A steady-state snow growth model (SGM) is formulated by microphysical growth processes of vapor d... more A steady-state snow growth model (SGM) is formulated by microphysical growth processes of vapor deposition, aggregation and riming. SGM is capable of predicting the temporal and vertical evolution of ice particle size distribution (PSD) by using radar reflectivity (Zw), supersaturation, temperature, liquid water content (LWC) and ice particle shape dependent mass-dimension power laws, and by solving the zeroth-and second-moment conservation equations with respect to mass. It appears that the riming process is essential in characterizing the snowfall rates and leads to the snowfall rates significantly greater than those produced by the vapor deposition and aggregation alone. Moreover, alteration in cloud condensation nuclei (CCN), due to aerosols, can modify cloud droplet SD (size distribution) and therefore change the snowfall rate. So, snowfall rate is sensitive to the shape of cloud droplet SD. Ice particle growth rates are uniquely formulated in the SGM in terms of ice particle mass-dimension (m-D) power laws (m = αD β), and in this way the impact of ice particle shape on particle growth rates and fall speeds is accounted for. These growth rates appear qualitatively consistent with empirical growth rates, with slower (faster) growth rates predicted for higher (lower) β values. It is well known that for a given ice particle habit, the m-D power law for the smallest ice particles differs considerably from the power law for the largest particles, with β being much larger for the smallest crystals. Our recent work quantitatively predicts β and α for frontal clouds as a function of maximum dimension D where the m-D expression is a second-order polynomial in log-log space. By tailoring the m-D power law to the relevant PSD moments, the SGM ice particle growth rates and fall speeds are represented more accurate and realistic. It is speculated that by implementing this new m-D treatment in any cloud resolving model or climate model, the ice particle growth rates will become more accurate. The predicted size spectra by SGM are in good agreement with observed spectra from aircraft measurement during Lagrangian spiral descents through frontal clouds.
American Meteorological Society 94th Annual Meeting, Feb 6, 2014
The North American Monsoon (NAM) is a seasonal change in the sub-tropical circulation that suppli... more The North American Monsoon (NAM) is a seasonal change in the sub-tropical circulation that supplies 60-80% of annual rainfall in northwestern Mexico and 30-40% in the US southwest. Regional climate models have shown that summer precipitation prediction over North America is poorest in the NAM region. An understanding of the NAM’s principal processes is essential for improving global and regional climate modeling. In this study, we suggest a partial mechanistic understanding of the NAM in the local scale that explains how the inversion over Gulf of California (GC) controls the low-level moisture. The proposed hypothesis is supported by satellite observations, ship soundings launched over the GC, and regional model (WRF) simulations. A set of carefully designed simulations of WRF is used to investigate the dependence of NAM precipitation, onset and circulation on SSTs along the Mexican coastline and in the GC. Enhanced observational data from North American Monsoon Experiment (NAME) field campaign in summer 2004 is used to evaluate the modeling results. WRF simulations show that warmer GC SSTs tend to enhance low-level moisture flux during this period and as a result more precipitation occurs over the foothills of Sierra Madre Occidental (SMO) and over US southwest. However, simulated inversions are stronger than rawinsonde observations and consequently show dry bias in free troposphere. This discrepancy may represent an opportunity to improve WRF performance over North America during summer.
Ice particle mass-and projected area-dimension (m-D and AD) power laws are commonly used in the t... more Ice particle mass-and projected area-dimension (m-D and AD) power laws are commonly used in the treatment of ice cloud microphysical and optical properties and the remote sensing of ice cloud properties. Although there has long been evidence that a single m-D or AD power law is often not valid over all ice particle sizes, few studies have addressed this fact. This study develops self-consistent m-D and AD expressions that are not power laws but can easily be reduced to power laws for the ice particle size (maximum dimension or D) range of interest, and they are valid over a much larger D range than power laws. This was done by combining ground measurements of individual ice particle m and D formed at temperature T < −20 • C during a cloud seeding field campaign with 2-D stereo (2D-S) and cloud particle imager (CPI) probe measurements of D and A, and estimates of m, in synoptic and anvil ice clouds at similar temperatures. The resulting m-D and AD expressions are functions of temperature and cloud type (synoptic vs. anvil), and are in good agreement with m-D power laws developed from recent field studies considering the same temperature range (−60 • C < T < −20 • C).
