Large-scale Vertical Motion Research Articles

Different Types of ENSO Influences on the Indian Summer Monsoon Variability

AbstractObservational analysis reveals three types of El Niño–Southern Oscillation (ENSO) influences on the Indian summer monsoon (ISM): indirect influence of the preceding winter [December–February (DJF)] eastern equatorial Pacific (EEP) sea surface temperature (SST) anomalies (DJF-only cases), direct influence of the concurrent summer [June–September (JJAS)] EEP SST anomalies (JJAS-only cases), and coherent influence of both the preceding winter and concurrent summer EEP SST anomalies (DJF&JJAS cases). The present study distinguishes the three types of ENSO influences and investigates the processes connecting ENSO to the ISM separately.In the DJF-only cases, the preceding winter EEP SST anomalies induce north Indian Ocean (NIO) SST anomalies through air–sea interaction processes in the tropical Indian Ocean. The SST anomalies over the western Indian Ocean alter the surface air humidity there. Both processes favor an anomalous ISM. In the JJAS-only cases, an anomalous ISM is directly induced by ENSO through large-scale circulation changes. The meridional thermal contrast may also contribute to an anomalous ISM. In the DJF&JJAS cases, the preceding winter EEP SST anomalies induce NIO SST anomalies and change the surface air humidity over the western Indian Ocean. Concurrent summer EEP SST anomalies induce large-scale vertical motion anomalies over South Asia. Together, they lead to an anomalous ISM. The anomalous meridional thermal contrast may contribute to an anomalous ISM in late summer.Impacts of the preceding winter EEP SST anomalies in the DJF and JJAS cases may contribute to the contemporaneous correlation between ISM and EEP SST. There are more DJF&JJAS cases before than after the late 1970s. This provides an alternative interpretation for the observed weakening in the ISM–ENSO relationship around the late 1970s.

The Relationship between Tropical Warm Pool Precipitation, Sea Surface Temperature, and Large-Scale Vertical Motion in IPCC AR4 Models

Abstract A regime sorting analysis is used to identify Caribbean and western Pacific precipitation, sea surface temperature, and large-scale vertical circulation relationships and biases within coupled and uncoupled Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) general circulation models. This analysis shows that an oversensitivity of precipitation to both SST and vertical circulation (as represented by ω500) is inherent in the atmospheric models in both regions, with models using a spectral-type convective parameterization performing best in the Caribbean, but less separation between convective parameterization groups is seen in the western Pacific. The error in magnitude of precipitation for a given SST and vertical circulation causes uncoupled models to overestimate Caribbean and western Pacific mean precipitation. In coupled models, however, errors in the frequency of occurrence of SSTs (the distribution is cold biased in both regions) and deep convective vertical circulations (reduced frequency) lead to an underestimation of Caribbean and western Pacific mean precipitation. In the western Pacific, increased frequency of subsidence regimes in coupled models leads to an overestimation of precipitation at ω500 values above 0 hPa day−1. The varied ability of convective parameterization groups in the two warm pool regions suggests that deficiencies in parameterization groups differ between the two regions, with improvements needed particularly in the deep convective regime in the Caribbean and subsidence regimes in the western Pacific.

Prediction of Northern Nigeria Rainfall Using Sea Surface Temperature

Large-scale vertical motion in the atmosphere in the tropics is tied to high sea surface temperatures (SST). Enhanced moisture content of the boundary layer over sea leads to enhanced moisture convergence because of the low pressure created over the high sea surface temperature regions. This leads to a feed-back mechanism that enhanced moisture convergence because vertical ascent of air through the release of latent heat in deep convection. All these phenomena are associated with high sea surface temperatures (SST) and are widely believed to induce large scale atmospheric circulation and heating anomalies. These anomalies in the tropical atmosphere can have consequences in locations far and remote from the direct sea surface temperatures (SST). In order to have data coverage sufficient and adequate for this study, the region was enlarged to include stations at the outskirts of northern Nigeria. All the stations lie approximately between latitudes 9°N and 15°N were used to analyse the data covering a period of 1901 to 2006. Sytat software was used to analyse rainfall and temperature data of cities of northern Nigeria and its outlying geographic areas for predicting rainfall. Correlation and regression was carried out on the data set to estimate the model. Using the results of the regression, models were constructed for April, May and June. Testing the models on the correlation between the forecast and the actual savanna July, August and September (JAS) rainfall shows that it can be predicted with 62.10% accuracy using April model, 74% accuracy using May and 64.5% accuracy using June model. The model showed a more robust prediction when more recent data (2000- 2006) are used. The result obtained for April was 70%, May was 90% and June was 80% when evaluated by common statistics, the model performed well as the magnitude of error was low.

