Specific Humidity Profiles Research Articles

Vertical Profiles of Ozone Concentration Collected by an Unmanned Aerial Vehicle and the Mixing of the Nighttime Boundary Layer over an Amazonian Urban Area

The nighttime boundary layer was studied in an urban area surrounded by tropical forest by use of a copter-type unmanned aerial vehicle (UAV) in central Amazonia during the wet season. Fifty-seven vertical profiles of ozone concentration, potential temperature, and specific humidity were collected from surface to 500 m above ground level (a.g.l.) at high vertical and temporal resolutions by use of embedded sensors on the UAV. Abrupt changes in ozone concentration with altitude served as a proxy of nighttime boundary layer (NBL) height for the case of a normal, undisturbed, stratified nighttime atmosphere, corresponding to 40% of the cases. The median height of the boundary layer was 300 m. A turbulent mixing NBL constituted 28% of the profiles, while the median height of the boundary layer was 290 m. The remaining 32% of profiles corresponded to complex atmospheres without clear boundary layer heights. The occurrence of the three different cases correlated well with relative cloud cover. The results show that the standard nighttime model widely implemented in chemical transport models holds just 40% of the time, suggesting new challenges in modeling of regional nighttime chemistry. The boundary layer heights were also somewhat higher than observed previously over forested and pasture areas in Amazonia, indicating the important effect of the urban heat island.

How Accurate Are Modern Atmospheric Reanalyses for the Data-Sparse Tibetan Plateau Region?

AbstractMore than 6000 independent radiosonde observations from three major Tibetan Plateau experiments during the warm seasons (May–August) of 1998, 2008, and 2015–16 are used to assess the quality of four leading modern atmospheric reanalysis products (CFSR/CFSv2, ERA-Interim, JRA-55, and MERRA-2), and the potential impact of satellite data changes on the quality of these reanalyses in the troposphere over this data-sparse region. Although these reanalyses can reproduce reasonably well the overall mean temperature, specific humidity, and horizontal wind profiles against the benchmark independent sounding observations, they have nonnegligible biases that can be potentially bigger than the analysis-simulated mean regional climate trends over this region. The mean biases and mean root-mean-square errors of winds, temperature, and specific humidity from almost all reanalyses are reduced from 1998 to the two later experiment periods. There are also considerable differences in almost all variables across different reanalysis products, though these differences also become smaller during the 2008 and 2015–16 experiments, in particular for the temperature fields. The enormous increase in the volume and quality of satellite observations assimilated into reanalysis systems is likely the primary reason for the improved quality of the reanalyses during the later field experiment periods. Besides differences in the forecast models and data assimilation methodology, the differences in performance between different reanalyses during different field experiment periods may also be contributed by differences in assimilated information (e.g., observation input sources, selected channels for a given satellite sensor, quality-control methods).

Assimilation of INSAT-3D Satellite Data in WRF Model

The three-dimensional variational data assimilation system of the Weather Research and Forecasting (WRF) model is used in this study to assimilate the INSAT-3D temperature and specific humidity profiles over west coast of India, Mumbai, during 0000 UTC of 11–12 July 2016. The analysis obtained after assimilation is compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis. After ECMWF assimilation, the results of the rainfall predictions show improvement over the Indian region. Overall, the assimilation of INSAT-3D is compared with ECMWF analysis, which shows quite good improvement in temperature and humidity analysis. Positive improvements are noticed in 24-h WRF model-predicted rainfall on assimilation of INSAT-3D thermodynamic profiles over west coast of India, Mumbai.

GPS RO precipitable water as a possible proxy for rainfall distribution over tropical South America: qualitative analysis

A principal component analysis (PCA) is applied to radio occultation vertical profiles of water vapor and rainfall data over tropical South America (SA). The low-level flow over this region is climatologically dominated by the Atlantic anticyclone and the trade winds. This results in an inflow of oceanic moist air to the continent from the east, which is forced to lift over the Andes range at the west. Precipitable water data (PW) for the period June 2006–February 2014 derived from the COSMIC/FORMOSAT-3 mission and monthly accumulated rainfall data (R) obtained from Tropical Rainfall Measuring Mission (TRMM) are compared. PCA in modes S and T is performed to find qualitative similarities and differences between both variables. This study shows that specific humidity profiles are able to reproduce a significant amount of the precipitation patterns over tropical SA, as they explain most of the variability during the June 2006–February 2014 period. Therefore, PW may be used as a proxy for rainfall data over the region.