This paper describes a new approach for representing ice microphysics in climate models. In contr... more This paper describes a new approach for representing ice microphysics in climate models. In contrast with most previous schemes, this approach does not include separate categories for cloud and precipitating ice and instead uses a single two-moment category to represent all solid hydrometeors. Thus, there is no need for an ice “autoconversion” size threshold parameter, which has a critical impact on simulated climate in the Community Atmosphere Model (CAM5) yet is poorly constrained by theory or observations. Further, in the new treatment, all ice microphysical processes and parameters, including ice effective radius and mean fall speed, are formulated self-consistently and flexibly based on empirical ice particle mass–size and projected area–size relationships. This means that the scheme can represent the physical coupling between bulk particle density, mean fall speed, and effective radius, which is not possible in current schemes. Two different methods for specifying these relationships based on observations are proposed. The new scheme is tested in global simulations using CAM5. Differences in simulations using the two methods for specifying the mass– and projected area–size relationships, particularly the cloud radiative forcing, are attributable mainly to the effects on mean ice particle fall speed, impacting sedimentation and ice water path. With some tuning of parameters involved in calculating homogeneous freezing it produces a similar climate compared to the simulations using the original CAM5 microphysics. Thus, it can produce a comparable climate while improving the physical basis and self-consistency of ice particle properties and parameters.
Abstract. There are two fundamental mechanisms through which cirrus clouds form; homo- and hetero... more Abstract. There are two fundamental mechanisms through which cirrus clouds form; homo- and heterogeneous ice nucleation (henceforth hom and het). The relative contribution of each mechanism to ice crystal production often determines the microphysical and radiative properties of a cirrus cloud. A new satellite remote sensing method is described in this study to estimate cirrus cloud ice particle number concentration and the relative contribution of hom and het to cirrus cloud formation as a function of altitude, latitude, season and surface type (e.g. land vs. ocean). This method uses co-located observations from the Infrared Imaging Radiometer (IIR) and from the CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) lidar aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) polar orbiting satellite, employing IIR channels at 10.6 μm and 12.05 μm. The method is applied here to single-layered clouds of visible optical depth between about 0.3 and 3. Two years of Version 3 data have been analyzed for the years 2008 and 2013, with each season characterized in terms of 532 nm cirrus cloud centroid altitude and temperature, the cirrus cloud ice particle number concentration, effective diameter, layer-average ice water content and visible optical depth. Using a conservative criterion for hom cirrus, on average, the sampled cirrus clouds formed through hom occur about 43 % of the time in the Arctic and 50 % of the time in the Antarctic, and during winter at mid-latitudes in the Northern Hemisphere, hom cirrus occur 37 % of the time. Elsewhere (and during other seasons in the Northern Hemisphere mid-latitudes), this hom cirrus fraction is lower. Processes that could potentially explain these observations are discussed, as well as the potential relevancy of these results to ice nucleation studies, climate modeling and jet-stream dynamics.