Tropical mid-tropospheric CO2 variability driven by the Madden-Julian oscillation.

Carbon dioxide (CO(2)) is the most important anthropogenic greenhouse gas in the present-day climate. Most of the community focuses on its long-term (decadal to centennial) behaviors that are relevant to climate change, but there are relatively few discussions of its higher-frequency forms of variability, and none regarding its subseasonal distribution. In this work, we report a large-scale intraseasonal variation in the Atmospheric Infrared Sounder CO(2) data in the global tropical region associated with the Madden-Julian oscillation (MJO). The peak-to-peak amplitude of the composite MJO modulation is ∼1ppmv, with a standard error of the composite mean <0.1ppmv. The correlation structure between CO(2) and rainfall and vertical velocity indicate positive (negative) anomalies in CO(2) arise due to upward (downward) large-scale vertical motions in the lower troposphere associated with the MJO. These findings can help elucidate how faster processes can organize, transport, and mix CO(2) and provide a robustness test for coupled carbon-climate models.

Modelling microphysical and meteorological controls on precipitation and cloud cellular structures in Southeast Pacific stratocumulus

Abstract. Microphysical and meteorological controls on the formation of open and closed cellular structures in the Southeast Pacific are explored using model simulations based on aircraft observations during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The effectiveness of factors such as boundary-layer moisture and temperature perturbations, surface heat and moisture fluxes, large-scale vertical motion and solar heating in promoting drizzle and open cell formation for prescribed aerosol number concentrations is explored. For the case considered, drizzle and subsequent open cell formation over a broad region are more sensitive to the observed boundary-layer moisture and temperature perturbations (+0.9 g kg−1; −1 K) than to a five-fold decrease in aerosol number concentration (150 vs. 30 mg−1). When embedding the perturbations in closed cells, local drizzle and pockets of open cell (POC) formation respond faster to the aerosol reduction than to the moisture increase, but the latter generates stronger and more persistent drizzle. A local negative perturbation in temperature drives a mesoscale circulation that prevents local drizzle formation but promotes it in a remote area where lower-level horizontal transport of moisture is blocked and converges to enhance liquid water path. This represents a potential mechanism for POC formation in the Southeast Pacific stratocumulus region whereby the circulation is triggered by strong precipitation in adjacent broad regions of open cells. A simulation that attempts to mimic the influence of a coastally induced upsidence wave results in an increase in cloud water but this alone is insufficient to initiate drizzle. An increase of surface sensible heat flux is also effective in triggering local drizzle and POC formation. Both open and closed cells simulated with observed initial conditions exhibit distinct diurnal variations in cloud properties. A stratocumulus deck that breaks up due solely to solar heating can recover at night. Precipitation in the open-cell cases depletes the aerosol to the extent that cloud formation is significantly suppressed within one diurnal cycle. A replenishment rate of cloud condensation nuclei of order 1 mg−1 h−1 is sufficient to maintain clouds and prevent the boundary layer from collapsing the following day, suggesting that some local and/or remote aerosol sources is necessary for POCs to be able to last for days.