ISOLESC: A Coupled Isotope‐LSM‐LES‐Cloud Modeling System to Investigate the Water Budget in the Atmospheric Boundary Layer

AbstractStable isotopes of water (H2O, HDO, and H218O) are tracers that provide powerful constraints on water transport processes in the atmosphere. This paper presents a description of an atmospheric boundary layer (ABL) simulation system called ISOLESC that couples water isotope fractionation processes with a land surface model, a large eddy simulation model, and a two‐moment cloud microphysics parameterization. Results from two model configurations—one with shallow precipitating cumulus and the other for a cloud‐free ABL—are presented to evaluate the model performance and determine its sensitivity to isotopic parameterizations. The coupled model successfully reproduces important ABL statistics (ABL height, cloud fraction, and cloud liquid water content), the expected effects of mixing and fractionation on the time evolution of ABL vapor isotopic composition, and observed diurnal variations of near‐surface water vapor isotopic composition. For the current configuration, nondiscriminating entrainment contributes 17% to the subdaily time variation of near‐surface vapor deuterium excess, while surface evapotranspiration contributes 83%. The isotopic compositions of water vapor and cloud water are insensitive to mesh resolution, but the profiles of cloud water specific humidity, rainwater specific humidity, and its isotopic ratios show moderate response to changes in grid size. Since ISOLESC resolves the energy containing scales of turbulent motions in the ABL and incorporates microphysical processes, it can be used for constraining ABL parameterizations. We find that a further improvement of raindrop reevaporation in the current cloud microphysical scheme is required in order to produce realistic near‐surface raindrop deuterium excess for the case simulated here. We suggest that ISOLESC provides a quantitative framework for utilizing vapor‐phase isotopic measurements to study local hydrological processes.

Influence of Wind Direction on Thermodynamic Properties and Arctic Mixed‐Phase Clouds in Autumn at Utqiaġvik, Alaska

Seven years of autumnal ground‐based observations (September–November 2002–2008) at the U.S. Department of Energy Atmospheric Radiation Measurement North Slope of Alaska site have been analyzed for addressing the occurrence frequency and macrophysical and microphysical properties of Arctic mixed‐phase clouds (AMC), as well as the relationship between environmental parameters and AMC properties. In September and October, AMC occurrence frequency is 20–30% lower during a southerly wind when compared to the other wind directions; in November, the variation of AMC occurrence frequency with wind direction is small. The mean liquid water path in November is about half of that in October and September. When the surface is snow free, temperature (T) and specific humidity (q) profiles during a northerly wind are warmer and moister than those for the southerly wind. Northerly wind profiles have a higher relative humidity to ice (RHi) and lower atmosphere stability. Furthermore, the AMC occurrence frequency has a positive relationship with RHi and a negative relationship with stability. These two points may explain the lower AMC occurrence frequency during a southerly wind. During a northerly wind, AMCs have larger radar reflectivity, wider spectrum width, and larger Doppler velocity signatures. The stronger precipitation for AMC during a northerly wind is possibly due to the cleaner air masses from the ocean (north). With the same amount of q, the radar spectrum width has a higher frequency in the larger bins during a northerly wind. Both T, q, and radar reflectivity, radar spectrum width profiles show evidence of deposition in the sub‐cloud layer in September and October.