A new satellite remote sensing method is described whereby the sensitivity of thermal infrared wa... more A new satellite remote sensing method is described whereby the sensitivity of thermal infrared wave resonance absorption to small ice crystals is exploited to estimate cirrus cloud ice-particle number concentration N, effective diameter D e and ice water content IWC. This method uses co-located observations from the Infrared Imaging Ra-diometer (IIR) and from the CALIOP (Cloud and Aerosol Li-dar with Orthogonal Polarization) lidar aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) polar orbiting satellite, employing IIR channels at 10.6 and 12.05 µm. Using particle size distributions measured over many flights of the TC4 (Tropical Composition, Cloud and Climate Coupling) and the mid-latitude SPARTI-CUS (Small Particles in Cirrus) field campaigns, we show for the first time that N/IWC is tightly related to β eff ; the ratio of effective absorption optical depths at 12.05 and 10.6 µm. Relationships developed from in situ aircraft measurements are applied to β eff derived from IIR measurements to retrieve N. This satellite remote sensing method is constrained by measurements of β eff from the IIR and is by essence sensitive to the smallest ice crystals. Retrieval uncertainties are discussed, including uncertainties related to in situ measurement of small ice crystals (D < 15 µm), which are studied through comparisons with IIR β eff. The method is applied here to single-layered semi-transparent clouds having a visible optical depth between about 0.3 and 3, where cloud base temperature is ≤ 235 K. CALIPSO data taken over 2 years have been analyzed for the years 2008 and 2013, with the dependence of cirrus cloud N and D e on altitude, temperature, latitude, season (winter vs. summer) and topography (land vs. ocean) described. The results for the mid-latitudes show a considerable dependence on season. In the high latitudes, N tends to be highest and D e smallest, whereas the opposite is true for the tropics. The frequency of occurrence of these relatively thick cirrus clouds exhibited a strong seasonal dependence in the high latitudes, with the occurrence frequency during Arctic winter being at least twice that of any other season. Processes that could potentially explain some of these micro-and macroscopic cloud phenomena are discussed.
Journal of Geophysical Research: Atmospheres , 2014
An understanding of the major governing processes of North American monsoon (NAM) is necessary to... more An understanding of the major governing processes of North American monsoon (NAM) is necessary to guide improvement in global and regional climate modeling of the NAM, as well as NAM's impacts on the summer circulation, precipitation and drought over North America. A mechanistic understanding of the NAM is suggested by incorporating local- and synoptic- scale processes. The local scale mechanism describes the effect of the temperature inversion over the Gulf of California (GC) on controlling low-level moisture during the 2004 NAM. The strong low-level inversion inhibits the exchange between the moist air in the marine boundary layer (MBL) and the overlying dry air. This inversion weakens with increasing sea surface temperatures (SSTs) in GC and generally disappears once SSTs exceed 29.5 °C, allowing the moist air, trapped in the MBL, to mix with free tropospheric air. This leads to a deep, moist layer that can be transported by across-gulf (along-gulf) flow towards the NAM core region (southwestern US) to form thunderstorms. On the synoptic scale, climatologies from 1983 to 2010 exhibit a temporal correspondence between coastal warm tropical surface water (TSW), NAM deep convection, NAM anticyclone center and NAM-induced strong descent. A hypothesis is proposed to explain this correspondence, based on limited soundings at the GC entrance (suggesting this local mechanism may also be active in that region), the climatologies and the relevant literature. The warmest SSTs moving up the coast may initiate NAM convection and atmospheric heating, advancing the position of the anticyclone and the region of descent northwards.
... Robert Rabin National Severe Storms Laboratory, Norman, Oklahoma. Timothy J. Brown andKelly R... more ... Robert Rabin National Severe Storms Laboratory, Norman, Oklahoma. Timothy J. Brown andKelly Redmond ... This results in a strong diurnal cycle of vigorous convection and rainfall, most active during late afternoonearly evening (Gourley et al. 1998; Douglas and Li 1996). ...