The Double-ITCZ Syndrome in Coupled General Circulation Models: The Role of Large-Scale Vertical Circulation Regimes

Abstract The double–intertropical convergence zone (DI) systematic error, affecting state-of-the-art coupled general circulation models (CGCMs), is examined in the multimodel Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) ensemble of simulations of the twentieth-century climate. The aim of this study is to quantify the DI error on precipitation in the tropical Pacific, with a specific focus on the relationship between the DI error and the representation of large-scale vertical circulation regimes in climate models. The DI rainfall signal is analyzed using a regime-sorting approach for the vertical circulation regimes. Through the use of this compositing technique, precipitation events are regime sorted based on the large-scale vertical motions, as represented by the midtropospheric Lagrangian pressure tendency ω500 dynamical proxy. This methodology allows partition of the precipitation signal into deep and shallow convective components. Following the regime-sorting diagnosis, the total DI bias is split into an error affecting the magnitude of precipitation associated with individual convective events and an error affecting the frequency of occurrence of single convective regimes. It is shown that, despite the existing large intramodel differences, CGCMs can be ultimately grouped into a few homogenous clusters, each featuring a well-defined rainfall–vertical circulation relationship in the DI region. Three major behavioral clusters are identified within the AR4 models ensemble: two unimodal distributions, featuring maximum precipitation under subsidence and deep convection regimes, respectively, and one bimodal distribution, displaying both components. Extending this analysis to both coupled and uncoupled (atmosphere only) AR4 simulations reveals that the DI bias in CGCMs is mainly due to the overly frequent occurrence of deep convection regimes, whereas the error on rainfall magnitude associated with individual convective events is overall consistent with errors already present in the corresponding atmosphere stand-alone simulations. A critical parameter controlling the strength of the DI systematic error is identified in the model-dependent sea surface temperature (SST) threshold leading to the onset of deep convection (THR), combined with the average SST in the southeastern Pacific. The models featuring a THR that is systematically colder (warmer) than their mean surface temperature are more (less) prone to exhibit a spurious southern intertropical convergence zone.

Modelling recent deformation of the Pannonian lithosphere: Lithospheric folding and tectonic topography

We investigated the role of lithospheric folding in the Quaternary inversion of the Pannonian basin by a series of analogue models. To this aim, build-up of stresses due to intraplate compression in the hot and weak Pannonian lithosphere, changes in the style of deformation and related surface processes were modelled. The primary response of the lithosphere to compression appears to be deformation in the form of large-scale folding. As a consequence of the folding, differential crustal motions occur, affecting present-day surface morphology and landscape processes. The analogue experiments examined folding mechanisms of the hot Pannonian lithosphere characterised by extremely low strengths except for a thin layer of brittle upper crust. Modelling results confirmed the existence of a large wavelength (∼ 350–400 km) component of deformation accounting for large-scale vertical crustal motions. The amplitude of folding is sufficient to generate the amount of observed uplift and subsidence. Our analogue models, supported by the results of stress analyses, suggest that despite the low rate of convergence between the Adriatic microplate (“Adria-push”) and the European plate, the weak Pannonian lithosphere has been an efficient transmitter of compression during the basin inversion. Crustal thickness variations are of key importance in governing regional deformation pattern and influence the timing and extent of the basin inversion. Effects of alternating strong and weak units in the brittle crust were also examined by means of two series of conceptual models, in which the order of thin and thick crustal blocks was opposite. Strain localisation in the brittle crust was strongly controlled by the moderate initial thickness variations. The concept gives a plausible explanation for the presence of anomalous rates of uplift and subsidence and multi-wavelength folding inside the basin. Models taking into account horizontal movements due to lateral extrusion were constructed with an oblique face of the indenter. This kinematic boundary condition resulted in a complex internal structure of the folded layers. The presented analogue experiments, together with previous numerical modelling studies, demonstrate the link between large-scale lithospheric folding and topography evolution in the Pannonian basin system.

Response of Humidity and Clouds to Tropical Deep Convection

Abstract Currently available satellite data can be used to track the response of clouds and humidity to intense precipitation events. A compositing technique centered in space and time on locations experiencing high rain rates is used to detail the characteristic evolution of several quantities measured from a suite of satellite instruments. Intense precipitation events in the convective tropics are preceded by an increase in low-level humidity. Optically thick cold clouds accompany the precipitation burst, which is followed by the development of spreading upper-level anvil clouds and an increase in upper-tropospheric humidity over a broader region than that occupied by the precipitation anomalies. The temporal separation between the convective event and the development of anvil clouds is about 3 h. The humidity increase at upper levels and the associated decrease in clear-sky longwave emission persist for many hours after the convective event. Large-scale vertical motions from reanalysis show a coherent evolution associated with precipitation events identified in an independent dataset: precipitation events begin with stronger upward motion anomalies in the lower troposphere, which then evolve toward stronger upward motion anomalies in the upper troposphere, in conjunction with the development of anvil clouds. Greater upper-tropospheric moistening and cloudiness are associated with larger-scale and better-organized convective systems, but even weaker, more isolated systems produce sustained upper-level humidity and clear-sky outgoing longwave radiation anomalies.