Vertical Structure of Moisture Content over Europe

The vertical structure of water vapor content in the atmosphere strongly affects the amount of solar radiation reaching the Earth’s surface and processes associated with the formation of clouds and atmospheric precipitation. The purpose of this study was to assess the vertical differentiation of water vapor over Europe on a seasonal basis and also to evaluate the role of atmospheric circulation in changes therein. Daily values of specific humidity (SHUM) for the time period 1981–2015 were obtained from pressure levels available from ECMWF Era-Interim reanalysis data and used in the study. Eight grid points were analyzed in detail. Each point is representative of a region with different moisture conditions. SHUM profiles were then used to identify cases of moisture inversion. Horizontal flux of specific humidity (SHUMF) was analyzed for principal pressure levels that occur in both inversion-type and inversion-free situations. In addition, SHUM and SHUMF anomalies were identified for advection directions. The research results showed the existence of differences in the vertical structure of water vapor content in the troposphere over Europe, and the Northeastern Atlantic and the presence of moisture inversions not only in areas north of 60°N but also in temperate and subtropical zones. Inversions can occur in two different forms—surface-based and elevated. The occurrence of inversions varies with the seasons. The role of atmospheric circulation is observable in the winter and triggers both surpluses and shortages of moisture via the effect of specific pressure system types (significant role of seasonal pressure high) and via advection directions. In addition, there exists a clear difference between the structure of moisture in the atmospheric boundary layer and in the free atmosphere.

High-resolution humidity profiles retrieved from wind profiler radar measurements

Abstract. The retrieval of humidity profiles from wind profiler radars has already been documented in the past 30 years and is known to be neither as straightforward and nor as robust as the retrieval of the wind velocity. The main constraint to retrieve the humidity profile is the necessity to combine measurements from the wind profiler and additional measurements (such as observations from radiosoundings at a coarser time resolution). Furthermore, the method relies on some assumptions and simplifications that restrict the scope of its application. The first objective of this paper is to identify the obstacles and limitations and solve them, or at least define the field of applicability. To improve the method, we propose using the radar capacity to detect transition levels, such as the top level of the boundary layer, marked by a maximum in the radar reflectivity. This forces the humidity profile from the free troposphere and from the boundary layer to coincide at this level, after an optimization of the calibration coefficients, and reduces the error. The resulting mean bias affecting the specific humidity profile never exceeds 0.25 g kg−1. The second objective is to explore the capability of the algorithm to retrieve the humidity vertical profiles for an operational purpose by comparing the results with observations from a Raman lidar.

A New Strategy for Extracting ENSO Related Signals in the Troposphere and Lower Stratosphere from GNSS RO Specific Humidity Observations

El Niño-Southern Oscillation related signals (ENSORS) in the troposphere and lower stratosphere (TLS) are the prominent source of inter-annual variability in the weather and climate system of the Earth, and are especially important for monitoring El Niño-Southern Oscillation (ENSO). In order to reduce the influence of quasi-biennial oscillations and other unknown signals compared with the traditional empirical orthogonal functions (EOF) method, a new processing strategy involving fusion of a low-pass filter with an optimal filtering frequency (hereafter called the optimal low-pass filter) and EOF is proposed in this paper for the extraction of ENSORS in the TLS. Using this strategy, ENSORS in the TLS over different areas were extracted effectively from the specific humidity profiles provided by the Global Navigation Satellite System (GNSS) radio occultation (RO) of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission from June 2006 to June 2014. The spatial and temporal responses of the extracted ENSORS to ENSO at different altitudes in the TLS were analyzed. The results show that the most suitable areas for extracting ENSORS are over the areas of G25 (−25°S–25°N, 180°W–180°E) −G65(−65°S–65°N, 180°W–180°E) in the upper troposphere (250–200 hpa) which show a lag time of 3 months relative to the Oceanic Niño index (ONI). In the troposphere, ENSO manifests as a major inter-annual variation. The ENSORS extracted from the N3.4 (−5°S to 5°N, 120°W to 170°W) area are responsible for 83.59% of the variability of the total specific humidity anomaly (TSHA) at an altitude of 250 hpa. Over all other defined areas which contain the N3.4 areas, ENSORS also explain the major variability in TSHA. In the lower stratosphere, the extracted ENSORS present an unstable pattern at different altitudes because of the weak ENSO effect. Moreover, the spatial and temporal responses of ENSORS and ONI to ENSO across the globe are in good agreement. Over the areas with strong correlation between ENSORS and ONI, the larger the correlation coefficient is, the shorter the lag time between them. Furthermore, the ENSORS from zonal-mean specific humidity monthly anomalies at different altitudes can clearly present the vertical structure of ENSO in the troposphere. This study provides a new approach for monitoring ENSO events.

