOX Geophysica aircraft, as well as high tropospheric chemical species concentrations in the TTL over land observed from space. Overall, injection of cold air by irreversible mixing of convective overshoots as proposed by Danielsen (1982) do not appear as episodic isolated features, but common and systematic events over a land convective area, that is a Stratospheric Fountain. Though the two-stages process proposed by Sherwood (2000) may also be operative, it offers a mechanism for producing the chemical, moisture and thermal properties observed in the stratosphere. The consistency between convective cooling of the TTL and weather radar echoes heights observed during the summer over South-East Brazil and the TRMM radar OPFs and LIS lightning events maximum frequencies, the latter showing also maximum events over Africa, South-East Asia, the Indonesian Islands and Northern Australia depending on the season (Liu and Zipser, 2005), suggests the existence of several "Stratospheric Fountains" over continents instead of the oceanic area of Micronesia as proposed by Newell and Gould-Stewart (1982), which appears a region of little overshoot.

The Use of High-Resolution Sounding Data to Evaluate and Optimize Nonlocal PBL Schemes for Simulating the Slightly Stable Upper Convective Boundary Layer

2019 ◽  
Vol 147 (10) ◽  
pp. 3825-3841 ◽  
Author(s):  
Xiao-Ming Hu ◽  
Ming Xue ◽  
Xiaolan Li
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Thermal Structure ◽  
Potential Temperature ◽  
Wrf Model ◽  
Radiosonde Data ◽  
Neutral Stability ◽  
Convective Boundary ◽  
Early Afternoon ◽  
Peak Value

Abstract Since the 1950s, a countergradient flux term has been added to some K-profile-based first-order PBL schemes, allowing them to simulate the slightly statically stable upper part of the convective boundary layer (CBL) observed in a limited number of aircraft soundings. There is, however, substantial uncertainty in inferring detailed CBL structure, particularly the level of neutral stability (zn), from such a limited number of soundings. In this study, composite profiles of potential temperature are derived from multiyear early afternoon radiosonde data over Beijing, China. The CBLs become slightly stable above zn ~ 0.31–0.33zi, where zi is the CBL depth. These composite profiles are used to evaluate two K-profile PBL schemes, the Yonsei University (YSU) and Shin–Hong (SH) schemes, and to optimize the latter through parameter calibration. In one-dimensional simulations using the WRF Model, YSU simulates a stable CBL above zn ~ 0.24zi, while default SH simulates a thick superadiabatic lower CBL with zn ~ 0.45zi. Experiments with the analytic solution of a K-profile PBL model show that adjusting the countergradient flux profile leads to significant changes in the thermal structure of CBL, informing the calibration of SH. The SH scheme replaces the countergradient heat flux term in its predecessor YSU scheme with a three-layer nonlocal heating profile, with fnl specifying the peak value and z*SL specifying the height of this peak value. Increasing fnl to 1.1 lowers zn, but to too low a value, while simultaneously increasing z*SL to 0.4 leads to a more appropriate zn ~ 0.36zi. The calibrated SH scheme performs better than YSU and default SH for real CBLs.

scholarly journals Seasonal and Diurnal Cycles of Surface Boundary Layer and Energy Balance in the Central Andes of Perú, Mantaro Valley

Atmosphere ◽  
2019 ◽  
Vol 10 (12) ◽  
pp. 779 ◽  
Author(s):  
José Luis Flores-Rojas ◽  
Joan Cuxart ◽  
Manuel Piñas-Laura ◽  
Stephany Callañaupa ◽  
Luis Suárez-Salas ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Energy Balance ◽  
Bowen Ratio ◽  
Heat Fluxes ◽  
Surface Boundary ◽  
Sensible Heat ◽  
Surface Boundary Layer ◽  
Mountain Valley ◽  
Arid Conditions

The present study presents a detailed analysis of the diurnal and monthly cycles the surface boundary layer and of surface energy balance in a sparse natural vegetation canopy on Huancayo observatory (12.04 ∘ S, 75.32 ∘ W, 3313 m ASL), which is located in the central Andes of Perú (Mantaro Valley) during an entire year (May 2018–April 2019). We used a set of meteorological sensors (temperature, relative humidity, wind) installed in a gradient tower 30 m high, a set of radiative sensors to measure all irradiance components, and a set of tensiometers and heat flux plate to measure soil moisture, soil temperatures and soil heat flux. To estimate turbulent energy fluxes (sensible and latent), two flux–gradient methods: the aerodynamic method and the Bowen-ratio energy-balance method were used. The ground heat flux at surface was estimated using a molecular heat transfer equation. The results show minimum mean monthly temperatures and more stable conditions were observed in June and July before sunrise, while maximum mean monthly temperatures in October and November and more unstable conditions in February and March. From May to August inverted water vapor profiles near the surface were observed (more intense in July) at night hours, which indicate a transfer of water vapor as dewfall on the surface. The patterns of wind direction indicate well-defined mountain–valley circulation from south-east to south-west especially in fall–winter months (April–August). The maximum mean monthly sensible heat fluxes were found in June and September while minimum in February and March. Maximum mean monthly latent heat fluxes were found in February and March while minimum in June and July. The surface albedo and the Bowen ratio indicate semi-arid conditions in wet summer months and extreme arid conditions in dry winter months. The comparisons between sensible heat flux ( Q H ) and latent heat flux ( Q E ), estimated by the two methods show a good agreement (R 2 above 0.8). The comparison between available energy and the sum of Q E and Q H fluxes shows a good level of agreement (R 2 = 0.86) with important imbalance contributions after sunrise and around noon, probably by advection processes generated by heterogeneities on the surface around the Huancayo observatory and intensified by the mountain–valley circulation.