Visualizing the Large-Scale Patterns of ENSO-Related Climate Anomalies in North America

Abstract The variations of large-scale climatic controls and surface responses through the annual cycle during strong positive (El Niño) and negative (La Niña) phase ENSO events are analyzed to assess the within-year and among-year variations of climate anomalies. Data from the NCEP–NCAR reanalysis project are presented as small-multiple maps to illustrate the spatial and temporal variability in North American climate associated with extreme phases of ENSO. Temperature, mean sea level pressure, 500-mb geopotential heights, and 850-mb specific humidity have composite-anomaly patterns that exhibit the greatest degree of spatial and temporal coherence. In general, the composite-anomaly patterns for El Niño and La Niña events are of opposite sign, with stronger, more spatially coherent anomalies occurring during El Niño events than during La Niña events. However, the strength and coherency of the precipitation anomaly patterns are reduced in the interior intermountain west during both positive and negative phase of ENSO. The variations in precipitation anomalies are compared to the 500-mb omega and 850-mb specific humidity composite-anomaly patterns, which provide information on the controls of precipitation by large-scale vertical motions and moisture availability thus providing information on the specific mechanisms associated with precipitation variability during ENSO events.

Dynamic Effects on the Tropical Cloud Radiative Forcing and Radiation Budget

Abstract Vertical velocity is used to isolate the effect of large-scale dynamics on the observed radiation budget and cloud properties in the tropics, using the methodology suggested by Bony et al. Cloud and radiation budget quantities in the tropics show well-defined responses to the large-scale vertical motion at 500 hPa. For the tropics as a whole, the ratio of shortwave to longwave cloud forcing (hereafter N) is about 1.2 in regions of upward motion, and increases to about 1.9 in regions of strong subsidence. If the analysis is restricted to oceanic regions with SST &amp;gt; 28°C, N does not increase as much for subsiding motions, because the stratocumulus regions are eliminated, and the net cloud forcing decreases linearly from about near zero for zero vertical velocity to about −15 W m−2 for strongly subsiding motion. Increasingly negative cloud forcing with increasing upward motion is mostly related to an increasing abundance of high, thick clouds. Although a consistent dynamical effect on the annual cycle of about 1 W m−2 can be identified, the effect of the probability density function (PDF) of the large-scale vertical velocity on long-term trends in the tropical mean radiation budget is very small compared to the observed variations. Observed tropical mean changes can be as large as ±3 W m−2, while the dynamical components are generally smaller than ±0.5 W m−2. For relatively small regions in the east and west Pacific, changes in the relative magnitude of longwave and shortwave cloud forcing can be related to the PDF of vertical velocity. The east Pacific in 1987 and 1998 showed large reductions of N in association with an increase in the fraction of the area in the domain with upward motion, and concomitant increases in high cloud. For the west Pacific in 1998, a large increase in N was caused not so much by a change in the mean vertical motion, but rather by a shift from top- to bottom-heavy upward motion.

Impact of Dynamics and Atmospheric State on Cloud Vertical Overlap

Abstract The observation and representation in general circulation models (GCMs) of cloud vertical overlap are the objects of active research due to their impacts on the earth’s radiative budget. Previous studies have found that vertically contiguous cloudy layers show a maximum overlap between layers up to several kilometers apart but tend toward a random overlap as separations increase. The decorrelation length scale that characterizes the progressive transition from maximum to random overlap changes from one location and season to another and thus may be influenced by large-scale vertical motion, wind shear, or convection. Observations from the U.S. Department of Energy Atmospheric Radiation Measurement program ground-based radars and lidars in midlatitude and tropical locations in combination with reanalysis meteorological fields are used to evaluate how dynamics and atmospheric state influence cloud overlap. For midlatitude winter months, strong synoptic-scale upward motion maintains conditions closer to maximum overlap at large separations. In the tropics, overlap becomes closer to maximum as convective stability decreases. In midlatitude subsidence and tropical convectively stable situations, where a smooth transition from maximum to random overlap is found on average, large wind shears sometimes favor minimum overlap. Precipitation periods are discarded from the analysis but, when included, maximum overlap occurs more often at large separations. The results suggest that a straightforward modification of the existing GCM mixed maximum–random overlap parameterization approach that accounts for environmental conditions can capture much of the important variability and is more realistic than approaches that are only based on an exponential decay transition from maximum to random overlap.

