Climatological Time Scales Research Articles

Seasonal Variability in Middepth Gyral Circulation Patterns in the Central East/Japan Sea as Revealed by Long-Term Argo Data

AbstractTrajectories of Argo floats deployed in the East/Japan Sea from 2001 to 2014 reveal that the middepth gyral circulation pattern of the Japan basin, the central part of the East/Japan Sea, undergoes a seasonal variation. The middepth circulation of the Japan basin is found to be characterized usually by the gyres trapped to the east of the Bogorov Rise (E-gyres) and those extending farther westward into the whole basin (BW-gyres). The E-gyre trajectories are generally associated with the turning of the floats toward deeper regions off the isobaths. This occurs in winter either on the northern or eastern side of the eastern Japan basin. It seems that the upstream part of the otherwise BW-gyre is subject to a strong negative wind stress curl in winter, and there the circulating water columns are driven toward the deeper region, thus triggering the formation of the E-gyre. The topographic effect associated with the Bogorov Rise seems to interfere thereafter in the process of determining the passage of the E-gyre. Otherwise, the water columns continue to flow along the isobaths, hence maintaining the BW-gyre. To the knowledge of the authors, this is the first observational evidence of seasonal variability in the middepth gyral circulation pattern in the East/Japan Sea. It suggests that oceanic middepth circulation, usually known to be quasi steady or slowly varying on climatological time scales, might also undergo a significant seasonal variation as it does in the East/Japan Sea.

ENSO influence on surface energy and mass balance at Shallap Glacier, Cordillera Blanca, Peru

Abstract. The El Niño/Southern Oscillation (ENSO) is a major driver of climate variability in the tropical Andes, where recent Niño and Niña events left an observable footprint on glacier mass balance. The nature and strength of the relationship between ENSO and glacier mass balance, however, varies between regions and time periods, leaving several unanswered questions about its exact mechanisms. The starting point of this study is a 4-year long time series of distributed surface energy and mass balance (SEB/SMB) calculated using a process-based model driven by observations at Shallap Glacier (Cordillera Blanca, Peru). These data are used to calibrate a regression-based downscaling model that links the local SEB/SMB fluxes to atmospheric reanalysis variables on a monthly basis, allowing an unprecedented quantification of the ENSO influence on the SEB/SMB at climatological time scales (1980–2013, ERA-Interim period). We find a stronger and steadier anti-correlation between Pacific sea-surface temperature (SST) and glacier mass balance than previously reported. This relationship is most pronounced during the wet season (December–May) and at low altitudes where Niño (Niña) events are accompanied with a snowfall deficit (excess) and a higher (lower) radiation energy input. We detect a weaker but significant ENSO anti-correlation with total precipitation (Niño dry signal) and positive correlation with the sensible heat flux, but find no ENSO influence on sublimation. Sensitivity analyses comparing several downscaling methods and reanalysis data sets resulted in stable mass balance correlations with Pacific SST but also revealed large uncertainties in computing the mass balance trend of the last decades. The newly introduced open-source downscaling tool can be applied easily to other glaciers in the tropics, opening new research possibilities on even longer time scales.

The Lofoten Vortex of the Nordic Seas

The Lofoten Basin is the largest reservoir of ocean heat in the Nordic Seas. A particular feature of the basin is ‘the Lofoten Vortex’, a most anomalous mesoscale structure in the Nordic Seas. The vortex resides in one of the major winter convection sites in the Norwegian Sea, and is likely to influence the dense water formation of the region. Here, we document this quasi-permanent anticyclonic vortex using hydrographic and satellite observations. The vortex’ uniqueness in the Nordic Seas, its surface characteristics on seasonal, inter-annual, and climatological time-scales, are examined together with the main forcing mechanisms acting on it. The influence of the vortex on the hydrography of the Lofoten Basin is also shown. We show that the Atlantic Water in the Nordic Seas penetrate the deepest inside the Lofoten Vortex, and confirm the persistent existence of the vortex in the deepest part of the Lofoten Basin, its dominant cyclonic drift and reveal seasonality in its eddy intensity with maximum during late winter and minimum during late autumn. Eddy merging processes are studied in detail, and mergers by eddies from the slope current are found to provide anticyclonic vorticity to the Lofoten Vortex.

