Hallett Mossop Research Articles

Cloud‐resolving simulations of intense tropical Hector thunderstorms: Implications for aerosol–cloud interactions

AbstractThe Hector thunderstorm is studied with a numerical cloud‐resolving model. Special attention is given to modelling the mixed‐phase and glaciated cloud microphysical processes (along with the implications of aerosols) and their influence on the resulting microphysical and dynamical storm structure. Radiative impacts are also calculated. Simulations are performed for a typical storm case from the EMERALD‐II convective cloud experiment in November and December 2002.It is found that, for intense thunderstorms, aerosol indirect effects are generally modified from recently proposed theoretical considerations. Specifically, the proposed ‘glaciation’ indirect effect, resulting from increasing ice nuclei concentrations, is small for intense convection. More importantly, increasing ice number concentrations results in a ‘collection’ indirect effect (where aggregation and accretion processes lead to precipitation) rather than the ‘glaciation’, Bergeron–Findeisen process. There is a ‘thermodynamic’ indirect effect for Hector, as increasing the cloud droplet number concentration from maritime to continental values resulted in a suppression of the heterogeneous freezing process. However, for extreme continental cases, liquid and raindrop freezing by collection processes acquires higher importance; hence there is an optimal value for strong cumulonimbus development. The ‘glaciation’ indirect effect is found to be similar to increasing the rate of ice production by the Hallett–Mossop process. Another aspect of this study shows that there is a significant impact of microphysics on cloud dynamics, and so studying aerosol–cloud effects must also consider dynamical feedback, a strong component of which arises from the latent heat released during homogeneous freezing.The important indirect effects may be well described by recent theory for smaller, more common stratiform and cumulus clouds; however, in the tropics, the importance of Hector‐type storms cannot be ignored as they, and other similar storms, provide a mechanism for the production of widespread cirrus and the release of a large amount of precipitation. Copyright © 2006 Royal Meteorological Society

Modelling the influence of rimer surface temperature on the glaciation of intense thunderstorms: The rime–splinter mechanism of ice multiplication

AbstractA modelling study is presented to look at the potential importance of ice multiplication in a deep Cb cloud observed during the second Egrett Microphysics Experiment with Radiation, Lidar and Dynamics (EMERALD‐II). Simulations with both a cloud‐resolving model (CRM) and an explicit‐microphysics model (EMM) are used. The CRM has a two‐moment bulk microphysics scheme and resolved cloud dynamics, while the EMM has detailed size‐resolved microphysics, but is driven by the CRM cloud dynamics. The simulations consider the sensitivity of the storms to the rime‐splinter, secondary ice production mechanism (or Hallett–Mossop process).Recent literature on modelling this process in Cb clouds suggests that it is of low importance to the cloud glaciation in intense storms. However, until now, numerical simulations in the literature have investigated this process by either switching it on or off in the model microphysics scheme. We show that if the rate of splinter production is doubled from that proposed by Hallett and Mossop's experiment—a reasonable factor—then for this simulated model cloud, there is a large direct impact on anvil glaciation. The approximate net radiative impact of this calculated with the CRM is 10 W m−2. Similar, ‘nonlinear’ results can also be expected from other ice nucleating mechanisms. Simulations are performed for both continental and maritime cloud condensation nuclei concentrations. In stark contrast to previous literature, we find that the Hallett–Mossop process is more important in the continental (‘polluted’) case than in the maritime (‘clean’) case. This is because of the extremely warm cloud base for this particular storm and consequently—because of the rapid production of rain—the scarcity of small cloud droplets for riming in the clean case. Copyright © 2006 Royal Meteorological Society

Numerical modelling of mixed-phase frontal clouds observed during the CWVC project

The Met Office's cloud-resolving model was used to model mixed-phase frontal clouds observed as part of the Clouds, Water Vapour and Climate programme between 1999 and 2001. The clouds were studied using the Met Office's C130 aircraft and the Chilbolton radar facility. In the model, precipitation forms in updraughts then falls, giving rise to regions of high radar reflectivity similar to those observed. The vertical motions were driven by conditional instability or shear instability. The sensitivity of the model results to the microphysics was examined by varying the number of primary ice nuclei and switching off the Hallett–Mossop process. It is demonstrated that the ice nucleation processes, both primary nucleation and secondary ice particle production, are key to the precipitation production in the cloud. This occurs both through the detail of cloud microphysics and the release of latent heat of fusion in the cloud. This is very sensitive to the details of the microphysics and can markedly change the cloud dynamics. In both cases it seems that the cloud dynamics can be simulated locally without considering the large-scale forcing. © Royal Meteorological Society, 2005. The contributions of P. R. A. Brown and P. R. Field are Crown copyright.