The complexity of predicting climate change is heightened by feedbacks resulting from clouds. Glo... more The complexity of predicting climate change is heightened by feedbacks resulting from clouds. Global Climate Models (GCMs) are highly sensitive to the representation of clouds and their feedbacks, which introduce the largest uncertainties in the prediction of climate sensitivity (i.e. the equilibrium response of global-mean surface temperature to a CO2 doubling). The Polar regions are predicted to be most affected
Cirrus cloud microphysical data from recent field programs using new instruments tend to minimize... more Cirrus cloud microphysical data from recent field programs using new instruments tend to minimize or remove the problem of ice particle shattering. These measurements suggest that in most instances, the anomalously high concentrations of small ice crystals reported in earlier in situ measurements are absent. These earlier measurements of small crystals indicated an abrupt increase in concentration for ice particle
PARAMETERIZING SIZE DISTRIBUTIONS IN ICE CLOUDS ; ; David L. Mitchell and Daniel H. DeSlover ; ; ... more PARAMETERIZING SIZE DISTRIBUTIONS IN ICE CLOUDS ; ; David L. Mitchell and Daniel H. DeSlover ; ; ABSTRACT ; An outstanding problem that contributes considerable uncertainty to Global Climate Model (GCM) predictions of future climate is the characterization of ice particle sizes in cirrus clouds. Recent parameterizations of ice cloud effective diameter differ by a factor of three, which, for
Global Climate Models (GCMs) are highly sensitive to the representation of clouds and their feedb... more Global Climate Models (GCMs) are highly sensitive to the representation of clouds and their feedbacks. As stated by Soden and Held (2006) the inter-model differences in cloud feedback are the largest source of uncertainty in current predictions of climate sensitivity. Sanderson et al. (2008) performed a principal component analysis across an ensemble of GCM runs and found that 70% of the ensemble variance in the global feedback parameter was due to two leading factors, the entrainment coefficient and the ice fall velocity (Vf). In spite of its importance, the ice fall velocity in climate models is highly uncertain due in part to its dependence on the ice particle size distribution (PSD) which has been plagued with measurement uncertainties associated with ice particle shattering on probe inlets. The focus of this research is to improve the parameterization of ice mass sedimentation rates in GCMs which is the product of the ice water content (IWC) and the mass weighted fall speed (Vm...
A steady-state snow growth model (SGM) is formulated by microphysical growth processes of vapor d... more A steady-state snow growth model (SGM) is formulated by microphysical growth processes of vapor deposition, aggregation and riming. SGM is capable of predicting the temporal and vertical evolution of ice particle size distribution (PSD) by using radar reflectivity (Zw), supersaturation, temperature, liquid water content (LWC) and ice particle shape dependent mass-dimension power laws, and by solving the zeroth-and second-moment conservation equations with respect to mass. It appears that the riming process is essential in characterizing the snowfall rates and leads to the snowfall rates significantly greater than those produced by the vapor deposition and aggregation alone. Moreover, alteration in cloud condensation nuclei (CCN), due to aerosols, can modify cloud droplet SD (size distribution) and therefore change the snowfall rate. So, snowfall rate is sensitive to the shape of cloud droplet SD. Ice particle growth rates are uniquely formulated in the SGM in terms of ice particle mass-dimension (m-D) power laws (m = αD β), and in this way the impact of ice particle shape on particle growth rates and fall speeds is accounted for. These growth rates appear qualitatively consistent with empirical growth rates, with slower (faster) growth rates predicted for higher (lower) β values. It is well known that for a given ice particle habit, the m-D power law for the smallest ice particles differs considerably from the power law for the largest particles, with β being much larger for the smallest crystals. Our recent work quantitatively predicts β and α for frontal clouds as a function of maximum dimension D where the m-D expression is a second-order polynomial in log-log space. By tailoring the m-D power law to the relevant PSD moments, the SGM ice particle growth rates and fall speeds are represented more accurate and realistic. It is speculated that by implementing this new m-D treatment in any cloud resolving model or climate model, the ice particle growth rates will become more accurate. The predicted size spectra by SGM are in good agreement with observed spectra from aircraft measurement during Lagrangian spiral descents through frontal clouds.
American Meteorological Society 94th Annual Meeting, Feb 6, 2014
The North American Monsoon (NAM) is a seasonal change in the sub-tropical circulation that suppli... more The North American Monsoon (NAM) is a seasonal change in the sub-tropical circulation that supplies 60-80% of annual rainfall in northwestern Mexico and 30-40% in the US southwest. Regional climate models have shown that summer precipitation prediction over North America is poorest in the NAM region. An understanding of the NAM’s principal processes is essential for improving global and regional climate modeling. In this study, we suggest a partial mechanistic understanding of the NAM in the local scale that explains how the inversion over Gulf of California (GC) controls the low-level moisture. The proposed hypothesis is supported by satellite observations, ship soundings launched over the GC, and regional model (WRF) simulations. A set of carefully designed simulations of WRF is used to investigate the dependence of NAM precipitation, onset and circulation on SSTs along the Mexican coastline and in the GC. Enhanced observational data from North American Monsoon Experiment (NAME) field campaign in summer 2004 is used to evaluate the modeling results. WRF simulations show that warmer GC SSTs tend to enhance low-level moisture flux during this period and as a result more precipitation occurs over the foothills of Sierra Madre Occidental (SMO) and over US southwest. However, simulated inversions are stronger than rawinsonde observations and consequently show dry bias in free troposphere. This discrepancy may represent an opportunity to improve WRF performance over North America during summer.