Theoretical aspects of the onset of Indian summer monsoon from perturbed orography simulations in a GCM

Abstract. A theory is proposed to determine the onset of the Indian Summer Monsoon (ISM) in an Atmospheric General Circulation Model (AGCM). The onset of ISM is delayed substantially in the absence of global orography. The impact of orography over different parts of the Earth on the onset of ISM has also been investigated using five additional perturbed simulations. The large difference in the date of onset of ISM in these simulations has been explained by a new theory based on the Surface Moist Static Energy (SMSE) and vertical velocity at the mid-troposphere. It is found that onset occurs only after SMSE crosses a threshold value and the large-scale vertical motion in the middle troposphere becomes upward. This study shows that both dynamics and thermodynamics play profound roles in the onset of the monsoon.

The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection

The LMDZ4 general circulation model is the atmospheric component of the IPSL–CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley–Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke’s convection scheme, used in previous versions, the Emanuel’s scheme improves the representation of the Hadley–Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke’s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.

Cirrus Clouds and the Large-Scale Atmospheric State: Relationships Revealed by Six Years of Ground-Based Data

AbstractThe properties of cirrus clouds observed at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) in Oklahoma are documented from a nearly continuous 6-yr record of 35-GHz cloud radar data. Cirrus frequency over the ACRF is 23% and 28% of the time in the warm (May–September) and cold seasons (November–March), respectively, with maxima and minima during the period studied of 30% and 16% in the warm season and 34% and 24% in the cold seasons. Cirrus, as defined here, reveal a seasonal oscillation in their macroscale properties that can be traced to the seasonal deepening of the troposphere in the Southern Plains region. While the average bulk microphysical properties do not change significantly from season to season, the variability of certain parameters demonstrates seasonal change. It is shown that the properties of cirrus clouds vary perceptively with the large-scale vertical motion. Using NCEP–NCAR reanalysis data to define the large-scale meteorological state when cirrus are observed at the ACRF, the authors find that cirrus tend to exist within a maximum in upper-tropospheric humidity and downstream of the peak upper-tropospheric vertical motion. Cirrus that exist in large-scale ascent upstream of the synoptic-scale middle-tropospheric ridge axis are shown to have higher water contents than cirrus that exist in large-scale subsidence downstream of the ridge axis, although the overall nature of the statistical distributions of water contents do not change greatly, suggesting that it may be difficult to parameterize the properties of cirrus based solely on large-scale vertical motion. The layer-mean particle size, on the other hand, shows no such sensitivity to the large-scale vertical motion.

Statistics of Absolute and Relative Dispersion in the Atmospheric Convective Boundary Layer: A Large-Eddy Simulation Study

