Humidity Advection Research Articles

What Are the Main Causes of Positive Subtropical Low Cloud Feedbacks in Climate Models?

AbstractWe investigate positive subtropical low cloud feedback mechanisms in climate models which have performed the CMIP6/CFMIP‐3 AMIP and AMIP uniform +4K experiments while saving CFMIP‐3 process diagnostics on model levels. Our analysis focuses on the trade cumulus/stratocumulus transition region between California and Hawaii, where positive low cloud feedbacks are present in the JJA season. We introduce a methodology to test various positive cloud feedback mechanisms proposed in the literature as the main causes of the low cloud responses in the models. Causal hypotheses are tested by comparing their predictions with the models' responses of clouds, cloud controlling factors, boundary layer depth and temperature/humidity tendencies to climate warming. Changes in boundary layer depth, relative humidity in the cloud layer, convective moistening rate and large‐scale humidity advection at the top of the boundary layer are shown to be crucial for identifying the main causes of the low cloud reductions in the models. For the cases examined, our approach narrows down the seven mechanisms considered to between one and three remaining candidates for each model. No single mechanism considered can explain the feedback in all of the models at the locations examined, but the surface latent heat flux/convective entrainment mechanism remains a candidate for BCC‐CSM2‐MR, IPSL‐CM6A‐LR, and MRI‐ESM2.0, while the surface upwelling longwave mechanism remains for CESM2, HadGEM3‐GC3.1‐LL, and MIROC6.

How Do Different Processes Shape Temperature Probability Distributions? A Percentile-Averaged Temperature Tendency Decomposition

Abstract Studying temperature probability distributions and the physical processes that shape them is important for understanding extreme temperature events. Previous work has used a conditional mean temperature framework to reveal whether horizontal temperature advection drives temperature to extreme or median values at a specific location as a method to dynamically interpret temperature probability distributions. In this paper, we generalize this method to study how other processes shape temperature probability distributions and explore the diverse effects of horizontal temperature advection on temperature probability distributions at different locations and different temperature percentiles. We apply this generalized method to several representative regions to demonstrate its use. We find that temperature advection drives temperatures toward more extreme values over most land in the midlatitudes (i.e., cold air advection occurs during cold anomalies and warm air advection occurs during warm anomalies). In contrast, we find that horizontal temperature advection dampens temperature anomalies in some coastal summer monsoon regions, where extreme temperatures result from other processes, such as horizontal humidity advection and vertical temperature advection. By calculating the mean of processes conditioned on the temperature percentile, this method enables composite analysis of processes that contribute to events for all percentiles and a range of processes. We show examples of composites at different percentiles for certain processes and regions to illustrate the conditional mean analysis. This general approach may benefit future studies related to temperature probability distributions and extreme events.

Classification of 2007 Winter Mist and Fog in São Paulo City, Brazil

Mist and fog observed at the Meteorological Station of the Institute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (EM/IAG) in the southeastern part of São Paulo city in the winter of 2007 were classified according to its formation processes: advection or radiation. Meteorological data from EM/IAG, Congonhas Airport and reanalysis from the European Center for Medium-Range Weather Forecast, as well as radiosondes from Campo de Marte Airport were used. In the 1981-2015 climatology, it was observed that these phenomena are more frequent in the winter, with an average of 11 events per month. For the winter of 2007, 21 advection and 28 radiation events were detected, with radiation fog occurring more frequently in June (13) and advection fog, in August (12). The South Atlantic Subtropical High favored the formation of radiation events in 24 cases; 15 of which had the contribution of humidity advected by the sea breeze. Post-frontal anticyclone was associated with the remaining 25 cases, of which 15 were classified as advection. Four cases were chosen for a more detailed analysis. On July 2, 2007, radiation mist occurred, with the injection of moisture by the sea breeze; the next day, there was also radiation mist, however, with no sea breeze. On the 20th of July, fog was caused by advection of humidity with north/northeast winds coming from the Serra da Cantareira and on the 27th, advection mist developed due to the intensification of winds associated with the movement of a post-frontal anticyclone.

