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Casey D. Burleyson and Sandra E. Yuter

Abstract

The spatial patterns of subtropical marine stratocumulus cloud fraction variability on diurnal time scales are examined using high-temporal-resolution cloud masks that are based on 30-min, 4 km × 4 km geosynchronous infrared data for 2003–10. This dataset permits comparison of the characteristics of variability in low cloud fraction among the three subtropical marine stratocumulus regions in the northeastern (NE) Pacific, southeastern (SE) Pacific, and SE Atlantic Oceans. In all three regions, the largest diurnal cycles and earliest time of cloud breakup occur on the edges of the cloud field where cloud fractions are generally lower. The rate at which the cloud breaks up during the day is tied to the starting cloud fraction at dawn, which determines the amount of longwave cooling that is initially available to offset shortwave radiative fluxes during the day. The maximum rate of cloud breakup occurs near 1200 LT. Cloud fraction begins to increase by 1600 LT (before the sun sets) and reaches its maximum value just before dawn. The diurnal-cycle characteristics of the SE Pacific and SE Atlantic marine stratocumulus cloud decks are more similar to each other than to those in the NE Pacific. The NE Pacific cloud deck has a smaller-amplitude diurnal cycle, slower rates of cloud breakup during the day for a given cloud fraction at dawn, and a higher probability of cloud breakup overnight.

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Casey D. Burleyson and Sandra E. Yuter

Abstract

This paper presents an analysis of subtropical marine stratocumulus cloud fraction variability using a 30-min and 3° × 3° cloud fraction dataset from 2003 to 2010. Each of the three subtropical marine stratocumulus regions has distinct diurnal characteristics, but the southeast (SE) Pacific and SE Atlantic are more similar to each other than to the northeast (NE) Pacific. The amplitude and season-to-season diurnal cycle variations are larger in the Southern Hemisphere regions than in the NE Pacific. Net overnight changes in cloud fraction on 3° × 3° scales are either positive or neutral >77% of the time in the NE Pacific and >88% of the time in the SE Pacific and SE Atlantic. Cloud fraction often increases to 100% by dawn when cloud fraction at dusk is >30%. In the SE Pacific and SE Atlantic, a typical decrease in cloud area (median ≤ −5.7 × 105 km2) during the day is equivalent to 25% or more of the annual-mean cloud deck area. Time series for 3° × 3° areas where cloud fraction was ≥90% sometime overnight and <60% at dawn, such as would result from nocturnal formation of pockets of open cells (POCs), only occur 1.5%, 1.6%, and 3.3% of the time in the SE Pacific, SE Atlantic, and NE Pacific, respectively. Comparison of cloud fraction changes to ship-based radar and satellite-derived precipitation intensity and area measurements shows a lack of sensitivity of cloud fraction to drizzle on time scales of 1–3 h and spatial scales of 100–300 km.

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Sandra E. Yuter and Robert A. Houze Jr.

Abstract

Raindrop images obtained on research flights of the NCAR Electra aircraft in the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) are analyzed. The drop size distributions, based on the images obtained in 6-s samples (about 750 m of flight track), are used to calculate both radar reflectivity Z and rain rate R. Airborne radar data from the NOAA P-3 aircraft flying in coordination with the Electra are used to categorize the particle-image data according to whether the drop images were obtained in regions of convective or stratiform precipitation.

Within stratiform precipitation, the same rain rate could be produced by a drop spectrum dominated by numerous small drops (lower reflectivity) or by a few large drops (higher reflectivity). The reflectivity values varied by as much as 9 dB for a given rain rate. Reflectivity data from the airborne radar and flight-level data reveal that the stratiform regions often contain fallstreaks of about 0.1–2 km in horizontal dimension. The fallstreaks are associated with large-drop spectra and local maxima in reflectivity up to approximately 40 dBZ and in rain rates up to 25 mm h−1. The fallstreaks extend downward from the melting band and bend with the low-level wind shear, but do not usually reach the surface. Thus, although relatively more uniform than convective regions, stratiform regions can be variable in reflectivity and rain rate at fine spatial scales in both the horizontal and vertical directions. Stratiform regions are therefore best characterized by their ensemble properties rather than the values of individual high-resolution measurements.