Spatiotemporal variation of long‐term drought propensity through reliability‐resilience‐vulnerability based Drought Management Index

AbstractThis paper characterizes the long‐term, spatiotemporal variation of drought propensity through a newly proposed, namely Drought Management Index (DMI), and explores its predictability in order to assess the future drought propensity and adapt drought management policies for a location. The DMI was developed using the reliability‐resilience‐vulnerability (RRV) rationale commonly used in water resources systems analysis, under the assumption that depletion of soil moisture across a vertical soil column is equivalent to the operation of a water supply reservoir, and that drought should be managed not simply using a measure of system reliability, but should also take into account the readiness of the system to bounce back from drought to a normal state. Considering India as a test bed, 5 year long monthly gridded (0.5° Lat × 0.5° Lon) soil moisture data are used to compute the RRV at each grid location falling within the study domain. The Permanent Wilting Point (PWP) is used as the threshold, indicative of transition into water stress. The association between resilience and vulnerability is then characterized through their joint probability distribution ascertained using Plackett copula models for four broad soil types across India. The joint cumulative distribution functions (CDF) of resilience and vulnerability form the basis for estimating the DMI as a five‐yearly time series at each grid location assessed. The status of DMI over the past 50 years indicate that drought propensity is consistently low toward northern and north eastern parts of India but higher in the western part of peninsular India. Based on the observed past behavior of DMI series on a climatological time scale, a DMI prediction model comprising deterministic and stochastic components is developed. The predictability of DMI for a lead time of 5 years is found to vary across India, with a Pearson correlation coefficient between observed and predicted DMI above 0.6 over most of the study area, indicating a reasonably good potential for drought management in the medium term water resources planning horizon.

Tropical Atlantic dust and smoke aerosol variations related to the Madden‐Julian Oscillation in MODIS and MISR observations

AbstractIn this study, Moderate Resolution Imaging Spectroradiometer (MODIS) fine mode fraction and Multi‐angle Imaging SpectroRadiometer (MISR) nonspherical fraction data are used to derive dust and smoke aerosol optical thickness (τdust and τsmoke) over the tropical Atlantic in a complementary way: due to its wider swath, MODIS has 3–4 times greater sampling than MISR, but MISR dust discrimination is based on particle shape retrievals, whereas an empirical scheme is used for MODIS. MODIS and MISR show very similar dust and smoke winter climatologies. τdust is the dominant aerosol component over the tropical Atlantic, accounting for 40–70% of the total aerosol optical thickness (AOT), whereas τsmoke is significantly smaller than τdust. The consistency and high correlation between these climatologies and their daily variations lends confidence to their use for investigating the relative dust and smoke contributions to the total AOT variation associated with the Madden‐Julian Oscillation (MJO). The temporal evolution and spatial patterns of the τdus anomalies associated with the MJO are consistent between MODIS and MISR: the magnitude of MJO‐realted τdust anomalies is comparable to or even larger than that of the total τ, while the τsmoke anomaly represents about 15% compared to the total, which is quite different from their relative magnitudes to the total τ on the climatological time scale. This suggests that dust and smoke are not influenced by the MJO in the same way. Based on correlation analysis, dust is strongly influenced by the MJO‐modulated trade wind and precipitation anomalies, and can last as long as one MJO phase, whereas smoke is less affected.

Climate and weather influences on spatial temporal patterns of mountain pine beetle populations in Washington and Oregon