Ice particle mass-and projected area-dimension (m-D and AD) power laws are commonly used in the t... more Ice particle mass-and projected area-dimension (m-D and AD) power laws are commonly used in the treatment of ice cloud microphysical and optical properties and the remote sensing of ice cloud properties. Although there has long been evidence that a single m-D or AD power law is often not valid over all ice particle sizes, few studies have addressed this fact. This study develops self-consistent m-D and AD expressions that are not power laws but can easily be reduced to power laws for the ice particle size (maximum dimension or D) range of interest, and they are valid over a much larger D range than power laws. This was done by combining ground measurements of individual ice particle m and D formed at temperature T < −20 • C during a cloud seeding field campaign with 2-D stereo (2D-S) and cloud particle imager (CPI) probe measurements of D and A, and estimates of m, in synoptic and anvil ice clouds at similar temperatures. The resulting m-D and AD expressions are functions of temperature and cloud type (synoptic vs. anvil), and are in good agreement with m-D power laws developed from recent field studies considering the same temperature range (−60 • C < T < −20 • C).
This paper describes a new approach for representing ice microphysics in climate models. In contr... more This paper describes a new approach for representing ice microphysics in climate models. In contrast with most previous schemes, this approach does not include separate categories for cloud and precipitating ice and instead uses a single two-moment category to represent all solid hydrometeors. Thus, there is no need for an ice “autoconversion” size threshold parameter, which has a critical impact on simulated climate in the Community Atmosphere Model (CAM5) yet is poorly constrained by theory or observations. Further, in the new treatment, all ice microphysical processes and parameters, including ice effective radius and mean fall speed, are formulated self-consistently and flexibly based on empirical ice particle mass–size and projected area–size relationships. This means that the scheme can represent the physical coupling between bulk particle density, mean fall speed, and effective radius, which is not possible in current schemes. Two different methods for specifying these relationships based on observations are proposed. The new scheme is tested in global simulations using CAM5. Differences in simulations using the two methods for specifying the mass– and projected area–size relationships, particularly the cloud radiative forcing, are attributable mainly to the effects on mean ice particle fall speed, impacting sedimentation and ice water path. With some tuning of parameters involved in calculating homogeneous freezing it produces a similar climate compared to the simulations using the original CAM5 microphysics. Thus, it can produce a comparable climate while improving the physical basis and self-consistency of ice particle properties and parameters.
Abstract. There are two fundamental mechanisms through which cirrus clouds form; homo- and hetero... more Abstract. There are two fundamental mechanisms through which cirrus clouds form; homo- and heterogeneous ice nucleation (henceforth hom and het). The relative contribution of each mechanism to ice crystal production often determines the microphysical and radiative properties of a cirrus cloud. A new satellite remote sensing method is described in this study to estimate cirrus cloud ice particle number concentration and the relative contribution of hom and het to cirrus cloud formation as a function of altitude, latitude, season and surface type (e.g. land vs. ocean). This method uses co-located observations from the Infrared Imaging Radiometer (IIR) and from the CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) lidar aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) polar orbiting satellite, employing IIR channels at 10.6 μm and 12.05 μm. The method is applied here to single-layered clouds of visible optical depth between about 0.3 and 3. Two years of Version 3 data have been analyzed for the years 2008 and 2013, with each season characterized in terms of 532 nm cirrus cloud centroid altitude and temperature, the cirrus cloud ice particle number concentration, effective diameter, layer-average ice water content and visible optical depth. Using a conservative criterion for hom cirrus, on average, the sampled cirrus clouds formed through hom occur about 43 % of the time in the Arctic and 50 % of the time in the Antarctic, and during winter at mid-latitudes in the Northern Hemisphere, hom cirrus occur 37 % of the time. Elsewhere (and during other seasons in the Northern Hemisphere mid-latitudes), this hom cirrus fraction is lower. Processes that could potentially explain these observations are discussed, as well as the potential relevancy of these results to ice nucleation studies, climate modeling and jet-stream dynamics.