scholarly journals Boundary Layer Quasi-Equilibrium Limits Convective Intensity Enhancement from the Diurnal Cycle in Surface Heating

2019 ◽  
Vol 77 (1) ◽  
pp. 217-237
Author(s):  
Zachary R. Hansen ◽  
Larissa E. Back ◽  
Peigen Zhou
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Large Scale ◽  
Deep Convection ◽  
Reanalysis Data ◽  
Surface Fluxes ◽  
Quasi Equilibrium ◽  
The Impact ◽  
Precipitation Enhancement ◽  
High Percentile

Abstract A combination of cloud-permitting model (CPM) simulations, satellite, and reanalysis data are used to test whether the diurnal cycle in surface temperature has a significant impact on the intensity of deep convection as measured by high-percentile updraft velocities, lightning, and CAPE. The land–ocean contrast in lightning activity shows that convective intensity varies between land and ocean independently from convective quantity. Thus, a mechanism that explains the land–ocean contrast must be able to do so even after controlling for precipitation variations. Motivated by the land–ocean contrast, we use idealized CPM simulations to test the impact of the diurnal cycle on high-percentile updrafts. In simulations, updrafts are somewhat enhanced due to large-scale precipitation enhancement by the diurnal cycle. To control for large-scale precipitation, we use statistical sampling techniques. After controlling for precipitation enhancement, the diurnal cycle does not affect convective intensities. To explain why sampled updrafts are not enhanced, we note that CAPE is also not increased, likely due to boundary layer quasi equilibrium (BLQE) occurring over our land area. Analysis of BLQE in terms of net positive and negative mass flux finds that boundary layer entrainment, and even more importantly downdrafts, account for most of the moist static energy (MSE) sink that is balancing surface fluxes. Using ERA-Interim data, we also find qualitative evidence for BLQE over land in the real world, as high percentiles of CAPE are not greater over land than over ocean.

scholarly journals Inclusion of a Drag Approach in the Town Energy Balance (TEB) Scheme: Offline 1D Evaluation in a Street Canyon

2008 ◽  
Vol 47 (10) ◽  
pp. 2627-2644 ◽  
Author(s):  
R. Hamdi ◽  
V. Masson
Keyword(s):  
Boundary Layer ◽  
Energy Balance ◽  
Street Canyon ◽  
Potential Temperature ◽  
Single Layer ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Surface Boundary ◽  
The Town ◽  
Meteorological Models

Abstract The Town Energy Balance module bridges the micro- and mesoscale and simulates local-scale urban surface energy balance for use in mesoscale meteorological models. Previous offline evaluations show that this urban module is able to simulate in good behavior road, wall, and roof temperatures and to correctly partition radiation forcing into turbulent and storage heat fluxes. However, to improve prediction of the meteorological fields inside the street canyon, a new version has been developed, following the methodology described in a companion paper by Masson and Seity. It resolves the surface boundary layer inside and above urban canopy by introducing a drag force approach to account for the vertical effects of buildings. This new version is tested offline, with one-dimensional simulation, in a street canyon using atmospheric and radiation data recorded at the top of a 30-m-high tower as the upper boundary conditions. Results are compared with simulations using the original single-layer version of the Town Energy Balance module on one hand and with measurements within and above a street canyon on the other hand. Measurements were obtained during the intensive observation period of the Basel Urban Boundary Layer Experiment. Results show that this new version produces profiles of wind speed, friction velocity, turbulent kinetic energy, turbulent heat flux, and potential temperature that are more consistent with observations than with the single-layer version. Furthermore, this new version can still be easily coupled to mesoscale meteorological models.

scholarly journals Evaluation of Near-Surface Variables and the Vertical Structure of the Boundary Layer in CMIP5 Models

2015 ◽  
Vol 28 (13) ◽  
pp. 5233-5253 ◽  
Author(s):  
Gunilla Svensson ◽  
Jenny Lindvall
Keyword(s):  
Boundary Layer ◽  
Great Plains ◽  
Diurnal Cycle ◽  
Vertical Structure ◽  
Heat Fluxes ◽  
Cmip5 Models ◽  
Turbulent Heat ◽  
Climate Change Scenarios ◽  
Near Surface ◽  
Turbulent Heat Fluxes

Abstract The diurnal cycles of near-surface variables and turbulent heat fluxes are evaluated in 16 models from phase 5 of CMIP (CMIP5) and compared with observations from 26 flux tower sites. The diurnal cycle of 2-m temperature agrees well in general with what is observed. The amplitude of the diurnal cycle of wind speed shows a large intermodel spread and is often overestimated at midlatitude grassland sites and underestimated at midlatitude forest sites. There is a substantial systematic negative bias in the nighttime net surface radiative flux, which is partly compensated for by the turbulent heat fluxes. Four models (CESM1, BCC_CSM1.1, HadGEM2-A, and IPSL-CM5A) are evaluated in more detail, including the vertical structure of the atmospheric boundary layer, at the ARM Southern Great Plains site in Oklahoma. At that site, all models tend to frequently overestimate the boundary layer depth and the wind turning in the boundary layer reveals large intermodel differences. In summer, these models exhibit a substantial warm bias with particularly high daytime temperatures. These high temperatures are associated with very small latent heat fluxes, indicating that the soil is too dry, which is likely to impact climate change scenarios.

Sign in / Sign up

Export Citation Format

Share Document