Widespread outbreaks of mountain pine beetle in North America have drawn the attention of scientists, forest managers, and the public. There is strong evidence that climate change has contributed to the extent and severity of recent outbreaks. Scientists are interested in quantifying relationships between bark beetle population dynamics and trends in climate. Process models that simulate climate suitability for mountain pine beetle outbreaks have advanced our understanding of beetle population dynamics; however, there are few studies that have assessed their accuracy across multiple outbreaks or at larger spatial scales. This study used the observed number of trees killed by mountain pine beetles per square kilometer in Oregon and Washington, USA, over the past three decades to quantify and assess the influence of climate and weather variables on beetle activity over longer time periods and larger scales than previously studied. Influences of temperature and precipitation in addition to process model output variables were assessed at annual and climatological time scales. The statistical analysis showed that new attacks are more likely to occur at locations with climatological mean August temperatures >15 degrees C. After controlling for beetle pressure, the variables with the largest effect on the odds of an outbreak exceeding a certain size were minimum winter temperature (positive relationship) and drought conditions in current and previous years. Precipitation levels in the year prior to the outbreak had a positive effect, possibly an indication of the influence of this driver on brood size. Two-year cumulative precipitation had a negative effect, a possible indication of the influence of drought on tree stress. Among the process model variables, cold tolerance was the strongest indicator of an outbreak increasing to epidemic size. A weather suitability index developed from the regression analysis indicated a 2.5x increase in the odds of outbreak at locations with highly suitable weather vs. locations with low suitability. The models were useful for estimating expected amounts of damage (total area with outbreaks) and for quantifying the contribution of climate to total damage. Overall, the results confirm the importance of climate and weather on the spatial expansion of bark beetle outbreaks over time.

The relative importance of rainfall, temperature and yield data for a regional-scale crop model

When projecting future crop production, the skill of regional scale (>100km resolution) crop models is limited by the spatial and temporal accuracy of the calibration and weather data used. The skill of climate models in reproducing surface properties such as mean temperature and rainfall patterns is of critical importance for the simulation of crop yield. However, the impact of input data errors on the skill of regional scale crop models has not been systematically quantified. We evaluate the impact of specific data error scenarios on the skill of regional scale hindcasts of groundnut yield in the Gujarat region of India, using observed input data with the GLAM crop model. Two methods were employed to introduce error into rainfall, temperature and crop yield inputs at seasonal and climatological timescales: (1) random temporal resequencing, and (2) biasing values.We find that, because the study region is rainfall limited, errors in rainfall data have the most significant impact on model skill overall. More generally, we find that errors in inter-annual variability of seasonal temperature and precipitation cause the greatest crop model error. Errors in the crop yield data used for calibration increased root mean square error by up to 143%. Given that cropping systems are subject both to a changing climate and to ongoing efforts to reduce the yield gap, both potential and actual crop productivity at the regional scale need to be measured.We identify three key endeavours that can improve the ability to assess future crop productivity at the regional scale: (i) increasingly accurate representation of inter-annual climate variability in climate models; (ii) similar studies with other crop models to identify their relative strengths in dealing with different types of climate model error; (iii) the development of techniques to assess potential and actual yields, with associated confidence ranges, at the regional scale.

Global Energy and Water Budgets in MERRA

Abstract Reanalyses, retrospectively analyzing observations over climatological time scales, represent a merger between satellite observations and models to provide globally continuous data and have improved over several generations. Balancing the earth’s global water and energy budgets has been a focus of research for more than two decades. Models tend to their own climate while remotely sensed observations have had varying degrees of uncertainty. This study evaluates the latest NASA reanalysis, the Modern Era Retrospective-Analysis for Research and Applications (MERRA), from a global water and energy cycles perspective, to place it in context of previous work and demonstrate the strengths and weaknesses. MERRA was configured to provide complete budgets in its output diagnostics, including the incremental analysis update (IAU), the term that represents the observations influence on the analyzed states, alongside the physical flux terms. Precipitation in reanalyses is typically sensitive to the observational analysis. For MERRA, the global mean precipitation bias and spatial variability are more comparable to merged satellite observations [the Global Precipitation and Climatology Project (GPCP) and Climate Prediction Center Merged Analysis of Precipitation (CMAP)] than previous generations of reanalyses. MERRA ocean evaporation also has a much lower value, which is comparable to independently derived estimate datasets. The global energy budget shows that MERRA cloud effects may be generally weak, leading to excess shortwave radiation reaching the ocean surface. Evaluating the MERRA time series of budget terms, a significant change occurs that does not appear to be represented in observations. In 1999, the global analysis increments of water vapor changes sign from negative to positive and primarily lead to more oceanic precipitation. This change is coincident with the beginning of Advanced Microwave Sounding Unit (AMSU) radiance assimilation. Previous and current reanalyses all exhibit some sensitivity to perturbations in the observation record, and this remains a significant research topic for reanalysis development. The effect of the changing observing system is evaluated for MERRA water and energy budget terms.