A new satellite remote sensing method is described whereby the sensitivity of thermal infrared wa... more A new satellite remote sensing method is described whereby the sensitivity of thermal infrared wave resonance absorption to small ice crystals is exploited to estimate cirrus cloud ice-particle number concentration N, effective diameter D e and ice water content IWC. This method uses co-located observations from the Infrared Imaging Ra-diometer (IIR) and from the CALIOP (Cloud and Aerosol Li-dar with Orthogonal Polarization) lidar aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) polar orbiting satellite, employing IIR channels at 10.6 and 12.05 µm. Using particle size distributions measured over many flights of the TC4 (Tropical Composition, Cloud and Climate Coupling) and the mid-latitude SPARTI-CUS (Small Particles in Cirrus) field campaigns, we show for the first time that N/IWC is tightly related to β eff ; the ratio of effective absorption optical depths at 12.05 and 10.6 µm. Relationships developed from in situ aircraft measurements are applied to β eff derived from IIR measurements to retrieve N. This satellite remote sensing method is constrained by measurements of β eff from the IIR and is by essence sensitive to the smallest ice crystals. Retrieval uncertainties are discussed, including uncertainties related to in situ measurement of small ice crystals (D < 15 µm), which are studied through comparisons with IIR β eff. The method is applied here to single-layered semi-transparent clouds having a visible optical depth between about 0.3 and 3, where cloud base temperature is ≤ 235 K. CALIPSO data taken over 2 years have been analyzed for the years 2008 and 2013, with the dependence of cirrus cloud N and D e on altitude, temperature, latitude, season (winter vs. summer) and topography (land vs. ocean) described. The results for the mid-latitudes show a considerable dependence on season. In the high latitudes, N tends to be highest and D e smallest, whereas the opposite is true for the tropics. The frequency of occurrence of these relatively thick cirrus clouds exhibited a strong seasonal dependence in the high latitudes, with the occurrence frequency during Arctic winter being at least twice that of any other season. Processes that could potentially explain some of these micro-and macroscopic cloud phenomena are discussed.
Journal of Geophysical Research: Atmospheres , 2014
An understanding of the major governing processes of North American monsoon (NAM) is necessary to... more An understanding of the major governing processes of North American monsoon (NAM) is necessary to guide improvement in global and regional climate modeling of the NAM, as well as NAM's impacts on the summer circulation, precipitation and drought over North America. A mechanistic understanding of the NAM is suggested by incorporating local- and synoptic- scale processes. The local scale mechanism describes the effect of the temperature inversion over the Gulf of California (GC) on controlling low-level moisture during the 2004 NAM. The strong low-level inversion inhibits the exchange between the moist air in the marine boundary layer (MBL) and the overlying dry air. This inversion weakens with increasing sea surface temperatures (SSTs) in GC and generally disappears once SSTs exceed 29.5 °C, allowing the moist air, trapped in the MBL, to mix with free tropospheric air. This leads to a deep, moist layer that can be transported by across-gulf (along-gulf) flow towards the NAM core region (southwestern US) to form thunderstorms. On the synoptic scale, climatologies from 1983 to 2010 exhibit a temporal correspondence between coastal warm tropical surface water (TSW), NAM deep convection, NAM anticyclone center and NAM-induced strong descent. A hypothesis is proposed to explain this correspondence, based on limited soundings at the GC entrance (suggesting this local mechanism may also be active in that region), the climatologies and the relevant literature. The warmest SSTs moving up the coast may initiate NAM convection and atmospheric heating, advancing the position of the anticyclone and the region of descent northwards.