Characteristics of Temperature and Humidity Inversions Based on High-Resolution Radiosonde Observations at Three Arctic Stations

AbstractThe Global Climate Observing System Reference Upper-Air Network (GRUAN) with high-vertical-resolution radiosonde data at three Arctic stations and European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data (ERA5) were used to investigate the characteristics of multiple temperature inversions (TI) and humidity inversions (HI) in this study. It is found that surface-based inversion (SBI) at two coastal stations exists throughout the whole year, mainly due to the surface cooling in cold months, advection warm months, and the orography of the stations. The seasonal variation of surfaced-based HI (SBHI) frequency is similar to that of SBI, and its intensity is greater in summer because of the larger air moisture content. The frequency of the first elevated TI (EI1) and HI (EHI1) are both higher than that of the surface-based ones. The second elevated TI/HI layer (EI2/EHI2) is shallower and weaker than that of the EI1/EHI1. At two coastal stations, EI1 caused by warm advection is thicker and stronger than that caused by subsidence. At the station farther from the coast, EI1 caused by subsidence is higher, thinner, and stronger. The top height and depth of the EHI2 both show seasonal variations, with larger values in the cold months. EHI1 tends to be formed by the TI, whereas EHI2 is dominant by humidity advection at all studied stations. HI under the influence of TI is usually thicker and stronger than that formed by humidity advection. The coexistence of EI and EHI is the most frequent inversion structure at these stations.

A Synthetic Study of the Position Difference of the Southern Branch Trough of the Qinghai-Tibet Plateau Based on Objective Identification

The southern branch trough (SBT) mainly appears in the winter half year (November to May of the following year), using the 4 times daily NCEP/NCAR re-analysis data nearly 41 years (1979-2019) to analyze the differences of the SBT distribution of spatial location, frequency in winter and spring, then selects the “eastern type” and “western type” of the 10 most typical SBT, using simplified vertical vorticity tendency equation, using simplified vertical vorticity tendency equation to diagnosis of the SBT in power, heat, water vapor and wave energy in different positions. The results show that: 1) The location of the SBT is more eastward in winter, and more westward in spring. 2) The diagnosis results of the vorticity equation show that the vorticity of the southern branch of the “western type” is mainly contributed by advection term; the vorticity of the “eastern type” south branch is mainly contributed by the non-adiabatic heating term. 3) The SBT of the “eastern type” has more obvious vorticity advection than the southern branch of the “western type”, and the dynamic action is stronger. The “western type” SBT has stronger Q1, specific humidity advection and water vapor flux than the “eastern type” SBT, which is greatly affected by thermal action and water vapor. When the “eastern type” and “western type” SBT occur, the T-N wave activity flux appears obvious abnormal energy fluctuation propagation.

How warmer and drier will the Mediterranean region be at the end of the twenty-first century?

Nearly all regions in the world are projected to become dryer in a warming climate. Here, we investigate the Mediterranean region, often referred to as a climate change “hot spot”. From regional climate simulations, it is shown that although enhanced warming and drying over land is projected, the spatial pattern displays high variability. Indeed, drying is largely caused by enhanced warming over land. However, in Northern Europe, soil moisture alleviates warming induced drying by up to 50% due to humidity uptake from land. In already arid regions, the Mediterranean Sea is generally the only humidity source, and drying is only due to land warming. However, over Sahara and the Iberian Peninsula, enhanced warming over land is insufficient to explain the extreme drying. These regions are also isolated from humidity advection by heat lows, which are cyclonic circulation anomalies associated with surface heating over land. The cyclonic circulation scales with the temperature gradient between land and ocean which increases with climate change, reinforcing the cyclonic circulation over Sahara and the Iberian Peninsula, both diverting the zonal advection of humidity to the south of the Iberian Peninsula. The dynamics are therefore key in the warming and drying of the Mediterranean region, with extreme aridification over the Sahara and Iberian Peninsula. In these regions, the risk for human health due to the thermal load which accounts for air temperature and humidity is therefore projected to increase significantly with climate change at a level of extreme danger.