The variability of stratiform drop size spectra arises primarily from the occurrence of fallstreaks and the discontinuous nature of regions favoring aggregation of snow crystals, and it implies that ZR distributions associated with convective and stratiform precipitation are not statistically distinct. Thus, separate ZR relations for convective and stratiform precipitation are not justified, and techniques to distinguish between convective and stratiform precipitation based solely on the characteristics of drop size distributions are not likely to be accurate.

The variability of the drop size spectra in tropical precipitation makes an exponential fit to the ZR relation sensitive to the spatial scale over which Z and R are determined. This sensitivity can be avoided by using a probability-matched ZR relation. The probability-matched ZR relation for all the raindrop image data from the Electra collected between altitudes of 2.7 and 3.3 km in TOGA COARE is similar to the ZR relation obtained at the sea surface in the Global Atmospheric Research Program Atlantic Tropical Experiment.

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Sandra E. Yuter and Robert A. Houze Jr.

Abstract

A statistical technique is employed to examine the evolving properties of the ensemble small-scale variability of high-resolution radar data collected in a multicellular Florida thunderstorm. This paper examines vertical mass transport and mass divergence and synthesizes these observations with results from the first two parts of the study into a self-consistent conceptual model that describes the convective-to-stratiform transition of the storm.

Vertical mass transport distributions indicate that the more numerous weak and moderate-strength upward and downward velocities, not the few strongest, accomplished most of the vertical mass transport in the storm. Hence, most of the mass of precipitation is condensed outside the areas of intense upward motion. These data thus suggest a change in the way we think about convection. Although the few regions of strongest vertical motion play a part in the overall storm evolution by dispersing particles throughout the depth of the storm, it is the more prevalent weak and moderate-strength upward velocities that are the more important determinants of the precipitation processes.

An extension of bubble-based conceptual models of convection is proposed to account for the convective-to-stratiform transition. Bubbles of positively buoyant air produced at low levels are weakened by varying amounts of entrainment and slowed down by pressure gradient forces as they rise. Thus many bubbles are slowed and stopped at mid- and upper levels. The weakened parcels flatten, encompass more area and, in the process, laterally spread their associated hydrometeors. As the weak updraft parcels congregate at mid- and upper levels of the storm, they create the region of weak mean ascent that is characteristic of stratiform mean vertical velocity profiles. Below the 0°C level, precipitation-associated downdrafts dominate the ensemble of smaller-scale drafts and create mean weak descent at low levels.

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Jeffrey G. Cunningham and Sandra E. Yuter

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The instability characteristics associated with different radar-derived mesoscale organization modes are examined using six cool seasons of operational scanning radar data near Portland, Oregon, and operational sounding data from Salem, Oregon. Additionally, several years of Microwave Rain Radar Ka-band vertically pointing radar data from Portland and Merwin, Washington, are used to characterize the nature and occurrence of generating cells and fall streaks. The combination of a new metric, convective-stratiform intermittency, with the classification of radar reflectivity maps into convective and stratiform precipitation types was applied to periods when the freezing level was >1.4-km altitude. This method distinguishes periods with embedded convective within stratiform mesoscale organization from those that were mostly convective or mostly stratiform. Mesoscale organization occurs in a continuum of states with predominantly stratiform structure occurring most frequently. Generating cells in the snow layer are common in cool-season storms and are primarily associated with potential instability aloft. For mostly stratiform and embedded convective within stratiform 3-h periods, the vertically pointing radar data showed nearly ubiquitous fall streaks in the snow layer originating above 3-km altitude. Stronger generating cells enhanced reflectivity in the rain layer consistent with a seeder mechanism. Stronger generating cells were more common during embedded convection within stratiform than in mostly stratiform periods. Nearly all embedded periods have active or latent (potential) instability. Hydrostatic instability more typically occurred at higher altitudes for embedded convective within stratiform periods compared to mostly convective periods. The occurrence of vertical wind shear instability was primary below 2-km altitude and was not typically associated with levels with generating cells.

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Sandra E. Yuter and Wendy S. Parker

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Fifteen rain measurement instruments were deployed on the National Oceanic and Atmospheric Administration Ship Ronald H. Brown during the 1997 Pan American Climate Studies (PACS) Tropical Eastern Pacific Process Study (TEPPS). To examine differences in rainfall catchment related to instrument design, three types of disdrometers, an optical rain gauge, a ship rain gauge, and a siphon gauge were clustered in one location to ensure similar exposure. To address exposure effects, eight siphon rain gauges were deployed on different sides of the ship and on several different levels.