... Robert Rabin National Severe Storms Laboratory, Norman, Oklahoma. Timothy J. Brown andKelly R... more ... Robert Rabin National Severe Storms Laboratory, Norman, Oklahoma. Timothy J. Brown andKelly Redmond ... This results in a strong diurnal cycle of vigorous convection and rainfall, most active during late afternoonearly evening (Gourley et al. 1998; Douglas and Li 1996). ...
The complexity of predicting climate change is heightened by feedbacks resulting from clouds. Glo... more The complexity of predicting climate change is heightened by feedbacks resulting from clouds. Global Climate Models (GCMs) are highly sensitive to the representation of clouds and their feedbacks, which introduce the largest uncertainties in the prediction of climate sensitivity (i.e. the equilibrium response of global-mean surface temperature to a CO2 doubling). The Polar regions are predicted to be most affected
Cirrus cloud microphysical data from recent field programs using new instruments tend to minimize... more Cirrus cloud microphysical data from recent field programs using new instruments tend to minimize or remove the problem of ice particle shattering. These measurements suggest that in most instances, the anomalously high concentrations of small ice crystals reported in earlier in situ measurements are absent. These earlier measurements of small crystals indicated an abrupt increase in concentration for ice particle
PARAMETERIZING SIZE DISTRIBUTIONS IN ICE CLOUDS ; ; David L. Mitchell and Daniel H. DeSlover ; ; ... more PARAMETERIZING SIZE DISTRIBUTIONS IN ICE CLOUDS ; ; David L. Mitchell and Daniel H. DeSlover ; ; ABSTRACT ; An outstanding problem that contributes considerable uncertainty to Global Climate Model (GCM) predictions of future climate is the characterization of ice particle sizes in cirrus clouds. Recent parameterizations of ice cloud effective diameter differ by a factor of three, which, for
Global Climate Models (GCMs) are highly sensitive to the representation of clouds and their feedb... more Global Climate Models (GCMs) are highly sensitive to the representation of clouds and their feedbacks. As stated by Soden and Held (2006) the inter-model differences in cloud feedback are the largest source of uncertainty in current predictions of climate sensitivity. Sanderson et al. (2008) performed a principal component analysis across an ensemble of GCM runs and found that 70% of the ensemble variance in the global feedback parameter was due to two leading factors, the entrainment coefficient and the ice fall velocity (Vf). In spite of its importance, the ice fall velocity in climate models is highly uncertain due in part to its dependence on the ice particle size distribution (PSD) which has been plagued with measurement uncertainties associated with ice particle shattering on probe inlets. The focus of this research is to improve the parameterization of ice mass sedimentation rates in GCMs which is the product of the ice water content (IWC) and the mass weighted fall speed (Vm...
To improve treatment of 2 nd aerosol indirect effect in mixed-phase clouds, it is required to kno... more To improve treatment of 2 nd aerosol indirect effect in mixed-phase clouds, it is required to know explicit treatment of snow-droplet collision efficiencies and impact of accretion on ice particle projected area (A) and mass (m). we developed a snow growth model (SGM) that explicitly treats ice particle accretion. Although riming effect on D is insignificant, it considerably change A and m. A method is introduced to calculate rimed A and m from unrimed A and m, and from maximum A and m that can be obtained by riming. It is well known that mass-dimension (m-D) and area-dimension (A-D) power laws (e.g. m=αD β) for small particles differ considerably from those for large particles. Our recent work predicts β and α for frontal clouds as a function of D where m-D expression is a 2 nd-order polynomial fit in log-log space. Since lowest radar reflectivity (Zw) over complex topography is often above cloud base, radar quantitative precipitation estimates (QPE) often underestimate precipitation at ground level. SGM can be initialized with Zw at lowest reliable radar echo and improves QPE at ground.
