Tropical convective clouds play an important part in the Earth's climate system. Convection and release of latent heat transports energy from the surface into the upper atmosphere. Clouds have a higher albedo than the underlying ocean, which causes more incoming solar radiation to be reflected back to space. Since the tops of tropical systems are much cooler than the surface of the Earth, the presence of high convective clouds cools the climate system.
The most recognizable cloud system in the tropics is the hurricane. In addition to the important climatic effects of tropical weather systems, hurricanes possess enough energy to cause massive death and destruction. Therefore, their accurate prediction is of utmost importance.
Cloud microphysics describe the structure and properties of clouds on the microscopic scale.
Background The Tropical Rainfall Measuring Mission (TRMM) was launched in 1997 to provide quantitative estimates of rainfall over the entire tropics. The satellite uses remote sensing techniques to convert the radiance recorded at the sensor to rainfall values. The most important variable used to constrain the measurements is the properties of the hydrometeors. Hurricanes are mixed-phase clouds, meaning that liquid and solid water (ice) are both present in the cloud. Typically, liquid water dominates at altitudes lower than the freezing level and solid water at altitudes where the temperature is colder than -40 °C. Between 0 °C and -40 °C water can exists in both phases simultaneously. In addition to the phase, the solid water hydrometeors can have different shapes and types that need to be accounted for in the radiative transfer calculations.
In Autumn 1999, the TRMM-Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) field experiment sampled continental and oceanic tropical clouds in Brazil. The goal of TRMM-LBA was to validate the rainfall in cloud resolving models. There have been several in-situ observations of cloud microphysics in tropical clouds which will be discussed here.
Cloud microphysics are the physical processes that describe the growth, decay, and fallout of precipitation particles. In terms of models, cloud microphysics occur on a scale smaller than the grid-scale of the model and have to be parameterized.Fovell, R.G., K.L. Corbosiero, and H.C. Kuo, 2009: Cloud Microphysics Impact on Hurricane Track as Revealed in Idealized Experiments. J. Atmos. Sci., 66, 1764–1778.
Hurricane track forecasts have been getting better in recent years. Looking at the example of Hurricane Rita, the forecast of the National Hurricane Center 36 hours before landfall shifted more than 130 kilometers from the previous forecast, causing an unneeded evacuation. There has been research that has shown that the choice of subgrid-scale parameterization schemes can influence hurricane intensity, track, speed, and precipitation rates. Microphysical assumptions may directly or indirectly modulated storm structure, which result in small changes in the hurricane track which can have societal consequences.
Hydrometeor formation and shapes The shape of liquid water drops is generally spherical because of the effects of surface tension. Depending on the size of the drop the friction of the air flowing past a falling drop may squish the bottom on the drop so that it is slightly non-spherical. However, solid ice does not generally form into nice spherical shapes. Ice crystals have a preference to form hexagonal structures by deposition, but can form odd shapes in the presence riming or aggregation into graupel.
The shape of ice particles is mostly dependent on the temperature and supersaturation where they form. The supersaturation is mostly dependent upon the speed in the updraft regions. In regions of high updraft, there are more hydrometeors formed. Graupel is found mostly in regions of weak updrafts. Particle size tends to decrease with increasing altitude because at lower altitudes the larger particles collide and aggregate with the smaller particles. Because updrafts are important for cloud microphysics, it is also necessary to consider how convection parameterization schemes may influence microphysics.
Small errors in the parameterization of the particle size distribution can have large impacts on the calculation of the terminal velocity. The composition, size, and number concentration of particles varies dramatically in stratiform and convective regions. The particle fall speed derived from observations of tropical cyclones varies significantly from those derived from midlatitude systems.McFarquhar, G.M., and R.A. Black, 2004: Observations of Particle Size and Phase in Tropical Cyclones: Implications for Mesoscale Modeling of Microphysical Processes. J. Atmos. Sci., 61, 422–439.
There have been many studies of the feasibility of modifying hurricanes so that they would not be as destructive. Rosenfeld et al. (2007) studied possible modification of Hurricane Katrina (2005). They simulated the seeding of the hurricane by suppressing all warm rain formation in the outer regions of the hurricane. They report that in their simulation the seeded hurricane initially weakened the surface winds in the region of seeding. The eye of the hurricane eventually contracted and became stronger, but the average of the total wind field was weaker. In this best case scenario, they report that seeding reduced the hurricane-force winds by 25%.Rosenfeld, D., A. Khain, B. Lynn, and W.L. Woodley, 2007: Simulation of hurricane response to suppression of warm rain by sub-micron aerosols. Atmos. Chem. Phys., 7, 3411-3424.
