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Catherine M. Naud and Brian H. Kahn

Abstract

Ice cloud properties in Northern Hemisphere winter extratropical cyclones are examined using the Atmospheric Infrared Sounder (AIRS), version 6, cloud products. The cloud thermodynamic phase product indicates that warm frontal clouds are dominated by ice, liquid-phase clouds occur outside of the warm frontal region, and supercooled or mixed-phase clouds are found in the southwestern quadrant of the cyclones. Stratiform ice clouds populate the warm frontal region and portions of the cold sector while convective ice clouds populate southeastern portions of the warm front and the southeastern quadrant. Total cloud cover is smaller in land cyclones than in ocean cyclones, especially in the southwestern quadrant and the warm frontal region. Ice cloud cover is less over land in the warm frontal region, because land cyclones are generally weaker and drier than ocean cyclones. The impact of cyclone average precipitable water (PW) and the magnitude of 850-hPa vertical ascent ω 850 on the thermodynamic phase, occurrence of stratiform or convective ice cloud, ice particle effective diameter, optical thickness, and cloud-top temperature are discussed. When comparing land and ocean cyclones with similar PW and ω 850, ice cloud coverage is found to be greater over land. Convective ice cloud occurs more often and is deeper over land. Supercooled cloud appears to persist to colder temperatures over ocean than over land, especially in the warm frontal region. These results suggest that, over land, ice cloud formation in warm fronts is possibly more efficient because of a greater aerosol amount from local or regional sources.

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Catherine M. Naud, Anthony D. Del Genio, and Mike Bauer

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The conditions under which supercooled liquid water gradually gives way to ice in the mixed-phase regions of clouds are still poorly understood and may be an important source of cloud feedback uncertainty in general circulation model projections of long-term climate change. Two winters of cloud phase discrimination, cloud-top temperature, sea surface temperature, and precipitation from several satellite datasets (the NASA Terra and Aqua Moderate Resolution Imaging Spectroradiometer, and the Tropical Rainfall Measuring Mission) for the North Atlantic and Pacific Ocean basins are analyzed to better understand these processes. Reanalysis surface pressures and vertical velocities are used in combination with a synoptic storm-tracking algorithm to define storm tracks, create composite storm dynamical and cloud patterns, and examine changes in storm characteristics over their life cycles. Characteristically different storm cloud patterns exist in the Atlantic and Pacific and on the west and east sides of each ocean basin. This appears to be related to the different spatial patterns of sea surface temperature in the two ocean basins. Glaciation occurs at very warm temperatures in the high, thick, heavily precipitating clouds typical of frontal ascent regions, except where vertical velocities are strongest, similar to previous field experiments. Outside frontal regions, however, where clouds are shallower, supercooled water exists at lower cloud-top temperatures. This analysis is the first large-scale assessment of cloud phase and its relation to dynamics on climatologically representative time scales. It provides a potentially powerful benchmark for the design and evaluation of mixed-phase process parameterizations in general circulation models and suggests that assumptions made in some existing models may negatively bias their cloud feedback estimates.

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Fayçal Lamraoui, James F. Booth, and Catherine M. Naud

Abstract

The present study explores the ability of the Weather Research and Forecasting (WRF) Model to accurately reproduce the passage of extratropical cold fronts at the DOE ARM eastern North Atlantic (ENA) observation site on the Azores. An analysis of three case studies is performed in which the impact of the WRF domain size, position of the model boundary relative to the ENA site, grid spacing, and spectral nudging conditions are explored. The results from these case studies indicate that model biases in the timing and duration of cold front passages change with the distance between the model domain boundary and the ENA site. For these three cases, if the western model boundary is farther than 1500 km from the site, the front becomes too meridional and fails to reach the site, making 1000 or 1500 km the optimal distances. In contrast, integrations with small distances (e.g., 500 km) between the site and domain boundaries have inadequate spatial spinup (i.e., the domain is too small for the model to properly stabilize). For all three cases, regardless of domain size, the model has biases in its upper-level circulation that impact the position and timing of the front. However, this issue is most serious for 4000-km2 domains and larger. For these domains, prolonged spectral nudging can correct cold front biases. As such, this analysis provides a framework to optimize the WRF Model configuration necessary for a realistic hindcast of a cold front passage at a fixed location centered in a domain as large as computationally possible.