Performance evaluation of regional climate model to simulate sub-seasonal variability of Indian Summer Monsoon

The study aims to evaluate the regional climate model (RegCM) over South Asian (SA) CORDEX domain to represent seasonal and sub-seasonal variability of Indian Summer Monsoon (ISM). The model’s ability is evaluated by conducting two sets of experiments using one-tier approach of coupling the RegCM with a simple mixed-layer slab ocean model (SOM) and the two-tier approach of prescribing sea surface temperature (SST) to RegCM. Two model experiments are initialized at 1st January 2000 for a period of 13 year continuous simulation at a spatial resolution of 50 km. It is found that, one-tier approach realistically represents the spatial distribution of precipitation with significant improvement noticed over central India (CI) and head Bay of Bengal (BoB) regions. In addition, it also fairly reproduced the observed mean meridional circulation response to the diabatic heating produced during ISM. Most importantly, in one-tier approach the model could able to represent the observed SST and precipitation (P) relationship with significant improvement in correlation and model response time. An important result is the representation of northwest-southeast tilt of precipitation anomalies during active/break phase of monsoon. Additionally, the lagged response of vertical profiles of specific humidity, omega, vorticity and divergence over CI with respect to peak rainfall anomaly (active phase) are relatively better represented in one-tier approach. In brief, coupling improves the performance of RegCM in simulating the space–time characteristics of monsoon ISO mode.

A New Framework to Compare Mass-Flux Schemes Within the AROME Numerical Weather Prediction Model

In the Application of Research to Operations at Mesoscale (AROME) numerical weather forecast model used in operations at Meteo-France, five mass-flux schemes are available to parametrize shallow convection at kilometre resolution. All but one are based on the eddy-diffusivity–mass-flux approach, and differ in entrainment/detrainment, the updraft vertical velocity equation and the closure assumption. The fifth is based on a more classical mass-flux approach. Screen-level scores obtained with these schemes show few discrepancies and are not sufficient to highlight behaviour differences. Here, we describe and use a new experimental framework, able to compare and discriminate among different schemes. For a year, daily forecast experiments were conducted over small domains centred on the five French metropolitan radio-sounding locations. Cloud base, planetary boundary-layer height and normalized vertical profiles of specific humidity, potential temperature, wind speed and cloud condensate were compared with observations, and with each other. The framework allowed the behaviour of the different schemes in and above the boundary layer to be characterized. In particular, the impact of the entrainment/detrainment formulation, closure assumption and cloud scheme were clearly visible. Differences mainly concerned the transport intensity thus allowing schemes to be separated into two groups, with stronger or weaker updrafts. In the AROME model (with all interactions and the possible existence of compensating errors), evaluation diagnostics gave the advantage to the first group.

Retrieval of humidity and temperature profiles over the oceans from INSAT 3D satellite radiances

In this study, retrieval of temperature and humidity profiles of atmosphere from INSAT 3D-observed radiances has been accomplished. As the first step, a fast forward radiative transfer model using an Artificial neural network has been developed and it was proven to be highly effective, giving a correlation coefficient of 0.97. In order to develop this, a diverse set of physics-based clear sky profiles of pressure (P), temperature (T) and specific humidity (q) has been developed. The developed database was further used for geophysical retrieval experiments in two different frameworks, namely, an ANN and Bayesian estimation. The neural network retrievals were performed for three different cases, viz., temperature only retrieval, humidity only retrieval and combined retrieval. The temperature/humidity only ANN retrievals were found superior to combined retrieval using an ANN. Furthermore, Bayesian estimation showed superior results when compared with the combined ANN retrievals.