Rango and Hobbs (2005) obtained in situ measurements of tropical convective systems. They found that the liquid water content was below adiabatic values. This was true even in newly formed updrafts, suggesting that collision-coalescence and/or entrainment mixing are efficient methods for the removal of liquid water. They noted that the effective radius started to decrease at altitudes above 2–4 km above cloud base, which they attribute to the warm rain process. Ice processes became important at temperatures between -4 °C and -10 °C, and they photographed different shapes including needles, frozen drops, and sheaths. In growing clouds, it was noted that the particle size often grew by riming.Rangno, A.L., and P.V. Hobbs, 2005: Microstructures and precipitation development in cumulus and small cumulonimbus clouds over the warm pool of the tropical Pacific Ocean. Q. J. R. Meteorol. Soc., 131, 639-673.
Tokay et al. (2008) studied the raindrop size distribution in topical cyclones. They found high concentrations of small and middle sized drops regardless if larger drops were present. The total number of droplets was between 600 and 1000 m−3, the liquid water content was around 1 g m−3, and a rain rate of approximately 20 mm per hour. The droplets had a mean mass diameter of ~1.6 mm and the maximum diameter recorded was 4.11 mm. There results indicate that prior rain rate estimates from tropical cyclones may have been underestimated due to the differences in microphysics between midlatitude and tropical storms.Tokay, A., P.G. Bashor, E. Habib, and T. Kasparis, 2008: Raindrop Size Distribution Measurements in Tropical Cyclones. Mon. Wea. Rev., 136, 1669–1685.
In-situ measurements of the microphysics of tropical clouds in the Amazon show that in regions of stronger updrafts contained smaller supercooled water droplets or ice particles than weaker updrafts. In stratiform anvil regions, aggregation into graupel was the main growth mechanism. The speed of the updraft determines if warm rain processes, riming, or aggregation are the primary mechanism of growth in updraft regions.Stith, J.L., J.E. Dye, A. Bansemer, A.J. Heymsfield, C.A. Grainger, W.A. Petersen, and R. Cifelli, 2002: Microphysical Observations of Tropical Clouds. J. Appl. Meteorol., 41, 97–117.
Heymsfield et al. (2002) also looked at the microphysics of tropical convection, but they limited themselves to the stratiform regions. They observed ice particles of many shapes and sizes. In particular, they noted that rimed particles were found near convective regions, small spheres were found in regions of "transient convection", and at low temperatures cirrus crystals formed. They constructed particle size distributions and noted that they fit particularly well to Gamma distributions and slightly less well to exponential distributions. They noted that their results were similar to results derived from midlatitude systems.Heymsfield, A.J., A. Bansemer, P.R. Field, S.L. Durden, J.L. Stith, J.E. Dye, W. Hall, and C.A. Grainger, 2002: Observations and Parameterizations of Particle Size Distributions in Deep Tropical Cirrus and Stratiform Precipitating Clouds: Results from in Situ Observations in TRMM Field Campaigns. J. Atmos. Sci., 59, 3457–3491.
Parameterization There are several different cloud microphysics parameterization schemes. Depending on the sophistication of the scheme, the number of ice-phase categories can vary. Many schemes use at least three categories: cloud ice, graupel, and snow. The classification of ice into categories is necessary because different forms of ice will fall at different velocities.
Typically, microphysics schemes will use a mass-weighted average for the fall velocity. McFarquhar and Black (2004) showed that different parameterizing methods results in dramatically different terminal velocities of the hydrometeors.
Intensity The presence of cloud condensation nuclei (CCN) influences the number of cloud drops that form in a cloud; the more CCN there are, the more cloud droplets that will form. Changes in the CCN concentration and their associated changes in the cloud drop distribution can redistribute the energy within a hurricane. This was known in the 1960s which led scientists to think that hurricanes could be modified by the addition of CCN to produce less intense hurricanes. It was proposed that by seeding with silver iodide outside the eyewall would freeze the supercooled water, release more latent heat, and increase convection.Willoughby, H., D. Jorgensen, R. Black, and S. Rosenthal, 1985, Project STORMFURY: A Scientific Chronicle 1962–1983. Bull. Amer. Meteor. Soc., 66, 505–514. This method was ultimately unsuccessful because of the lack of supercooled water in the tropical cyclone. A different approach seeds the clouds with a large number of small hygroscopic aerosols. The large number of CCN leads to smaller raindrops, less collision-coalescence, and thus less rainout. This water is convected above the freezing level, leading to warming in the upper atmosphere and greater convection.
There have been several modeling studies on the effects of increased CCN on hurricane intensity. Rosenfeld et al.. (2007) used the Weather Research Model (WRF) to simulate Hurricane Katrina, and then turned off the warm rain processes to approximate the effects of adding a large number of CCN. They report that they were successful in simulating key features of Katrina in the control run including the minimum central pressure and maximum wind speeds. When the warm rain processes were removed, the cloud water content naturally increased and the rain water content decreased. The warm rain suppression also changes the thermodynamic structure of the hurricane: temperatures are decreased at low levels at the outer edge of the hurricane. Later, the peak winds decreased along with the central pressure.