Implementation and evaluation of aerosol and cloud microphysics in a regional climate model

[1] A new aerosol modeling framework is presented within the regional climate model COSMO-CLM. The model accounts for the microphysical interactions of internally and externally mixed aerosol particles. Sulfate, black carbon, particulate organic matter, sea salt, and mineral dust are considered. The model is applied over Europe at a horizontal resolution of 50 km. The lateral boundary conditions are given by the ERA-Interim reanalysis for the meteorological fields and by a global ECHAM5-HAM simulation for the aerosols. Present-day AeroCom emissions are used for the evaluation period from 1997 to 2003. The model largely reproduces the annual mean pattern of the aerosol optical depth derived from satellite data over Europe (model and observed domain mean is 0.17, but it is 0.37 with standard model version). The annual cycle is overestimated in COSMO-CLM in some regions due to strong dust transport across the Mediterranean in late spring. Day-to-day variability in aerosol optical depth and the Angstrom exponent is also captured by the model. The corresponding correlations of the daily mean time series between measurements from AERONET stations and the model range from 0.17 to 0.74. In comparison with the standard model version, which does not account for aerosol transport and indirect aerosol effects and uses an outdated aerosol climatology, the mid-European summer cold bias disappears with the new framework. The new framework allows studies of mesoscale interactions between aerosols, clouds, precipitation, and radiation on climatological time scales due to the advanced physical representation of the underlying processes.

Can ensembles of regional climate model simulations improve results from sensitivity studies?

Intrinsic variability (IV) in regional climate models (RCMs) is often assumed to be small because at climatological timescales, the model solutions tend to be dominated by the model’s lateral boundary conditions. Recent studies have indicated that this IV may actually be large in certain instances for some variables. Direct interpretation of anomalies from RCM sensitivity studies relies on the assumption that differences between model simulations are entirely due to a physical forcing. However, if IV is as large or larger than the physical signal, then this assumption is violated. Using a 20 member ensemble of RCM simulations, we verify that IV of precipitation within a RCM can be large enough to violate the sensitivity study assumption, and we show that generating ensembles of simulations can help reduce the level of IV. We also present two indicators that can rule out the influence of IV when it is ambiguous whether anomalies within a sensitivity study are due to the sensitivity perturbation or whether they are due to IV.

Diagnosing Land–Atmosphere Interaction from a Regional Climate Model Simulation over West Africa

Abstract Land–atmosphere interaction at climatological time scales in a large area that includes the West African Sahel has been explicitly explored in a regional climate model (RegCM) simulation using a range of diagnostics. First, areas and seasons of strong land–atmosphere interaction were diagnosed from the requirement of a combined significant correlation between soil moisture, evaporation, and the recycling ratio. The northern edge of the West African monsoon area during June–August (JJA) and an area just north of the equator (Central African Republic) during March–May (MAM) were identified. Further analysis in these regions focused on the seasonal cycle of the lifting condensation level (LCL) and the convective triggering potential (CTP), and the sensitivity of CTP and near-surface dewpoint depressions HIlow to anomalous soil moisture. From these analyses, it is apparent that atmospheric mechanisms impose a strong constraint on the effect of soil moisture on the regional hydrological cycle.

From Observations to Forecasts – Part 3. Key principles and recent developments in satellite observations

On 1 April 1960, two and a half years after the launch of the first satellite, Sputnik-1, the Television InfraRed Observation Satellite (TIROS-1) became the first meteorological satellite. Since then, meteorology has been at the forefront of Earth observation. In 1963, the World Weather Watch programme was set up by the World Meteorological Organization (WMO) to establish a satellite observation network of (nominally) five geostationary and two polar orbiting meteorological satellites. The first geostationary meteorological observations started in 1966, followed by the launch of a series of meteorological satellites to observe and monitor our atmosphere (see Brugge and Stuttard, 2003). Key to the success of satellite observations was the ability to acquire routine synoptic-scale observations of the Earth and its atmosphere at relatively fine resolutions. By interpreting the imagery and monitoring any changes, these observations have provided crucial information to aid our understanding and knowledge of meteorological and atmospheric processes. Furthermore, the longevity of such observations now allows such data to be used on climatological timescales. Meteorological satellites provide observations of the Earth from low-Earth orbits or from geostationary orbits: observations taken by satellite sensors in these orbits complement each other to provide global im agery on a routine basis. Figure 1 shows the current operational meteorological satellites of the Global Observing System (GOES).