Synoptic analysis of unexampled super-heavy rainfall on April 1, 2019, in west of Iran

The aim of this study is to analyze the synoptic and analyzing the mechanisms of flood occurrence on April 1, 2019, in the western half of Iran. In order to statistically analyze super-heavy precipitation in western Iran, the daily meteorological data from April 2019 for 13 synoptic stations in Kermanshah, Hamadan, and Lorestan Provinces were received and used by the Meteorological Organization. The results show that on the peak day of precipitation in the western half of the country, most stations recorded more than 100 mm of precipitation, the highest of which, 127 mm, occurred at Nahavand station. The arrangement of sea level pressure patterns also indicates the existence of severe pressure gradients between the low pressure in northern Saudi Arabia and western Iran and the high pressure of Siberian and European cold cores, which have created contradictory weather conditions and a strong front at sea level. Also, in the middle and upper levels of the troposphere, with the penetration of cold air flow and cyclonic from Russia and its settlement in West Asia and the region in the form of cutoff low-pressure blocking, severe positive vorticity and divergent and humid ascent flows at all analyzed levels in the western half of the country have been created. Humidity advection maps of 500–1000 hPa levels show that at the 850 and 1000 hPa levels a humidity supply is provided from various sources in the Gulf of Aden, Red Sea, and Mediterranean Sea. But at levels of 500 and 700 hPa, only the Mediterranean Sea plays a key role in supplying and injecting humidity.

Analysis of an extreme precipitation event in the Atacama Desert on January 2020 and its relationship to humidity advection along the Southeast Pacific

An extreme precipitation event took place during the second half of January of 2020 in the Atacama Desert. From Tacna, Perú to Iquique, Chile (18-21ºS) rainfall extended for several days producing floods, major damage to infrastructure, and affectation to the population in one of the driest deserts of the world. Analysis of surface-weather stations, sounding, satellite data and reanalysis suggests that the most intense precipitation occurred in precordillera (2,000 to 3,600 m.a.s.l). In a historical context, several weather stations between the pampas and precordillera recorded the highest daily rainfall intensity on record. The analysis of this EP event also suggests that at least four major factors were present to produce record-breaking precipitation in the Atacama Desert: (i) a low-level circulation off-shore Atacama Desert, potentially linked to the southward displacement of the Bolivian High and a high troposphere trough subtropical southeast Pacific Perú, (ii) the advection of humidity through an atmospheric river like structure, trapped to the coast in the front of the low-level cyclonic circulation, leading to increases in precipitable water vapor over the Atacama Desert, (iii) above normal sea surface temperatures that favor moist conditions in the boundary layer and (iv) a reinforced local circulation, where low level winds transport humid wet parcels toward the east, producing terrain-forced ascend at the foothill of the west slope of the Andes Cordillera, triggering precipitation and thunderstorm mostly in precordillera, but also extended to the coast, pampas and Altiplano. Analysis of previous austral summer precipitation days from 2008 to 2020 suggests that this thermo-dynamic mechanism is highly linked to the majority of the EP days in Southern Perú and Northern Chile, becoming in an important configuration to predict future EP events in the Atacama Desert.

Intraseasonal modulation of spring‐strong wind events associated with convection in northeastern Argentina

AbstractThe relationship between intraseasonal variability (IS; 10–90 days) and days which registered convection‐associated strong surface wind events (CSSWE) over northeastern Argentina (NEA) was studied. The climatological behaviour of these strong wind events showed a higher duration and occurrence in austral spring. CSSWE were categorized as a function of the wet and dry phases of the spring‐season intraseasonal (spring‐SIS) index, which describes the activity of the leading pattern of IS‐filtered outgoing longwave radiation (IS‐OLR) during that season in eastern South America. A modulation of the IS variability over localized and mesoscale phenomena as the CSSWE was found, showing significant peaks of wind variability in that timescale, and especially the submonthly timescale. The CSSWE were categorized according to the phases of the spring‐SIS pattern and most of them occurred before or during a wet phase, especially for the longer CSSWE. Moreover, the detection of CSSWE days during and before a dry phase was scarce. Rossby wave trains were observed to organize the circulation on intraseasonal timescales that configure regional cyclonic anomalies in such way that favours the development of CSSWE, promoting mid‐level ascents over NEA and northerly advection of humidity to the region. Together with the composites of IS‐OLR anomalies and the spectra of wind velocity, they support the fact that the higher‐frequency IS variability is the primary influence for the development of CSSWE.