Validation of eight atmospheric reanalyses in the Antarctic Peninsula region

AbstractEight atmospheric reanalyses were compared against observed vertical profiles of temperature, specific humidity and wind speed collected by two research aircraft in February–March 2010 in the Antarctic Peninsula region. These data offered a rare possibility to validate reanalyses against independent in situ data which have not been assimilated into the reanalyses. The reanalyses had generally too moist profiles with too low wind speeds, but otherwise the errors in the reanalyses had large spatial differences. On the eastern side of the peninsula, the near‐surface temperatures were largely overestimated. None of the reanalyses outperformed the others in all variables, at all altitudes and on both sides of the peninsula. Generally, NCEP‐CFSR and MERRA had the smallest errors in temperature profiles, JRA‐55 had marginally the most accurate specific humidity profiles and NCEP‐CFSR had the best wind profiles. The reanalyses were coherent, although biased, on the western side of the Antarctic Peninsula, but on the eastern side the spread was large. All the reanalyses underestimated the variability between the individual profiles of temperature and wind speed. The modern reanalyses with a sufficient spatial resolution and an adequate data assimilation method outperformed the others, especially on the eastern side.

Simulations and parameterisation of shallow volcanic plumes of Piton de la Fournaise, Réunion Island, using Méso-NH version 4-9-3

Abstract. In mesoscale models (resolution ~ 1 km) used for regional dispersion of pollution plumes the volcanic heat sources and emissions of gases and aerosols, as well as the induced atmospheric convective motions, are all sub-grid-scale processes (mostly true for weak effusive eruptions) which need to be parameterised. We propose a modified formulation of the EDMF scheme (eddy diffusivity/mass flux) proposed by Pergaud et al. (2009) which is based on a single sub-grid updraft model. It is used to represent volcano induced updrafts tested for a case study of the January 2010 summit eruption of Piton de la Fournaise (PdF) volcano. The validation of this modified formulation using a reference large eddy simulation (LES) focuses on the ability of the model to transport tracer concentrations up to 1–2 km above the ground in the lower troposphere as is the case of majority of PdF eruptions. The modelled volcanic plume agrees reasonably with the profiles of SO2 (sulfur dioxide) tracer concentrations and specific humidity found from the reference LES. Sensitivity tests performed for the modified formulation of the EDMF scheme emphasise the sensitivity of the parameterisation to ambient fresh air entrainment at the plume base.

Ground-based direct-sun DOAS and airborne MAX-DOAS measurements of the collision-induced oxygen complex, O<sub>2</sub>O<sub>2</sub>, absorption with significant pressure and temperature differences

Abstract. The collision-induced O2 complex, O2O2, is a very important trace gas for understanding remote sensing measurements of aerosols, cloud properties and atmospheric trace gases. Many ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of the O2O2 optical depth require correction factors of 0.75 ± 0.1 to reproduce radiative transfer modeling (RTM) results for a nearly pure Rayleigh atmosphere. One of the potential causes of this discrepancy is uncertainty in laboratory-measured O2O2 absorption cross section temperature and pressure dependencies due to difficulties in replicating atmospheric conditions in the laboratory environment. This paper presents ground-based direct-sun (DS) and airborne multi-axis (AMAX) DOAS measurements of O2O2 absorption optical depths under actual atmospheric conditions in two wavelength regions (335–390 and 435–490 nm). DS irradiance measurements were made by the Washington State University research-grade Multi-Function Differential Spectroscopy Instrument instrument from 2007 to 2014 at seven sites with significant pressure (778 to 1013 hPa) and O2O2 profile-weighted temperature (247 to 275 K) differences. Aircraft MAX-DOAS measurements were conducted by the University of Colorado (CU) AMAX-DOAS instrument on 29 January 2012 over the Southern Hemispheric subtropical Pacific Ocean. Scattered solar radiance spectra were collected at altitudes between 9 and 13.2 km, with O2O2 profile-weighted temperatures of 231 to 244 K and nearly pure Rayleigh scattering conditions. Due to the well-defined DS air-mass factors during ground-based measurements and extensively characterized atmospheric conditions during the aircraft AMAX-DOAS measurements, O2O2 "pseudo" absorption cross sections, σ, are derived from the observed optical depths and estimated O2O2 column densities. Vertical O2O2 columns are calculated from the atmospheric sounding temperature, pressure and specific humidity profiles. Based on the ground-based atmospheric DS observations, there is no pressure dependence of the O2O2 σ within the measurement errors (3%). Two data sets are combined to derive the peak σ temperature dependence of the 360 and 477 nm dimer absorption bands from 231 to 275 K. DS and AMAX-derived peak σ ( O2O2) as a function of T can be described by a quadratic function at 360 nm and linear function at 477 nm with about 9% ± 2.5% per 44 K rate. Recent laboratory-measured O2O2 cross sections by Thalman and Volkamer (2013) agree with these "DOAS apparent" peak σ( O2O2) at 233, 253 and 273 K within 3%. Changes in the O2O2 spectral band shape at colder temperatures are observed for the first time in field data. Temperature effects on spectral band shapes can introduce errors in the retrieved O2O2 column abundances if a single room temperature σ( O2O2) is used in the DOAS analysis. Simultaneous fitting of σ( O2O2) at temperatures that bracket the ambient temperature range can reduce such errors. Our results show that laboratory-measured σ( O2O2) (Hermans, 2011, at 296 K and Thalman and Volkamer, 2013) are applicable for observations over a wide range of atmospheric conditions. Column densities derived using Hermans (2011) σ at 296 K require very small correction factors (0.94 ± 0.02 at 231 K and 0.99 ± 0.02 at 275 K) to reproduce theoretically calculated slant column densities for DS and AMAX-DOAS measurements. Simultaneous fitting of σ( O2O2) at 203 and 293 K further improved the results at UV and visible wavelengths for AMAX-DOAS.