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Catherine M. Naud, James F. Booth, and Anthony D. Del Genio

Abstract

Using NASA Aqua MODIS and AIRS data, the relationship between low-level cloud cover (cloud top below the 700-hPa level) and boundary layer stability is explored in post-cold-frontal conditions. A linear relationship is found between seasonal cloud cover and two separate measures of inversion strength, the lower-tropospheric stability (LTS) and the estimated inversion strength (EIS), for two specific regions in the North Atlantic and Pacific in quiescent and weakly subsiding conditions. The relationship barely changes when considering dynamically active and subsiding post-cold-frontal conditions for the same regions. To explore the generality of this result and increase sample size, cold-front-centered composites of cloud cover and stability are constructed. The Northern and Southern Hemisphere seasonal cloud cover and stability distributions in the post-cold-frontal regions are then compared. A fairly good correlation between cloud cover and EIS is found in both hemispheres across all seasons, suggesting that a linear relationship between cloud cover and inversion strength proposed for quiescent conditions exists also in more dynamically active subsiding post-cold-frontal conditions. However, for a given season and hemisphere, the correlation between cloud cover and EIS degrades in post-cold-frontal regions, especially in the Northern Hemisphere. At these scales, other large-scale factors tend to correlate better with cloud cover.

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Catherine M. Naud, Juan A. Crespo, and Derek J. Posselt

Abstract

Surface latent and sensible heat fluxes are important for extratropical cyclone evolution and intensification. Because extratropical cyclone genesis often occurs at low latitudes, Cyclone Global Navigation Satellite System (CYGNSS) surface latent and sensible heat flux retrievals are composited to provide a mean picture of their spatial distribution in low-latitude oceanic extratropical cyclones. CYGNSS heat fluxes are not affected by heavy precipitation and offer observations of storms with frequent revisit times. Consistent with prior results obtained for cyclones in the Gulf Stream region, the fluxes are strongest in the wake of the cold fronts and are weakest to negative in the warm sector in advance of the cold fronts. As cyclone strength increases or mean precipitable water decreases, the maximum in surface heat fluxes increases while the minimum decreases. This affects the changes in fluxes during cyclone intensification: the post-cold-frontal surface heat flux maximum increases as a result of the increase in near-surface winds. During cyclone dissipation, the fluxes in this sector decrease because of the decrease in winds and in temperature and humidity contrast. The warm-sector minimum decreases throughout the entire cyclone lifetime and is mostly driven by sea–air temperature and humidity contrast changes. However, during cyclone dissipation, the surface heat fluxes increase along the cold front in a narrow band to the east, independent from changes in the cyclone characteristics. This result suggests that, during cyclone dissipation, energy transfers from the ocean to the atmosphere are linked to frontal processes in addition to synoptic-scale processes.

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Catherine M. Naud, James F. Booth, and Anthony D. Del Genio

Abstract

The Southern Ocean cloud cover modeled by the Interim ECMWF Re-Analysis (ERA-Interim) and Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalyses are compared against Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) observations. ERA-Interim monthly mean cloud amounts match the observations within 5%, while MERRA significantly underestimates the cloud amount. For a compositing analysis of clouds in warm season extratropical cyclones, both reanalyses show a low bias in cloud cover. They display a larger bias to the west of the cyclones in the region of subsidence behind the cold fronts. This low bias is larger for MERRA than for ERA-Interim. Both MODIS and MISR retrievals indicate that the clouds in this sector are at a low altitude, often composed of liquid, and of a broken nature. The combined CloudSatCloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) cloud profiles confirm these passive observations, but they also reveal that low-level clouds in other parts of the cyclones are also not properly represented in the reanalyses. The two reanalyses are in fairly good agreement for the dynamic and thermodynamic characteristics of the cyclones, suggesting that the cloud, convection, or boundary layer schemes are the problem instead. An examination of the lower-tropospheric stability distribution in the cyclones from both reanalyses suggests that the parameterization of shallow cumulus clouds may contribute in a large part to the problem. However, the differences in the cloud schemes and in particular in the precipitation processes, which may also contribute, cannot be excluded.

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Juan A. Crespo, Catherine M. Naud, and Derek J. Posselt

Abstract

Latent and sensible heat fluxes over the oceans are believed to play an important role in the genesis and evolution of marine-based extratropical cyclones (ETCs) and affect rapid cyclogenesis. Observations of ocean surface heat fluxes are limited from existing in situ and remote sensing platforms, which may not offer sufficient spatial and temporal resolution. In addition, substantial precipitation frequently veils the ocean surface around ETCs, limiting the capacity of spaceborne instruments to observe the surface processes within maturing ETCs. Although designed as a tropics-focused mission, the Cyclone Global Navigation Satellite System (CYGNSS) can observe ocean surface wind speed and heat fluxes within a notable quantity of low-latitude extratropical fronts and cyclones. These observations can assist in understanding how surface processes may play a role in cyclogenesis and evolution. This paper illustrates CYGNSS’s capability to observe extratropical cyclones manifesting in various ocean basins throughout the globe and shows that the observations provide a robust sample of ETCs winds and surface fluxes, as compared with a reanalysis dataset.