Our recent publication in JGR describes and provides evidence for a partial mechanistic understan... more Our recent publication in JGR describes and provides evidence for a partial mechanistic understanding of the North American monsoon (NAM). This presentation would focus on the oceanographic aspects of that research and relate them to the air-sea interactions (documented in our paper) that help transfer abundant moisture from the Gulf of California (GC) into northern Mexico, the USA and as far north as Canada. This moisture transfer and subsequent convection provides a source region for stationary Rossby waves that affect the large-scale circulation and precipitation patterns of over North America during summer.
Regarding the air-sea mechanism, an intrusion of tropical surface water (TSW) into the GC occurs during May-June that coincides with a slackening of the coastal wind field off Baja California and central Mexico. Satellite measurements of sea surface height (SSH) further show a reconfiguration of SSH fields and geostrophic currents at the time of this wind slackening, with TSW from the eastern Pacific warm pool directed poleward into the GC. Subsequently GC sea surface temperatures (SSTs) typically reach 29-30°C by mid-July, with this SST increase warming and humidifying the GC marine boundary layer (MBL) air, eroding the MBL inversion once SSTs exceed ~ 29°C. This allows the very humid GC MBL air to mix with free tropospheric air, producing a deep (several km) moist layer that can be advected inland to produce thunderstorms. This entire process may depend on the annual cycle of the North Pacific High. A NAM paleoclimate study (Barron et al., 2012), supported by these ideas, addresses the potential impact of a warming climate on the NAM.
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Papers by David L . Mitchell
In this study, we suggest a partial mechanistic understanding of the NAM in the local scale that explains how the inversion over Gulf of California (GC) controls the low-level moisture. The proposed hypothesis is supported by satellite observations, ship soundings launched over the GC, and regional model (WRF) simulations.
A set of carefully designed simulations of WRF is used to investigate the dependence of NAM precipitation, onset and circulation on SSTs along the Mexican coastline and in the GC. Enhanced observational data from North American Monsoon Experiment (NAME) field campaign in summer 2004 is used to evaluate the modeling results. WRF simulations show that warmer GC SSTs tend to enhance low-level moisture flux during this period and as a result more precipitation occurs over the foothills of Sierra Madre Occidental (SMO) and over US southwest. However, simulated inversions are stronger than rawinsonde observations and consequently show dry bias in free troposphere. This discrepancy may represent an opportunity to improve WRF performance over North America during summer.
In this study, we suggest a partial mechanistic understanding of the NAM in the local scale that explains how the inversion over Gulf of California (GC) controls the low-level moisture. The proposed hypothesis is supported by satellite observations, ship soundings launched over the GC, and regional model (WRF) simulations.
A set of carefully designed simulations of WRF is used to investigate the dependence of NAM precipitation, onset and circulation on SSTs along the Mexican coastline and in the GC. Enhanced observational data from North American Monsoon Experiment (NAME) field campaign in summer 2004 is used to evaluate the modeling results. WRF simulations show that warmer GC SSTs tend to enhance low-level moisture flux during this period and as a result more precipitation occurs over the foothills of Sierra Madre Occidental (SMO) and over US southwest. However, simulated inversions are stronger than rawinsonde observations and consequently show dry bias in free troposphere. This discrepancy may represent an opportunity to improve WRF performance over North America during summer.
Regarding the air-sea mechanism, an intrusion of tropical surface water (TSW) into the GC occurs during May-June that coincides with a slackening of the coastal wind field off Baja California and central Mexico. Satellite measurements of sea surface height (SSH) further show a reconfiguration of SSH fields and geostrophic currents at the time of this wind slackening, with TSW from the eastern Pacific warm pool directed poleward into the GC. Subsequently GC sea surface temperatures (SSTs) typically reach 29-30°C by mid-July, with this SST increase warming and humidifying the GC marine boundary layer (MBL) air, eroding the MBL inversion once SSTs exceed ~ 29°C. This allows the very humid GC MBL air to mix with free tropospheric air, producing a deep (several km) moist layer that can be advected inland to produce thunderstorms. This entire process may depend on the annual cycle of the North Pacific High. A NAM paleoclimate study (Barron et al., 2012), supported by these ideas, addresses the potential impact of a warming climate on the NAM.