Assessing the performance of GPS radio occultation measurements in retrieving tropospheric humidity in cloudiness: A comparison study with radiosondes, ERA-Interim, and AIRS data sets

We assess the impact that the Global Positioning System radio occultations (GPSRO) measurements have on complementing different data sets in characterizing the lower-to-middle tropospheric humidity in cloudy conditions over both land and oceans using data from 1 August 2006 to 31 October 2006. We use observations from rawinsondes, Global Positioning System radio occultations (GPSRO), Atmospheric Infrared Sounder (AIRS), and the European Center for Medium-Range Weather Forecasts Reanalysis Interim (ERA-Interim). During the selected time period, Constellation Observing System for Meteorology, Ionosphere, and Climate data were not assimilated in ERA-Interim. From each data set, we estimate a zonally averaged tropospheric specific humidity profile at tropical, middle, and high latitudes. Over land, we use rawinsondes as the ground truth and quantify the specific humidity differences and root-mean-square-errors (RMSEs) of the GPSRO, AIRS, and ERA-Interim profiles. GPSRO are beneficial in retrieving lower tropospheric humidity than upper tropospheric profiles, due to their loss of sensitivity at high altitudes. Blending GPSRO with ERA-Interim produces profiles with smaller humidity biases outside the tropics, but GPSRO data do not improve the humidity RMSE when compared to rawinsondes. Combining GPSRO with AIRS leads to smaller humidity bias at the tropics and high latitudes, while reducing humidity's RMSEs. Over oceans, no rawinsonde information is available, and we use ERA-Interim as a reference. Combining GPSRO with AIRS leads to smaller humidity RMSEs than AIRS. We conclude that cross-comparisons and synergies among multi-instrument observations are promising in advancing our knowledge of the tropospheric humidity in cloudy conditions. GPSRO data can contribute to improving humidity retrievals over cloud-covered regions, especially over land and within the boundary layer.

Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer

Abstract. Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations. The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget. Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations. Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.

The Impact of Radiation on the GABLS3 Large-Eddy Simulation through the Night and during the Morning Transition

Large-eddy simulation in the GABLS3 intercomparison is concerned with the developed stable boundary layer (SBL) and the ensuing morning transition. The impact of radiative transfer on simulations of this case is assessed. By the time of the reversal of the surface buoyancy flux, a modest reduction of the lapse rate in the developed SBL is apparent in simulations that include longwave radiation. Subsequently, with radiation, the developing mixed layer grows significantly more quickly, so that four hours after the transition the mixed layer is roughly 40 % deeper; the resulting profiles of potential temperature and specific humidity are in better agreement with observations. The inclusion of radiation does not substantively alter the shape of turbulent spectra, but it does indirectly reduce the variance of temperature fluctuations in the mixed layer. The deepening of the mixed layer is interpreted as a response to the reduction of the strength of the capping inversion, resulting from cumulative radiative cooling in the residual layer and around the top of the former SBL. Sensitivity studies are performed to separate the two effects. Solar radiative heating of the atmosphere has a smaller impact on the development of the mixed layer than does longwave radiative cooling and slightly reduces its rate of growth, compared to simulations including longwave radiation alone. These simulations demonstrate that nocturnal radiative processes have an important effect on the morning transition and that they should be considered in future large-eddy simulations of the transition.