Impact of dynamical regionalization on precipitation biases and teleconnections over West Africa

West African societies are highly dependent on the West African Monsoon (WAM). Thus, a correct representation of the WAM in climate models is of paramount importance. In this article, the ability of 8 CMIP5 historical General Circulation Models (GCMs) and 4 CORDEX-Africa Regional Climate Models (RCMs) to characterize the WAM dynamics and variability is assessed for the period July-August-September 1979–2004. Simulations are compared with observations. Uncertainties in RCM performance and lateral boundary conditions are assessed individually. Results show that both GCMs and RCMs have trouble to simulate the northward migration of the Intertropical Convergence Zone in boreal summer. The greatest bias improvements are obtained after regionalization of the most inaccurate GCM simulations. To assess WAM variability, a Maximum Covariance Analysis is performed between Sea Surface Temperature and precipitation anomalies in observations, GCM and RCM simulations. The assessed variability patterns are: El Nino-Southern Oscillation (ENSO); the eastern Mediterranean (MED); and the Atlantic Equatorial Mode (EM). Evidence is given that regionalization of the ENSO–WAM teleconnection does not provide any added value. Unlike GCMs, RCMs are unable to precisely represent the ENSO impact on air subsidence over West Africa. Contrastingly, the simulation of the MED–WAM teleconnection is improved after regionalization. Humidity advection and convergence over the Sahel area are better simulated by RCMs. Finally, no robust conclusions can be determined for the EM–WAM teleconnection, which cannot be isolated for the 1979–2004 period. The novel results in this article will help to select the most appropriate RCM simulations to study WAM teleconnections.

Analysis of Strengthening and Dissipating Mesoscale Convective Systems Propagating off the West African Coast

Abstract A large number of Atlantic tropical depressions are generated in the eastern basin in relation to the African easterly wave (AEW) and embedded mesoscale convective systems (MCSs) coming from the African continent. In this paper, the structures of strengthening and dissipating MCSs evolving near the West African coast are analyzed, including the role of the ocean surface conditions in their evolution. Satellite infrared brightness temperature and meteorological radar data over seven summer seasons between 1993 and 2006 are used to subjectively select 20 cases of strengthening and dissipating MCSs in the vicinity of the Senegal coast. With these observed MCSs, a lagged composite analysis is then performed using Interim ECMWF Re-Analysis (ERA-Interim) and Climate Forecast System Reanalysis (CFSR). It is shown that the strengthening MCS is generally preceded by prior passage of an AEW near the West African coast. This previous wave trough is associated with a convective cyclonic circulation in the low and middle troposphere, which enhances the southwesterly flow and then provides humidity to the strengthening MCS, located in the vicinity of the subsequent AEW trough. This is favored by the contraction of the wavelength associated with the two troughs. The sea surface contributes to the MCS enhancement through surface evaporation flux. But this contribution is found to be less important than advection of humidity from the previous wave trough. These conditions are almost not found in the dissipating MCS cases, which dissipate in a dry environment dominated by a subsident and anticyclonic circulation, with generally no interaction with a previous wave trough.