Air–sea fluxes and river discharges in the Black Sea with a focus on the Danube and Bosphorus

Climatological variations of thermal and haline buoyancy fluxes are investigated in the Black Sea. Analyses are performed to determine whether or not thermal buoyancy flux due to net heat flux (or haline buoyancy flux due to freshwater flux) dominates net buoyancy flux in the Black Sea. The effect of the two types of buoyancy flux are examined using a ≈ 3.2 km resolution HYbrid Coordinate Ocean Model (HYCOM). In the Black Sea, salinity is increased by Bosphorus inflow and decreased by precipitation and the inflow from six major rivers. Thus, the monthly mean discharge values from six major rivers are used as additions to the precipitation field in the model. River discharges are obtained from four climatologies: (1) River DIScharge (RivDIS), (2) Perry, (3) University Corporation for Atmospheric Research (UCAR) and (4) Naval Research Laboratory (NRL). Statistical evaluations of climatological river discharges from these products result in similar annual mean values. However, there are differences in the seasonal cycle. In the case of Danube, which has the largest annual river discharge contribution of ≈ 6365 m 3 s − l , RMS differences for river discharge values over the seasonal cycle are within ≈ 2% among all products. The Black Sea HYCOM simulation uses climatological monthly mean atmospheric forcing (wind and thermal forcing) from European Centre for Medium-range Weather Forecast (ECMWF) Re-analyses. Buoyancy flux fields obtained from the HYCOM simulation demonstrate that thermal buoyancy flux dominates haline buoyancy flux in all months except March when the basin-averaged absolute ratio of former to the latter is 0.5 in the Black Sea. On the contrary, large buoyancy ratio values of ≫ 1 in other months explain the buoyancy is much more sensitive to variations in heating. It is also found that near the Bosphorus Strait in the Black Sea the strongly concentrated source of salty water typically penetrates into the deeper layers as a plume. Further analyses of mean buoyancy fluxes reveal nonexistence of deep convection in the Black Sea on climatological time scales.

A new tropospheric and stratospheric Chemistry and Transport Model MOCAGE-Climat for multi-year studies: evaluation of the present-day climatology and sensitivity to surface processes

Abstract. We present the configuration of the Météo-France Chemistry and Transport Model (CTM) MOCAGE-Climat that will be dedicated to the study of chemistry and climate interactions. MOCAGE-Climat is a state-of-the-art CTM that simulates the global distribution of ozone and its precursors (82 chemical species) both in the troposphere and the stratosphere, up to the mid-mesosphere (~70 km). Surface processes (emissions, dry deposition), convection, and scavenging are explicitly described in the model that has been driven by the ECMWF operational analyses of the period 2000–2005, on T21 and T42 horizontal grids and 60 hybrid vertical levels, with and without a procedure that reduces calculations in the boundary layer, and with on-line or climatological deposition velocities. Model outputs have been compared to available observations, both from satellites (TOMS, HALOE, SMR, SCIAMACHY, MOPITT) and in-situ instrument measurements (ozone sondes, MOZAIC and aircraft campaigns) at climatological timescales. The distribution of long-lived species is in fair agreement with observations in the stratosphere putting aside the shortcomings associated with the large-scale circulation. The variability of the ozone column, both spatially and temporarily, is satisfactory. However, because the Brewer-Dobson circulation is too fast, too much ozone is accumulated in the lower to mid-stratosphere at the end of winter. Ozone in the UTLS region does not show any systematic bias. In the troposphere better agreement with ozone sonde measurements is obtained at mid and high latitudes than in the tropics and differences with observations are the lowest in summer. Simulations using a simplified boundary layer lead to larger ozone differences between the model and the observations up to the mid-troposphere. NOx in the lowest troposphere is in general overestimated, especially in the winter months over the Northern Hemisphere, which may result from a positive bias in OH. Dry deposition fluxes of O3 and nitrogen species are within the range of values reported by recent inter-comparison model exercises. The use of climatological deposition velocities versus deposition velocities calculated on-line had greatest impact on HNO3 and NO2 in the troposphere.