Abstract The influence of the different scales of turbulent motion on plume dispersion in the atmospheric convective boundary layer (CBL) is studied by means of a large-eddy simulation (LES). In particular, the large-scale (meandering) and small-scale (relative diffusion) contributions are separated by analyzing dispersion in two reference systems: the absolute (fixed) coordinate system and the coordinate system relative to the plume’s instantaneous center of mass. In the relative coordinate system, the (vertically) inhomogeneous meandering motion is removed, and only the small-scale, homogeneous turbulent motion contributes to the dispersion process. First, mean plume position, dispersion parameters (variance), and skewness of the plume position are discussed. The analysis of the third-order moments shows how the structure and the symmetry of scalar distribution are affected with respect to the turbulent characteristics of the CBL (inhomogeneity of the large-scale vertical motion) and the presence of the boundary conditions (surface and top of the CBL). In fact, the reflection of the plume by the CBL boundaries generates the presence of nonlinear cross-correlation terms in the balance equation for the third-order moments of the plume position. As a result, the third-order moment of the absolute position is not balanced by the sum of the third-order moments of the meandering and relative plume position. Second, mean concentration and concentration fluctuations are calculated and discussed in both coordinate systems. The intensity of relative concentration fluctuation icr, which quantifies the internal (in plume) mixing, is explicitly calculated. Based on these results, a parameterization for the probability distribution function (PDF) of the relative concentration is proposed, showing very good agreement with the LES results. Finally, the validity of Gifford’s formula, which relates the absolute concentration’s high-order moments to the relative concentration and the PDF of the plume centerline, is studied. It is found that due to the presence of the CBL boundaries, Gifford’s formula is not able to reproduce correctly the value of the absolute mean concentration near the ground. This result is analyzed by showing that, when the plume is reflected by the CBL boundaries, the instantaneous relative plume width z′2r(t) departs from its mean value σ2r. By introducing the skewness of the relative plume position into a parameterization for the relative mean concentration, the results for the calculated mean concentration are improved.

Evidence of mesospheric bore formation from a breaking gravity wave event: simultaneous imaging and lidar measurements

A large wave event was observed in the three upper-mesospheric (80–105 km) airglow emissions of O( 1S), Na and OH by the Boston University all-sky imager, at the Arecibo Observatory, during the night of 3 May 2003. The airglow structures appeared to be due to a large upward propagating internal gravity wave, which subsequently became unstable near the 95 km height level and produced large-scale vertical motions and mixing. Simultaneous density and temperature lidar measurements indicated the presence of a large temperature inversion of 80 K valley-to-peak between 88 and 96 km during the time of the event. Near-simultaneous temperature profiles, made by the TIMED SABER instrument, provided evidence that the horizontal extent of the inversion was localized to within 500 km of Arecibo during the wave event. As the gravity wave dissipated, an internal bore was generated, apparently due to the deposition of momentum and energy into the region by the original wave. Although mesospheric gravity wave breaking has been reported previously (Swenson and Mende, 21(1994); Hecht et al., 102(1997); Yamada et al., 28(2001), for example), this was the first time that the phenomenon has been associated with the generation of an internal mesospheric bore. The event suggested that the breaking of a large mesospheric gravity wave can lead to the generation of an internal bore, as suggested by Dewan and Picard 106(2001). Such behavior is of particular interest since little is known of their origins.

Sensitivities of Cumulus-Ensemble Rainfall in a Cloud-Resolving Model with Parameterized Large-Scale Dynamics

Abstract The problem of closure in cumulus parameterization requires an understanding of the sensitivities of convective cloud systems to their large-scale setting. As a step toward such an understanding, this study probes some sensitivities of a simulated ensemble of convective clouds in a two-dimensional cloud-resolving model (CRM). The ensemble is initially in statistical equilibrium with a steady imposed background forcing (cooling and moistening). Large-scale stimuli are imposed as horizontally uniform perturbations nudged into the model fields over 10 min, and the rainfall response of the model clouds is monitored. In order to reduce a major source of artificial insensitivity in the CRM, a simple parameterization scheme is devised to account for heating-induced large-scale (i.e., domain averaged) vertical motions that would develop in nature but are forbidden by the periodic boundary conditions. The effects of this large-scale vertical motion are parameterized as advective tendency terms that are ap...