Cross-ship differences in hourly rainfall accumulation were negligible when relative wind speeds were less than 3 m s−1 and became significant at greater than 5 m s−1, especially when the relative wind direction was 20° or greater from the bow. Instruments with both horizontal and vertical catchment surfaces yielded a measurable collection advantage over instruments with only horizontal catchment surfaces.

Analysis of data collected during TEPPS using a multiple-instrument, multiple-location approach yields the following recommendations for reducing uncertainty in rain measurement at sea. The first two of the four recommendations apply to rain measurements on buoys as well as on ships. 1) Deploy experimental rain measurement instrumentation paired with a baseline minimum siphon gauge or other trusted instrument. Comparison of the rain-rate time series between the baseline gauge measurements and the experimental instrument data permits detection of erratic behavior and bias. 2) Apply an appropriate wind correction. To do this step properly, both a wind correction formula derived for the specific gauge type and a nearby measurement of relative wind are needed. These features are already incorporated into the ship rain gauge. 3) Locate gauges where distortion of the airflow by the ship is locally minimized and relative wind speeds are as low as possible. This analysis confirms previous recommendations for placement of rain instrumentation at lower locations as long as the location is protected against direct spray from the sea without being shadowed by higher objects. 4) Place instrumentation on both sides of ship and along centerline. Airflow distortion by the ship itself can induce significant differences between port and starboard accumulations at high wind speeds and high angle of wind attack to the bow. Multiple locations aid in constraining error, because relative wind direction and speed vary during a cruise and there is no one perfect location on ship for rain instrumentation.

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Sandra E. Yuter and Robert A. Houze Jr.

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This paper is the first in a three-part study that examines the kinematic and microphysical evolution of Florida cumulonimbus and focuses on the convective-to-stratiform transition of the storm. This first paper lays the groundwork for the subsequent papers by defining the problem under study, delineating the setting for the storm, and describing the spatial distribution of updrafts, downdrafts, and precipitation.

High-resolution radar data of a typical line of storms associated with the Florida sea breeze is the centerpiece of this study. The high-resolution data reveal details of the internal structure of the squall line that were beyond the resolution of previous squall-line studies. Radar reflectivity filled in between cells at upper levels as the storm evolved. Reflectivity values were only weakly associated with updraft and downdraft magnitude. The updrafts and downdrafts in the storm tended to be irregular in their three-dimensional shape and less than 5 km in horizontal extent. At any given time, updrafts and downdrafts at a variety of strengths were present at all levels throughout the storm. The stronger drafts were usually closer to the leading edge of the storm. Upper-level downdrafts were often located alongside upper-level updrafts. Updrafts tended to drift upward from lower levels and weaken as they aged.

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Sandra E. Yuter and Robert A. Houze Jr.

Abstract

High-resolution radar data collected in Florida during the Convection and Precipitation/Electrification Experiment are used to elucidate the microphysical and kinematic processes occurring during the transition of a multicellular storm from convective to stratiform stages. A statistical technique is employed to examine the evolving properties of the ensemble small-scale variability of radar reflectivity, vertical velocity, and differential reflectivity over the entire storm.

Differential radar reflectivity data indicate that the precipitation at upper levels was nearly glaciated early in the storm's lifetime. Dual-Doppler radar data show that throughout the storm's lifetime both updrafts and down-drafts were present at all altitudes and that most of the volume of the radar echo contained vertical velocities incapable of supporting precipitation-size particles. Thus, the ensemble microphysical properties of the storm were increasingly dominated by particles falling in an environment of weak vertical velocity, and the radar reflectivity began to take on a statistically stratiform character during the early stages of the storm. This stratiform structure became more distinct as the storm aged.

Two dynamically distinct downdrafts were indicated. Lower-level downdrafts were associated with precipitation. Upper-level downdrafts were dynamically associated with the stronger upper-level updrafts and were likely primarily a consequence of the pressure gradient forces required to maintain mass continuity in the presence of buoyant updrafts.