Utility of Radiosonde Wind Data in Representing Climatological Variations of Tropospheric Temperature and Baroclinicity in the Western Tropical Pacific

Abstract The utility of the thermal wind equation (TWE) in relating tropospheric (850–300 hPa) wind and temperature on climatological time scales is assessed, based on data from 59 radiosonde stations in the western tropical Pacific during 1979–2004. Observed long-term mean and seasonal variations closely obey geostrophic balance; incorporating additional (ageostrophic) terms yields negligible improvement. The authors conclude that observed winds offer a useful constraint on the horizontal structure of monthly and longer temperature variations (although the reverse is not true close to the equator where f → 0). This conclusion is also supported by general circulation model output. Wind data show a slowing of the midlatitude jets in the Maritime Continent region since 1979, indicating that tropical thicknesses and temperature have increased less than those poleward of 25°N/S. This pattern is consistent with Microwave Sounding Unit temperature trends in the region but is exaggerated south of the equator in trends obtained directly from the temperature data. The latter are however sensitive to which stations are used and how the data are averaged, and the discrepancy is fairly small in a homogenized climatology from the Hadley Centre (HadAT). The discrepancy is most easily explained by spurious cooling at stations in the near-equatorial western Pacific. These results support the use of the wind field as a way of overcoming heterogeneities in the temperature records in the monitoring of climate change patterns.

Accumulation Rates over the Past 500 Years Recorded in Ice Cores from the Northern and Southern Tibetan Plateau, China

ABSTRACT Based on the Guliya, Dunde, and Dasuopu ice cores and direct observations, we investigated the spatial and temporal variations in precipitation on the Tibetan Plateau over the past 500 yr. The variations in accumulation rates showed significant periodicities of 12.7, 7.6, 6.2, 5.4, 4.4, and 2.1 yr in the Guliya ice core, 9.5, 6.8, 5.7, and 2.1 yr in the Dunde ice core, and 12.3, 7.5, 6.3, 5.3, and 2.4 yr in the Dasuopu ice core. The periodicities displayed in these three ice core records are similar and correspond to the periodicities of the Quasi-Biennial Oscillation, the Southern Oscillation, the North Atlantic Oscillation, and the sunspot cycle. However, the accumulation rate from the Guliya and Dunde ice cores exhibited a generally decreasing trend, while the records from the Dasuopu ice core show a generally increasing trend over the entire period of interest. Our study also indicates that there is a strong negative correlation between the accumulation rates in the ice cores from the northern and southern Tibetan Plateau, especially on climatological (multidecadal or longer) time-scales. Modern meteorological observation data suggest that a dividing line between the northern and southern Tibetan Plateau with respect to variations in precipitation is located at ∼32–33°N. This dividing line coincides with other atmospheric, geographical, geological, and geophysical discontinuities. This suggests that interactions among these phenomena might help to understand the spatial patterns of climate over the Tibetan Plateau.

A simple equilibrium model for shallow-cumulus-topped mixed layers

A new equilibrium model for shallow-cumulus-topped mixed layers is presented. A variant on the w∗ closure for the shallow-cumulus mass flux is applied that retains the convective area-fraction in its formu- lation. As opposed to being constant, the fraction is explicitly modeled using a statistical closure as a function of the saturation deficit and humidity variance at cloud base. As a consequence, important new interactions are introduced between the convective transport, humidity, and depth of the mixed layer. This mechanism, which we call the mass-flux humidity feedback, helps determine the character of the equilibrium state such that the mixed-layer top is maintained close to the cloud-base height. Due to the strong sensitivity of the mass flux to the area fraction, the latter thus acts as a regulator or valve mechanism on moist convective transport. As a consequence, the mixed-layer model is able to explain the robustness of many aspects of the shallow-cumulus boundary layer that is typically found in observations and large-eddy simulations (LESs). The model is evaluated for a single-LES case as well as for global climatology obtained from a 40-year reanalysis of meteorological data by the European Centre for Medium-range Weather Forecasts (ECMWF). LES characteristics of convective mass flux, cloud fraction, humidity variance, cloud-base height, and surface fluxes of heat and humidity are reproduced. The solution on reanalysis fields reproduces the spatial structure of mixed-layer temperature and humidity and their associated surface fluxes in the subtropical Atlantic and Pacific trade wind regions. Furthermore, the spatial structure of the convective area-fraction matches that of synoptic surface observations of frequency of occurrence of shallow cumulus. Particularly striking is the smooth onset of the convective area-fraction and mass flux along the trade-wind trajectory that is reproduced, from zero to typical trade-wind values. The cumulus onset represents the necessity for shallow-cumulus mass flux to occur in order to close the mixed-layer budgets of heat, moisture, and mass, as a response to the changing magnitude of large-scale subsidence and free tropospheric humidity along the trajectory. Finally, the mass flux model is implemented in an intermediate-complexity tropical climate model to study its behavior when fully interactive with the larger-scale flow. A climate run then shows that the model is stable, due to the mass-flux humidity feedback acting to keep the shallow-cumulus boundary layer close to its equilibrium state for long, climatological timescales.