Low Visibility Formation and Forecasting on the Northern Coast of Brazil

Visibility analysis and forecast at the Maceio International Airport in the Brazilian Northeast (NEB) was the principal goal of this investigation. Surface meteorological data of the Maceio International Airport were used for low visibility frequency study. Low visibility in NEB was provoked more frequently by light fog (LF) formation (1,098 or 92 h month−1 on average). Haze and fog were very rare (81 h and one event per year, respectively on average). Light fog with a visibility less than 2 km usually was detected together with rain or drizzle. Low visibility was observed more frequently at night and during the rainy season. Applications of the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model for light fog forecast were tested. Thermodynamic processes were studied by vertical profile, elaborated by: (1) National Centers for Environmental Prediction (NCEP) reanalysis data for Maceio (because of some radiosonde absence) and (2) forecast vertical temperature and humidity profiles were produced, using Air Parcels Trajectories of the HYSPLIT model at the pattern levels. The synoptic situations before and during low visibility phenomena were analyzed using different products of NCEP reanalysis, the high resolution (10 km) ETA model and infrared satellite images. Wave disturbance in the trade winds field, localized on the northwest periphery of the South Atlantic subtropical High, usually accompanied the phenomena. A humidity advection, weak ascendant movement and thermal inversion absence at the low levels were created by these waves. The middle level’s descendent movement provoked the humidity accumulation at levels below. Satisfactory results of the HYSPLIT model applications for light fog forecast were obtained with 12 h antecedence. In particular, stable level forecast by the ETA model was forecast satisfactorily with 12 h antecedence; vertical movements were predicted better with up to 48 h antecedence. The PSU/NCAR mesoscale model (MM5) and PAFOG models were tested for analysis and forecast of an intensive fog event. Intensive fog provoked a fatal accident of a small airplane near the Maceio Airport in 2007. These fog formation processes were studied by NCEP reanalysis data, the high resolution regional model MM5, and satellite and radar data. Fog formation was simulated by PAFOG model and satisfactory results were obtained with 10 h antecedence.

Evaluation of simulated aerosol properties with the aerosol-climate model ECHAM5-HAM using observations from the IMPACT field campaign

Abstract. In May 2008, the measurement campaign IMPACT for observation of atmospheric aerosol and cloud properties was conducted in Cabauw, The Netherlands. With a nudged version of the coupled aerosol-climate model ECHAM5-HAM we simulate the size distribution and chemical composition of the aerosol and the associated aerosol optical thickness (AOT) for the campaign period. Synoptic scale meteorology is represented realistically through nudging of the vorticity, the divergence, the temperature and the surface pressure. Simulated concentrations of aerosol sulfate and organics at the surface are generally within a factor of two from observed values. The monthly averaged AOT from the model is 0.33, about 20% larger than observed. For selected periods of the month with relatively dry and moist conditions discrepancies are approximately −30% and +15%, respectively. Discrepancies during the dry period are partly caused by inaccurate representation of boundary layer (BL) dynamics by the model affecting the simulated AOT. The model simulates too strong exchange between the BL and the free troposphere, resulting in weaker concentration gradients at the BL top than observed for aerosol and humidity, while upward mixing from the surface layers into the BL appears to be underestimated. The results indicate that beside aerosol sulfate and organics also aerosol ammonium and nitrate significantly contribute to aerosol water uptake. The simulated day-to-day variability of AOT follows synoptic scale advection of humidity rather than particle concentration. Even for relatively dry conditions AOT appears to be strongly influenced by the diurnal cycle of RH in the lower boundary layer, further enhanced by uptake and release of nitric acid and ammonia by aerosol water.

The Atlantic inflow to the Saharan heat low: observations and modelling

AbstractThe inflow of relatively cold and stably stratified air from the Atlantic Ocean into western Mauritania and into the southwestern part of the Saharan heat low is studied using the mesoscale COSMO model. This model was used to provide operational forecasts for the GERBILS field campaign, which was conducted by the Met Office in West Africa in June 2007. The forecasts were validated against airborne measurements as well as satellite imagery and were found to represent the main synoptic features of the region accurately.A complex mesoscale feature in western Mauritania, which we call the Atlantic Inflow, was identified in the COSMO model output. The main component of the Atlantic Inflow is the sea breeze and associated front. The sea breeze interacts with larger‐scale, higher‐altitude fluctuations in the thermal and humidity advection. During the day the balance between horizontal advection of cool maritime air and turbulence in the convective boundary layer over land results in a stationary sea breeze front at the coast. Once turbulence dies down in the evening, the sea breeze front penetrates inland. Above the sea breeze layer, thermal advection in the Saharan Atmospheric Boundary Layer (SABL) also controls the structure of the Atlantic Inflow. A marked baroclinic zone was observed, in which the temperature and humidity made a relatively smooth transition from values typical of the Atlantic air to values characteristic of the SABL. Budget calculations showed that, through its cooling and occasional moistening at low levels, the Atlantic Inflow has an important impact on the regional heat and moisture budgets. Copyright © Royal Meteorological Society and Crown Copyright, 2009

Modelling the Interaction Between a River Surface and the Atmosphere at the Bottom of a Valley