Sensitivity of freezing drizzle formation in stably stratified clouds to ice processes

This study examines the impact of ice formation and growth processes on freezing drizzle formation in stably stratified clouds. In particular we investigate the reason why freezing drizzle is rarely observed in clouds with top temperatures less than −15 °C. We also investigate the sensitivity of freezing drizzle formation to the Hallett Mossop secondary ice process (Hallet and Mossop, 1974). The evaluation is performed by simulating cloud formation over a two-dimensional idealized mountain using a detailed microphysical scheme. The height and width of the two-dimensional mountain were designed to produce an updraft pattern with extent and magnitude similar to documented freezing drizzle cases. The simulations show that: (i) drizzle formation is very sensitive to the ice crystal concentration, with a significant reduction in the area over which drizzle forms and the maximum drizzle water content as the cloud top temperature decreases below −10 °C, and (ii) secondary ice crystal formation has a significant effect on drizzle formation at cloud top temperatures below −10 °C.

Determination of precipitation rates and yields from lightning measurements

This paper is concerned with the determination of relationships between precipitation rates p and lightning flash-rates in thunderstorms. Previous calculations predicting that the lightning frequency f is proportional to the product of the downflux of solid precipitation p and the mass upflux I of ice crystals, have been extended in order to derive direct relationships between f and p. The form of the f/ p relationship depends upon the dominant glaciation mechanism. If primary nucleation is dominant f is proportional to the first power of p, but if Hallett–Mossop nucleation prevails the exponent is roughly 1.5. In order to test these predictions adequately, significant additional field-data analysis is required. The estimated values of precipitation yield per lightning flash, Y, are 10 7 and 2×10 6 kg/flash, respectively. These predictions are in reasonable agreement with field data. Improved predictions of Y should further increase the reported important role of lightning data—when assimilated into mesoscale models—in improving the accuracy of quantitative precipitation forecasting and shortterm forecasts of flooding.

Simulations of the glaciation of a frontal mixed‐phase cloud with the Explicit Microphysics Model

AbstractSimulations with the Explicit Microphysics Model (EMM) of a case of lightly precipitating, glaciated stratiform cloud are presented. This frontal cloud was observed by the UK Met Office C‐130 aircraft and the dual‐polarization radar at Chilbolton in southern England.The Hallett–Mossop (H‐M) process was found to cause extremely high number concentrations of crystal columns of up to almost 1000 l−1 in the H‐M region (−3 to −8°C) of the updraught in the EMM control simulation. Similarly high number concentrations of ice particles were seen in, and in the vicinity of, ascending thermals in the aircraft observations. Such concentrations are orders of magnitude higher than the ice nucleus (IN) concentration. Moreover, the dendritic growth of highly planar particles of primary ice produces a peak in differential reflectivity of almost 4 dB just below the cloud top (−15°C). Primary ice particles are generally larger than H‐M splinters and mostly determine the simulated radar properties of the model cloud.Tests with the EMM revealed significant sensitivities of the average ice number concentration, the cloud‐ and ice‐water paths, and the surface precipitation rate, to atmospheric concentrations of IN and cloud condensation nuclei for this frontal‐cloud case. The layer of supercooled cloud‐water located just below cloud top in the control is depleted by evaporation when the IN concentration is augmented. © Royal Meteorological Society, 2003. P. R. Field's contribution is Crown copyright

The influence of aerosol concentrations on the glaciation and precipitation of a cumulus cloud