Equilibration mechanisms in an adjoint ocean general circulation model

We examine the equilibrated and time-evolving adjoint solutions of an ocean general circulation model. Adjoint models calculate the sensitivity of a diagnostic, (here, the strength of the meridional overturning) to all forcing fields in a single integration. The time evolution of the sensitivity patterns demonstrates the validity of the adjoint modeling approach over climatological time scales in coarse-resolution ocean models. Our objective is to identify the principle adjustment mechanisms through which the meridional overturning strength adapts to perturbations in wind and buoyancy forcing. The adjoint approach is shown to be a valuable alternative to traditional perturbation methods in highlighting the processes and time scales important to ocean and climate modeling.

Episodes of alpine heavy precipitation with an overlying elongated stratospheric intrusion: a climatology

This study discusses the role of stratospheric intrusions (determined as potential vorticity (PV) streamers) as upper-level instigators of heavy precipitation along the Swiss Alpine south side (AS) on a climatological timescale. A climatology of streamers is used compiled on the basis of the ECMWF 40-year re-analysis data set (ERA-40). Days of extreme and heavy precipitation along the Swiss AS are determined from an existing observational Alpine precipitation climatology. For these days, the presence of streamers over western Europe as well as their location and orientation is recorded. On 73% of the extreme precipitation days, a streamer is situated over western Europe. The mean spatial frequency distribution of the streamers on the extreme precipitation days exhibits a structure that resembles in its form and location the ‘archetypal heavy precipitation streamer’ known from case studies. The frequency maximum is situated over southern England and the west coast of France. The same analysis is applied to three sub-domains (Valais, Grisons, Ticino) along the Swiss AS. Significant differences in the location and the orientation of the streamers for the sub-domains are found. The majority of streamers associated with heavy rain in the western-most sub-domain (Valais) are oriented in a cyclonically-sheared fashion, while for the Ticino the streamers are more anti-cyclonically orientated. Differences for events of increasing severity are analysed by comparing the form, location, amplitude (PV), and persistence of the streamers on extreme and heavy precipitation days. The precipitation distribution is shifted to higher intensities for more persistent streamers. There is no detectable difference found in the form parameters, but the southerly moisture flux into the domain is significantly larger during extreme precipitation days than during heavy precipitation days. Likewise, the seasonal variation in the percentage of streamer-related heavy precipitation, which is highest in autumn (85%), can be related to the seasonal variation of southerly moisture fluxes. Copyright  2006 Royal Meteorological Society.

Parameter estimation using chaotic time series

We show how the response of a chaotic model to temporally varying external forcing can be efficiently tuned via parameter estimation using time series data, extending previous work in which an unforced climatologically steady state was used as the tuning target. Although directly fitting a long trajectory of a chaotic deterministic model to a time series of data is generally not possible even in principle, this is not actually necessary for useful prediction on climatological time-scales. If the model and data outputs are averaged over suitable time-scales, the effect of chaotic variability is effectively converted into nothing more troublesome than some statistical noise. We show how tuning of models to unsteady time series data can be efficiently achieved with an augmented ensemble Kalman filter, and we demonstrate the procedure with application to a forced version of the Lorenz model. The computational cost is of the order of 100 model integrations, and so the method should be directly applicable to more sophisticated climate models of at least moderate resolution.