The role of a river of small dimensions in driving the surface exchange of sensible and latent heat fluxes at the bottom of a valley is investigated using large-eddy simulation (LES). Simulations were performed using different valley topographies, river widths and large-scale wind speed and direction. In all cases, the river acted as a sink of both sensible and latent heat during daytime. Despite the general agreement concerning the flux direction above the river surface, specific differences exist between the simulations. The topography enhances the wind divergence caused by the river, and the larger negative surface fluxes above the river occur when there are no slopes, a consequence of larger wind speeds above the river. For large-scale winds aligned with the valley axis, the surface fluxes depend on the large-scale wind speed, but this dependence is reduced if the large-scale wind is perpendicular to the valley axis. There is a minimum of temperature and a maximum of specific humidity above the river surface. The scalar budgets show that sensible heat flux converges above the river, being balanced by the warm air subsidence at the centre of the valley. Latent heat fluxes, on the other hand, converge above the river surface, and they are balanced by the horizontal advection of humidity towards the river margins.

Relationships between autumn precipitation anomalies in southeastern South America and El Niño event classification

AbstractThe classification of El Niño events was performed based on the time evolution of sea surface warming in the tropical Pacific Ocean during the period 1950–2000. Two sets of events were constructed: one in which the warming core migrates eastward along the tropical Pacific until April–June of the following year, and another one in which it evolves westward until November–January. The first type has associated positive precipitation anomalies over southeastern South America during April–June. It results from a favourable combination of cyclonic vorticity advection and humidity convergence. At high levels, cyclonic vorticity advection is explained in terms of an eastward extension of the subtropical jet. Enhanced humidity advection takes place by an increased low‐level northwesterly flow to the east of the Andes. It provides enough moisture availability that, in combination with the upper‐level cyclonic vorticity advection, supports heavy precipitation during April–June. The second type of event exhibits slight negative or near‐normal precipitation anomalies over the same region. Both low and high‐level circulation anomalies are also weaker in this case. The 1997–1998 El Niño is analysed separately because it cannot be classified into any of the previously described event types.The observed distribution of both types of events along the analysed period changes after the 1970s. Comparison with other authors' results suggests the influence of low‐frequency processes such as the Pacific Decadal Oscillation (PDO). Copyright © 2008 Royal Meteorological Society

Rainfall yield characteristics of electrical storm observed in the Spanish Basque Country area during the period 1992–1996

This paper is focused on the study of rainfall yield characteristics of electrical storms observed over the Northern Iberian Peninsula during 1992–1996. To this aim Principal Components Analysis (PCA) and Self-Organizing Maps (SOM) method have been used. The SOM method is a group of artificial neural networks based on the topological properties of the human brain. Results clearly suggest that there exist three different meteorological patterns that are linked to the characteristics of electrical events found in the study area. In winter, most of the electrical events are formed under oceanic advection (NW air fluxes). On these cases, mean rainfall yield estimates reach values of 700 10 4 m 3 per cloud to ground lightning flash (CG flash). During summer most frequent electrical storms are associated to local instability shooting by surface heating with advection of humidity coming from the Iberian Peninsula. Under these meteorological situations, rain is scarcer if compared with oceanic events but lightning CG counts reach the maximum values found in the area (about 10 CG counts per 20 × 20 km 2 and day) giving this way the smallest rainfall yield with a mean value of 15 10 4 m 3 per CG flash. Iberian air fluxes associated with cold air in upper parts of the atmosphere represent the third meteorological pattern found. This pattern is most common in spring and autumn but is not unusual in the rest of the seasons. In those cases mean rainfall yield in the area is about 150 10 4 m 3 per CG flash. In all electrical episodes K instability index is greater than 15 °C but in the most lightning producing events, this index reaches in the area values greater than 24 °C. PCA results pointed out that there exists a relationship between rain and CG counts expressed by the first principal component computed from standardized data. However, we must notice that no event is solely linked to this axis, since a seasonal influence which decreases lightning production when rain increases is always present. Results found are of great interest for short term forecasting of flashfloods in mountainous areas like the Spanish Basque Country region.