AbstractThe response of the glaciation and precipitation of a multi‐thermal cumulus cloud to changes in the aerosol concentration has been assessed in a series of sensitivity tests with the UMIST Explicit Microphysics Model (EMM). A simulation of this cloud from the Met Office cloud‐resolving model (CRM) has been utilized in these tests. This cumulus cloud was observed by aircraft during the initial stage of its growth over New Mexico on 10 August 1987. The growth of the simulated cloud is divided into two parts: a shallow phase followed by a deep phase. Maximum values of the cloud depth in these two phases were 5 and 9 km, respectively.In the EMM simulations including only the shallow phase, the precipitation efficiency was found to decrease substantially with increasing atmospheric concentrations of cloud condensation nuclei (CCN). Also, the graupel mixing ratio and total ice concentration were found to decrease as normalized CCN concentrations were increased above typical continental values. These changes are explicable in terms of: (1) the Hallett–Mossop (H–M) process at −3 to −8°C and the freezing of supercooled raindrops in collisions with ice splinters dominating the glaciation; and (2) the warm‐rain process being more significant for the overall precipitation production than the ice process in these particular simulations. The almost complete suppression of precipitation by extreme CCN concentrations corresponding to a forest‐fire plume in the EMM simulation is consistent with the analysis by Rosenfeld of satellite images of Indonesian cumuli engulfed by smoke from biomass burning.A clear tendency for ice crystals to be smaller and more numerous in the anvil was found with increasing CCN concentrations beyond typical continental values in long‐term simulations that included the deep phase. The sensitivity of the precipitation rate to the normalized CCN concentration was found to be relatively low in these deep cases. Copyright © 2002 Royal Meteorological Society.

Properties of embedded convection in warm‐frontal mixed‐phase cloud from aircraft and polarimetric radar

AbstractResults are presented from a case‐study in which the macrophysical and microphysical characteristics of a warm‐frontal mixed‐phase cloud were investigated using simultaneous aircraft and polarimetric‐radar measurements. A region of embedded convection was located and sampled at various stages of its ascent through the cloud from −5° to −11°, and evidence is presented that it was triggered by Kelvin–Helmholtz instability near the melting layer. High concentrations of small crystals were observed in and above narrow convective turrets, around 1 km across, that contained supercooled liquid‐water droplets with an effective diameter of 24 µm and riming ice particles. The area surrounding the turrets was found to contain pristine columns, and was strikingly visible to the radar as a broad plume of high differential reflectivity ZDR (around 3 dB), contrasting with the 0–0.5 dB range usually found in ice clouds. The columns observed in situ had axial ratios of around 5:1, in close agreement with the values predicted theoretically from ZDR assuming horizontal alignment of the crystals. We argue that the high concentrations of small ice crystals (up to 103 l−1) observed in this case were splinters produced via the Hallett–Mossop mechanism in the riming process, and these then grew rapidly by deposition in the water‐saturated environment into pristine columns. Later in the ascent the picture was complicated by the presence of a fallstreak containing large, low‐ZDR aggregates which appeared to rapidly accrete the columns. A number of plumes of high ZDR were observed by the radar on the same day, each of which persisted for at least half an hour. In the vicinity of cloud top −15°, the regions of high ZDR tended to spread out horizontally, and attain values as high as 7 dB. At these temperatures plates and dendrites are known to grow, so these ZDR observations are in good agreement with our theoretical finding that columns alone cannot produce values of ZDR greater than 4 dB, no matter how extreme their axial ratios, whereas planar crystals can attain values up to 10 dB. Embedded convection could clearly be important in determining the distribution of ice and liquid water in frontal clouds, which affects both cloud glaciation and the radiation budget, and is important for aircraft icing. Copyright © 2002 Royal Meteorological Society

A laboratory study of the effect of velocity on Hallett–Mossop ice crystal multiplication

Laboratory experiments into the effect of ice multiplication in clouds by the Hallett–Mossop (H–M) mechanism have been extended to higher velocities than in earlier studies. The secondary ice particle production rate per milligram of rime accreted is found to be dependent on the velocity of the graupel pellet as it accretes supercooled droplets. In the range 1.5 to 12 m s −1, the maximum secondary ice particle ejection occurs at 6 m s −1 with a rate of one crystal per 70 mg of accreted rime. The relevance of these new results to the particle production mechanism and to behaviour in natural clouds is discussed.