Scalar Flux–Gradient Relationships Under Unstable Conditions over Water in Coastal Regions

The scalar flux–gradient relationships of temperature ( $$\phi _{T}$$ ) and specific humidity ( $$\phi _{q}$$ ) under unstable conditions are investigated using eddy-covariance measurements of air–sea turbulent fluxes and vertical profiles of temperature and specific humidity collected from a marine meteorological platform. The gradients of temperature and specific humidity are obtained from measurements at five heights above the sea surface using the log-square fitting method and the simpler first-order approximation method. The two methods yield similar results. The proposed flux–gradient relationships $$\phi _{T}$$ and $$\phi _{q}$$ covers a wide range of instability: the stability parameter $$\zeta $$ ranges from $$-$$ 0.1 to $$-$$ 50. The functional form of the proposed flux–gradient relationships is an interpolation between the Businger–Dyer relation and the free convection relation, which includes the “ $$-$$ 1/2” and “ $$-$$ 1/3” scaling laws at two different stability regimes. The widely used COARE 3.0 algorithm, which is an interpolation between the integrals of the Businger–Dyer and the free convection relations, is also evaluated and compared. The analysis and comparisons show that both schemes generate reasonable values of $$\phi _{q}$$ in the whole unstable regime. The COARE 3.0 algorithm, however, overestimates $$\phi _{T}$$ values under very unstable conditions. The errors in the flux–gradient relationships induced by the random errors in the turbulence measurements are assessed. When the random errors are taken into account, the observations agree with predictions of various schemes fairly well, implying that the dominant transport mechanism is adequately captured by the Monin–Obukhov similarity theory. The study also shows that $$\phi _{q}$$ is significantly $${>}\phi _{T}$$ under unstable conditions and that the ratio $$\phi _{q}/\phi _{T}$$ increases with $$-\zeta $$ . The ratio of $$\phi _{q}$$ to $$\phi _{T}$$ and the ratio of turbulent transport efficiencies of heat and water vapour ( $$R_{wT}/R_{wq}$$ ) suggest that heat is transported more efficiently than water vapour under unstable conditions.

Structural evolution of the Madden‐Julian Oscillation from COSMIC radio occultation data

The atmospheric temperature and specific humidity profiles derived from 4 years of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements during the boreal winter (November thru April in 2006–2010) were employed to study the large‐scale vertical structure of the Madden‐Julian Oscillation (MJO). In a composited MJO cycle, both RO temperature and moisture anomalies propagate eastward along with the enhanced MJO convection over the Indo‐Pacific region. In the same region, the RO temperature anomaly is positively correlated with the convection (Tropical Rainfall Measuring Mission (TRMM) rainfall) anomaly in the middle and upper troposphere (800 hPa–200 hPa) and negatively correlated in the lower troposphere and at the tropopause. A positive RO moisture anomaly has a westward tilt below 300 hPa and is well associated with the convection anomaly. RO bending angle and refractivity anomalies show vertical structure similar to that of temperature MJO anomaly above ∼300 hPa and to that of the moisture anomaly below that. These salient MJO features agree well with those revealed by global reanalysis data and meteorological satellite observations, such as the Atmospheric Infrared Sounder (AIRS), even though RO anomalies exhibit sharper structure at the tropopause region due to the higher vertical resolution of the RO. Investigation of the cold‐point tropopause anomaly indicates that the enhanced convection is preceded by cooling of the tropopause and followed by lowering of the tropopause height. The MJO moisture structures during two boreal winters, November 2007–April 2008 (La Niña) and November 2009–April 2010 (El Niño), are individually presented to demonstrate the El Niño‐Southern Oscillation (ENSO) effect on the MJO structure.