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Brian A. Colle and Sandra E. Yuter

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The modification of precipitation by the coastal land areas of Long Island (LI), New York, and southern Connecticut (CT) is examined for an extratropical cyclone over the northeast United States on 1 December 2004, which produced strong southerly flow (15–30 m s−1) below 900 mb and heavy precipitation over LI. The differential surface roughness at the coast and the hills of LI (30–80 m) and southern CT (100–250 m) enhanced the surface precipitation by 30%–50% over these regions compared with the nearby water region of LI Sound. The three-dimensional precipitation structures are shown using composite Weather Surveillance Radar-1988 Doppler radar data interpolated to a Cartesian grid, which is compared with a 4-km simulation using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). As the low-level stratification and flow increased at low levels, the MM5 produced a terrain-forced gravity wave over LI and CT upward through 6 km MSL. Precipitation enhancement (2–3 dBZ) occurred from the surface upward to around the freezing level (3 km MSL) across central LI and southern CT, while there was a localized precipitation minimum over LI Sound. A factor separation on a few sensitivity MM5 runs was performed to isolate the impact of small hills and differential friction across the LI coastline. Both the hills and frictional effects have similar contributions to the total precipitation enhancement and the vertical circulations below 3 km. The hills of LI enhanced the gravity wave circulations slightly more than the differential friction above 3 km, while there was little flow and precipitation interaction between the hills and differential friction. A sensitivity simulation without an ice/snow cloud above 3 km MSL revealed that the seeder-feeder process enhanced surface precipitation by about a factor of 4.

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Sandra E. Yuter and Robert A. Houze Jr.

The Pan American Climate Studies Tropical Eastern Pacific Process Study (TEPPS) obtained a comprehensive set of observations of the structure of clouds and precipitating storms over the eastern tropical Pacific from 28 July to 6 September 1997. The TEPPS data can address a wide range of problems involving tropical oceanic clouds and precipitation. The main goal of the project was to understand why passive microwave satellite algorithms indicate an E–W gradient in the precipitation pattern in the tropical Pacific with heavier rainfall in the east while infrared satellite algorithms indicate heavier rainfall in the west. Satellite-derived precipitation estimates are based on characteristics of the vertical structure of precipitating clouds: in the case of infrared methods, cloud-top temperature, and in the case of microwave methods, the vertically integrated ice scattering and/or water absorption determined by the vertical profile of hydrometeors. The premise of the expedition was that by obtaining surface-based radar measurements of the vertical structure of precipitation where and when the differences between the infrared and microwave precipitation estimates were large, it could be determined which satellite method yielded a more accurate pattern of precipitation in the Pacific. This paper describes the types of observations obtained during TEPPS and some preliminary results.

A single, well-equipped vessel on its maiden voyage, the National Oceanic and Atmospheric Administration ship Ronald H. Brown, was the base for all observations. Scanning C-band Doppler radar and cloud photography documented the three-dimensional structure of clouds and precipitation in the vicinity of the ship. Upper-air soundings were obtained at ≤ 4 h intervals. Surface meteorological and oceanographic instruments and vertically pointing 915-MHz and S-band profilers characterized conditions at the ship itself. During 28.5 days in the eastern Pacific ITCZ, the shipborne radar observed echoes larger than 50 km in maximum horizontal dimension within 100-km radius of the ship 71% of the time and larger than 100 km 55% of the time. The ship spent 16 days on station at 7.8°N, 125°W and 4 days in the vicinity of Hurricane Guillermo.

Samples of surface atmospheric and oceanic data collected during the cruise illustrate the difficulty of interpreting short timescale buoy data time series in the absence of the mesoscale context provided by radar data. The ship sounding data show that the larger-scale, longer-lived convective precipitation activity and organization on timescales of days in the eastern Pacific ITCZ is closely associated with the presence of stronger southerly winds, which in turn suggests that large-scale atmospheric processes such as easterly waves or inertial stability oscillations are a regulating mechanism.

Comparison of the ship radar data, satellite IR data, and satellite microwave data shows that part of the reason why the IR and microwave-derived precipitation maps differ is that in the eastern Pacific ITCZ IR cold cloudiness resolves only a subset of the precipitation detected by microwave data. Large precipitating systems (> 100 km scale) of long duration (> 24 h; i.e., the mesoscale organized systems) were reliably associated with cold cloudiness < 235 K. Precipitating systems of shorter duration and/or smaller scale (i.e., the less-organized convection) sometimes reached 235 K and sometimes did not. Satellite microwave data generally agreed with the radar data regarding the location and areal coverage of precipitating regions larger than ~10 km in horizontal scale. However, the microwave algorithm examined in this study had varying degrees of skill in locating the heavier rainfall areas within rainy regions.

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