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James F. Booth, Catherine M. Naud, and Jeff Willison

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The representation of extratropical cyclone (ETC) precipitation in general circulation models (GCMs) and the Weather Research and Forecasting (WRF) Model is analyzed. This work considers the link between ETC precipitation and dynamical strength and tests if parameterized convection affects this link for ETCs in the North Atlantic basin. Lagrangian cyclone tracks of ETCs in ERA-Interim (ERAI), GISS and GFDL CMIP5 models, and WRF with two horizontal resolutions are utilized in a compositing analysis. The 20-km-resolution WRF Model generates stronger ETCs based on surface wind speed and cyclone precipitation. The GCMs and ERAI generate similar composite means and distributions for cyclone precipitation rates, but GCMs generate weaker cyclone surface winds than ERAI. The amount of cyclone precipitation generated by the convection scheme differs significantly across the datasets, with the GISS model generating the most, followed by ERAI and then the GFDL model. The models and reanalysis generate relatively more parameterized convective precipitation when the total cyclone-averaged precipitation is smaller. This is partially due to the contribution of parameterized convective precipitation occurring more often late in the ETC’s life cycle. For reanalysis and models, precipitation increases with both cyclone moisture and surface wind speed, and this is true if the contribution from the parameterized convection scheme is larger or not. This work shows that these different models generate similar total ETC precipitation despite large differences in the parameterized convection, and these differences do not cause unexpected behavior in ETC precipitation sensitivity to cyclone moisture or surface wind speed.

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James F. Booth, Catherine M. Naud, and Anthony D. Del Genio

Abstract

This study analyzes characteristics of clouds and vertical motion across extratropical cyclone warm fronts in the NASA Goddard Institute for Space Studies general circulation model. The validity of the modeled clouds is assessed using a combination of satellite observations from CloudSat, Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E), and the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis. The analysis focuses on developing cyclones, to test the model's ability to generate their initial structure. To begin, the extratropical cyclones and their warm fronts are objectively identified and cyclone-local fields are mapped into a vertical transect centered on the surface warm front. To further isolate specific physics, the cyclones are separated using conditional subsetting based on additional cyclone-local variables, and the differences between the subset means are analyzed. Conditional subsets are created based on 1) the transect clouds and 2) vertical motion; 3) the strength of the temperature gradient along the warm front, as well as the storm-local 4) wind speed and 5) precipitable water (PW). The analysis shows that the model does not generate enough frontal cloud, especially at low altitude. The subsetting results reveal that, compared to the observations, the model exhibits a decoupling between cloud formation at high and low altitudes across warm fronts and a weak sensitivity to moisture. These issues are caused in part by the parameterized convection and assumptions in the stratiform cloud scheme that are valid in the subtropics. On the other hand, the model generates proper covariability of low-altitude vertical motion and cloud at the warm front and a joint dependence of cloudiness on wind and PW.

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Catherine M. Naud, James F. Booth, Matthew Lebsock, and Mircea Grecu

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Using cyclone-centered compositing and a database of extratropical-cyclone locations, the distribution of precipitation frequency and rate in oceanic extratropical cyclones is analyzed using satellite-derived datasets. The distribution of precipitation rates retrieved using two new datasets, the Global Precipitation Measurement radar–microwave radiometer combined product (GPM-CMB) and the Integrated Multisatellite Retrievals for GPM product (IMERG), is compared with CloudSat, and the differences are discussed. For reference, the composites of AMSR-E, GPCP, and two reanalyses are also examined. Cyclone-centered precipitation rates are found to be the largest with the IMERG and CloudSat datasets and lowest with GPM-CMB. A series of tests is conducted to determine the roles of swath width, swath location, sampling frequency, season, and epoch. In all cases, these effects are less than ~0.14 mm h−1 at 50-km resolution. Larger differences in the composites are related to retrieval biases, such as ground-clutter contamination in GPM-CMB and radar saturation in CloudSat. Overall the IMERG product reports precipitation more often, with larger precipitation rates at the center of the cyclones, in conditions of high precipitable water (PW). The CloudSat product tends to report more precipitation in conditions of dry or moderate PW. The GPM-CMB product tends to systematically report lower precipitation rates than the other two datasets. This intercomparison provides 1) modelers with an observational uncertainty and range (0.21–0.36 mm h−1 near the cyclone centers) when using composites of precipitation for model evaluation and 2) retrieval-algorithm developers with a categorical analysis of the sensitivity of the products to PW.

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