An Idealized Two-Dimensional Framework to Study the West African Monsoon. Part II: Large-Scale Advection and the Diurnal Cycle

The idealized 2D model developed in Part I of this study is used to study the West African monsoon sensitivity to large-scale forcing. Using ECWMF reanalyses, a large-scale forcing is introduced in the 2D model in terms of temperature and humidity advection. A coherent structure of cooling–moistening near the surface and drying–warming in the 2–4-km layer is found in the Saharan heat low region. The effect of the advective forcing is to block the monsoon propagation by strengthening the northerly flux and by an increase of convective inhibition. The heat low thus appears to play a key role in the monsoon northward penetration through its temperature and humidity budget. Ultimately, warmer low levels and/or more moist midlevels in the heat low favor a more northerly position of the ITCZ. A detailed view of the continental diurnal cycle is also presented. Potential temperature and humidity budgets are performed in the deep convective and heat low area. The moistening process to sustain deep convection is made through nocturnal advection at low levels and daytime turbulence that redistributes humidity vertically. The same mechanism occurs in the heat low except that the vertical transfers by turbulence help maintain the dryness of the low levels. A possible mechanism of interaction between the deep convective zone and the Saharan heat low is also proposed that involves gravity waves in the upper troposphere.

A Bayesian hierarchical model for local precipitation by downscaling large‐scale atmospheric circulation patterns

AbstractPrecipitation over the Western part of Iberian Peninsula is known to be related to the large‐scale sea level pressure field and thus to advection of humidity into this area. The major problem is to downscale this synoptic atmospheric information to local daily precipitation patterns. One way to handle this problem is by weather‐state models, where, based on the pressure field, each day is classified into a weather state and precipitation is then modeled within each weather state via multivariate distributions.In this paper, we propose a spatiotemporal Bayesian hierarchical model for precipitation. Basic objective and novelty of the paper is to capture and model the essential spatiotemporal relationships that exist between large‐scale sea level pressure field and local daily precipitation. A specific local spatial ordering that mimics the essential large‐scale patterns is used in the likelihood. The model is then applied to a network of rain gauge stations in the river Tagus valley. The inference is then carried out using appropriate MCMC methods. Copyright © 2006 John Wiley & Sons, Ltd.

Dynamic Stabilization of Atmospheric Single Column Models

Single column models (SCMs) provide an economical framework for assessing the sensitivity of atmospheric temperature and humidity to natural and imposed perturbations, and also for developing improved representations of diabatic processes in weather and climate models. Their economy is achieved at the expense of ignoring interactions with the circulation dynamics; thus, advection by the large-scale flow is either prescribed or neglected. This artificial decoupling of the diabatic and adiabatic tendencies can often cause rapid error growth in SCM integrations, especially in the Tropics where large-scale vertical advection is important. As a result, SCMs can quickly develop highly unrealistic thermodynamic structures, making it pointless to study their subsequent evolution. This paper suggests one way around this fundamental difficulty through a simple coupling of the diabatic and adiabatic tendencies. In essence, the local vertical velocity at any instant is specified by a formula that links the local vertical temperature advection to the evolution of SCM-generated diabatic heating rates up to that instant. This vertical velocity is then used to determine vertical humidity advection, and also horizontal temperature and humidity advection under an additional assumption that the column is embedded in a uniform environment. The parameters in the formula are estimated in a separate set of calculations, from the approach to equilibrium of a linearized global primitive equation model forced by steady heat sources. As a test, the parameterized dynamics are used to predict the linear model’s local response to oscillating heat sources, and found to perform remarkably well over a wide range of space and time scales. In a second test, the parameterization is found to capture important aspects of a general circulation model’s vertical advection and temperature tendencies and their lead‐lag relationships with diabatic heating fluctuations at convectively active locations in the Tropics. When implemented in the NCAR SCM, the dynamically coupled SCM shows a clear improvement over its uncoupled counterpart for tropical conditions observed during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). Coupling effectively stabilizes the SCM. As a result, short-term prediction errors are substantially reduced, the ensemble spread is reduced in ensemble runs, and the SCM is able to maintain realistic thermodynamic structures in extended runs. Such a dynamically coupled SCM should therefore be more useful not only for isolating physical parameterization errors in weather and climate models, but also for economical simulations of regional climate variability.