The glaciation of a cumulus cloud over New Mexico

AbstractThe Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi‐thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud.The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett‐Mossop (H‐M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud‐base temperature, entrainment, and the freezing and splintering of supercooled raindrops. These tests with the EMM demonstrate that reductions in the mean droplet diameter can inhibit the rates of H‐M splinter production and auto‐conversion, reducing the rate of accumulation of precipitation at the ground and reducing the concentration of ice particles. The warm‐rain process in the EMM is fundamental to the production of graupel, H‐M splinters and precipitation.Good agreement was found between the predictions of the CRM and the available dynamical and microphysical field observations. Analysis of results from both models indicated that the cloud glaciation is explicable in terms of the H‐M process, with ice production being dominated by the freezing of supercooled raindrops in the H‐M band, and the immediate and continuous production of ice splinters as supercooled droplets freeze onto them.

The glaciation of a cumulus cloud over New Mexico

The Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi-thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud. The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett-Mossop (H-M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud-base temperature, entrainment, and the freezing and splintering of supercooled raindrops. These tests with the EMM demonstrate that reductions in the mean droplet diameter can inhibit the rates of H-M splinter production and auto-conversion, reducing the rate of accumulation of precipitation at the ground and reducing the concentration of ice particles. The warm-rain process in the EMM is fundamental to the production of graupel, H-M splinters and precipitation. Good agreement was found between the predictions of the CRM and the available dynamical and microphysical field observations. Analysis of results from both models indicated that the cloud glaciation is explicable in terms of the H-M process, with ice production being dominated by the freezing of supercooled raindrops in the H-M band, and the immediate and continuous production of ice splinters as supercooled droplets freeze onto them.

An Investigation of Ice Production Mechanisms in Small Cumuliform Clouds Using a 3D Model with Explicit Microphysics. Part II: Case Study of New Mexico Cumulus Clouds

Abstract A new 3D model with explicit liquid- and ice-phase microphysics and a detailed treatment of ice nucleation and multiplication processes is applied to study ice formation and evolution in cumulus clouds. Simulation results are compared with in situ observations collected by the National Center for Atmospheric Research King Air aircraft in a cloud over the Magdalena Mountains in New Mexico on 9 August 1987. The model reproduces well the observed cloud in terms of cloud geometry, liquid water content, and concentrations of cloud drops and ice particles (IP). Primary ice nucleation is shown to produce IP in concentrations on the order of 103 m−3 (1 L−1) once the cloud top reaches −10° to −12°C. At mature and early dissipating stages of cloud development, ice production is dominated by the rime-splintering (Hallett–Mossop) mechanism, which in some regions generates up to 5 × 104 m−3 (50 L−1) IP in about 10 min. The predicted maximum of IP concentration is in agreement with observations. The sampling t...

Relationships between lightning activity and various thundercloud parameters: satellite and modelling studies