Evaluating climate model simulations of tropical cloud

The representation of tropical cloud and its radiative effects in the Hadley Centre climate model are evaluated using a combination of Earth observation data and meteorological reanalyses. It is shown that useful information regarding the model’s physical parametrizations can be obtained by considering cloud radiative effects and cloud types in terms of ‘dynamical regimes’, defined in terms of sea surface temperature and large-scale vertical motion. In addition to comparisons with observed top-of-atmosphere radiation budget parameters and total cloud amount, information is obtained through direct comparisons of International Satellite Cloud Climatology Project (ISCCP) cloud types, defined according to cloud top pressure and optical depth, with corresponding model diagnostics. An analysis of the atmosphereonly model, HadAM3, demonstrates how errors in the albedo and outgoing long-wave radiation can be related to the simulation of particular cloud types in the different dynamical regimes. Inconsistencies between the simulations of the various cloud types and the top-of-atmosphere radiation budget are also highlighted. A version of the model including several new cloud-related parametrizations is then examined. A more consistent comparison with the observed radiation budget and cloud amounts is obtained, although deficiencies in the simulation still remain. A parametrization for the radiative effects of convective anvils and the impact of a new boundary layer mixing scheme are examined in more detail. Finally, it is shown how the climate model’s ability to simulate the observed interannual variability of cloud in the equatorial Pacific follows directly from the analysis according to dynamical regimes.

Evaluating climate model simulations of tropical cloud

The representation of tropical cloud and its radiative effects in the Hadley Centre climate model are evaluated using a combination of Earth observation data and meteorological reanalyses. It is shown that useful information regarding the model’s physical parametrizations can be obtained by considering cloud radiative effects and cloud types in terms of ‘dynamical regimes’, defined in terms of sea surface temperature and large-scale vertical motion. In addition to comparisons with observed top-of-atmosphere radiation budget parameters and total cloud amount, information is obtained through direct comparisons of International Satellite Cloud Climatology Project (ISCCP) cloud types, defined according to cloud top pressure and optical depth, with corresponding model diagnostics. An analysis of the atmosphereonly model, HadAM3, demonstrates how errors in the albedo and outgoing long-wave radiation can be related to the simulation of particular cloud types in the different dynamical regimes. Inconsistencies between the simulations of the various cloud types and the top-of-atmosphere radiation budget are also highlighted. A version of the model including several new cloud-related parametrizations is then examined. A more consistent comparison with the observed radiation budget and cloud amounts is obtained, although deficiencies in the simulation still remain. A parametrization for the radiative effects of convective anvils and the impact of a new boundary layer mixing scheme are examined in more detail. Finally, it is shown how the climate model’s ability to simulate the observed interannual variability of cloud in the equatorial Pacific follows directly from the analysis according to dynamical regimes.

Numerical Simulations of a Landfalling Cold Front Observed during COAST: Rapid Evolution and Responsible Mechanisms*

This paper identifies mechanisms that led to the observed rapid evolution of a landfalling weak cold front along the steep mountainous northern California coast on 1 December 1995. This event was simulated down to 3-km horizontal grid spacing using the Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5). The MM5 simulation reproduced the basic features such as the timing, location, and orientation of the cold front and associated precipitation evolution, as well as the tendency for enhanced precipitation to extend ∼50–100 km upwind of the coastal barrier, with the heaviest amounts occurring over the windward slopes (0–20 km inland); locally, however, the model underestimated the magnitude of the prefrontal terrain-enhanced flow by as much as 30% since the simulated low-level static stability was weaker than observed. The MM5 simulations illustrate the complex thermal, wind, and precipitation structures in the coastal zone. Upstream flow blocking by the steep coastal terrain led to the development of a mesoscale pressure ridge and prefrontal terrain-enhanced winds exceeding 25 m s−1. Because of the irregular coastline and highly three-dimensional terrain, the low-level winds were not uniform along the coast. Rather, prefrontal southerly flow was significantly reduced downwind of the major capes (viz. Mendocino and Blanco), while there were localized downgradient accelerations adjacent to regions of higher topography along uninterrupted stretches of coastline. Terrain–front interactions resulted in a slowing of the front as the system made landfall, and blocking contributed to a “tipped forward” baroclinic structure below 800 mb. The MM5 was used to investigate some of the reasons for the rapid intensification of the frontal temperature gradient and banded precipitation in the coastal zone. During this event the large-scale vertical motions increased in an environment favorable for moist convection, and a simulation without coastal topography illustrated rapid development of coastal precipitation even in the absence of local terrain influences. The coastal topography helped to further enhance and collapse the thermal gradient and associated cold-frontal rainband through enhanced deformation frontogenesis associated with the prefrontal terrain-enhanced flow. Diabatic effects from precipitation are also shown to have been important in organizing the precipitation in the coastal zone and further enhancing the frontal temperature gradient.