The lightning frequency model developed by Baker et al. [Baker, M.B., Christian, H.J., Latham, J., 1995. A computational study of the relationships linking lightning frequency and other thundercloud parameters, Q. J. R. Meteorol. Soc., 121, 1525–1548] has been refined and extended, in an effort to provide a more realistic framework from which to examine computationally the relationships that might exist between lightning frequency f (which is now being routinely measured from a satellite, using the NASA/MSFC Optical Transient Detector (OTD)) and a variety of cloud physical parameters. Specifically, superior or more comprehensive representations were utilised of: (1) glaciation via the Hallett–Mossop (H–M) process; (2) the updraught structure of the model cloud; (3) the liquid-water-content structure of the model cloud; (4) the role of the reversal temperature T rev in influencing lightning characteristics; (5) the critical breakdown field for lightning initiation; and (6) the electrical characteristics of the ice crystal anvil of the model cloud. Although our extended studies yielded some new insights into the problem, the basic pattern of relationships between f and the other parameters was very close to that reported by Baker et al. (1995). The more elaborate treatment of T rev restricted somewhat the range of conditions under which reverse-polarity lightning could be produced if the cloud glaciated via H–M, but confirmed the earlier conclusion that such lightning would not occur if the glaciation was of the Fletcher type. The computations yielded preliminary support for the hypothesis that satellite measurements of f might be used to determine values of the ice-content of cumulonimbus anvils: a parameter of climatological importance. The successful launch and continuing satisfactory functioning of the OTD [Christian, H.J., Goodman, S., 1992. Global observations of lightning from space, Proc. 9th Int. Conf. on Atmospheric Electricity, St. Petersburg, pp. 316–321; Christian, H.J., Blakesee, R.J., Goodman, S.J., 1992. Lightning imaging sensor (LIS) for the earth observing system. NASA Tech. Memorandum, 4350] make it possible—with a high degree of precision—to measure lightning location, occurrence time and frequency f over extensive areas of the Earth's surface. Measured global distributions of lightning and associated lightning stroke radiance demonstrate that: lightning activity is particularly pronounced over the tropics, much greater over land than over the oceans, and exhibits great seasonal variability; lightning radiance tends to be greater over the oceans, less when lightning activity is high, and greater in the Northern Hemisphere winter than summer.

A parametrization of the ice water content observed in frontal and convective clouds

The properties of the ice phase in a number of cloud types are investigated to improve the ice phase parametrization in atmospheric global-climate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North Atlantic, and continental convective clouds over New Mexico and Montana in the USA are studied. Ice concentrations are seen to be several orders of magnitude higher than those which could be attributed to primary nucleation of ice nuclei at cloud-top temperatures. Thus secondary ice multiplication processes must be operating in each cloud type. Evidence suggests that the process of ice splinter production during riming, the Hallett–Mossop process which operates at temperatures around −6°C, is the dominant mechanism operating. The data analysed are parametrized as phase ratios, the fraction of cloud condensed water found in the liquid phase, and the variation of this phase ratio with temperature is examined. The greatest differences are observed between frontal and convective clouds, although smaller differences between continental and maritime clouds of the same type are also seen. In general, frontal clouds possess very high fractions of ice across a wide range of temperature. In contrast, convective clouds exhibit a wide range of phase ratio across the whole temperature range observed. These differences are attributed to the greater vertical wind velocities present in convective clouds. These parametrizations have been used in the UK Meteorological Office Global Climate Model. They are valid for clouds which span the Hallett-Mossop splinter-production temperature range.

A parametrization of the ice water content observed in frontal and convective clouds

AbstractThe properties of the ice phase in a number of cloud types are investigated to improve the ice phase parametrization in atmospheric global‐climate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North Atlantic, and continental convective clouds over New Mexico and Montana in the USA are studied.Ice concentrations are seen to be several orders of magnitude higher than those which could be attributed to primary nucleation of ice nuclei at cloud‐top temperatures. Thus secondary ice multiplication processes must be operating in each cloud type. Evidence suggests that the process of ice splinter production during riming, the Hallett–Mossop process which operates at temperatures around −6°C, is the dominant mechanism operating.The data analysed are parametrized as phase ratios, the fraction of cloud condensed water found in the liquid phase, and the variation of this phase ratio with temperature is examined. The greatest differences are observed between frontal and convective clouds, although smaller differences between continental and maritime clouds of the same type are also seen. In general, frontal clouds possess very high fractions of ice across a wide range of temperature. In contrast, convective clouds exhibit a wide range of phase ratio across the whole temperature range observed. These differences are attributed to the greater vertical wind velocities present in convective clouds.These parametrizations have been used in the UK Meteorological Office Global Climate Model. They are valid for clouds which span the Hallett‐Mossop splinter‐production temperature range.

Possible High Ice Particle Production during Graupel–Graupel Collisions

Abstract In a cold room collisions of two ice spheres having different growth modes were studied to examine ice production. The production of ice particles through this process was high, with a maximum rate more than 800 per collision at −16°C. This ice particle production mechanism is different from the Hallett–Mossop ice multiplication process and may help to explain the high concentrations of ice crystals observed in maritime winter cumuli.