Sea breezes are frequently observed in the South Carolina/Georgia region of the Southeastern United States (SEUS) and can reach upwards of 150km inland. This region is unique among the places frequently affected by sea breeze due to it being a continental location with relatively flat topography. The thermal gradient between land and water environments is a factor in introducing the sea breeze, but its role in the inland extent of sea breeze propagation isn’t as well known. We investigate the role of the thermal gradient in previously catalogued sea breeze events observed at the Savannah River Site (SRS) by taking differences of temperature measurements at inland and coastal weather stations for the days that the events occurred. We saw that the temperature differences for those days were much higher than in the non-sea breeze days during the mornings and afternoon. Numerical models were also used to conduct a sensitivity study on a sea breeze case, using simple modifications of the temperature gradient. We found that while the modifications did not stop the generation of a sea breeze circulation, the extent of the inland propagation was dependent on the magnitude of the thermal gradient.
Daily weather types (WTs) over the Southeast United States have been analyzed using 850 hPa winds from reanalysis data from March to October of 1979–2019. Six WTs were obtained. WTs 1–3 represent mid-latitude synoptic systems propagating eastward. WT4 is a summer-type pattern predominantly occurring in June–August, with the center of the North Atlantic Subtropical High (NASH) along the Gulf coast in the southern United States. WT5 is most frequent from August to middle October, with the NASH pushed further north and southerly winds over the northern Great Plains. An anticyclone centered at the Carolina coast characterizes WT6, which occurs in all months but is slightly more frequent in the spring and fall, especially in October, corresponding to fair weather in the region. WTs 1, 2 and 3 can persist for only a few days. WTs 4, 5 and 6 can have long spells of persistence. Besides self-persistence, the most observed progression loop is WT1 to WT2, to WT3, and then back to WT1, corresponding to eastward-propagating waves. WTs 4 and 5 are likely to show persistence, with long periods of consecutive days. WT6 usually persists but can also transfer to WT3, i.e., a change from fair weather in the Southeast U.S. to rainy weather in the Mississippi River Valley. A diurnal cycle of precipitation is apparent for each WT, especially over coastal plains. The nocturnal precipitation in central U.S. is associated with WT3. WTs 1–3 are more frequent in El Niño years, corresponding to stronger westerly wave activities and above normal rainfall in the Southeast U.S. in the spring. The positive rainfall anomaly in the Mississippi and Ohio River valley in El Niño years is also associated with more frequent WT3.
The goal of this research is to describe the inland penetrating sea breeze and interactions with atmospheric phenomena across multiple scales, thereby establishing a baseline of understanding. The land-ocean-atmosphere interface in the southeastern US drives regional sea breeze circulations that impact clouds, convection, and precipitation. We analyzed a variety of atmospheric observations to identify the characteristics and frequency of sea breezes. Sea breezes were identified all along the southeastern Atlantic coast (63% of days from March to September in 2019) at times impacting large areas of the southeastern US. They were found to penetrate inland to the Savannah River Site on about 27% of all days during the typical sea breeze season (March to October) from 2015 to 2020, often leaving a residual layer the following day. Future work focuses on modeling studies, analysis of additional data, and climate impacts on sea breezes.
Abstract The shapes of high‐resolution cloud condensation nucleus (CCN) spectra are compared with cloud and precipitation characteristics observed in the Ice in Clouds Experiment–Tropical. These high‐resolution spectra often revealed bimodality that is usually caused by in‐cloud processing. Bimodal CCN spectra were associated with clouds that had a third of the droplet concentrations, twice as broad droplet spectra and 2 to 3 orders of magnitude more drizzle than clouds associated with unimodal CCN spectra. These findings are opposite of a similar earlier study of stratus clouds that suggested enhancement of the indirect aerosol effect. The observations were consistent over the full range of cloud droplet liquid water content (LWC c ) thresholds and bands. But the largest differences between clouds associated with bimodal and unimodal CCN were at intermediate LWC c bins where drizzle and droplet spectral width was greatest in the clouds associated with bimodal CCN.
Abstract High resolution extended‐range cloud condensation nuclei (CCN) spectral comparisons with cloud microphysics and drizzle of the Physics of Stratocumulus Tops (POST) field experiment confirmed results in the Marine Stratus/Stratocumulus Experiment (MASE). Both of these stratus cloud projects demonstrated that bimodal CCN spectra typically caused by cloud processing were associated with clouds that exhibited higher concentrations of smaller droplets with narrower distributions and less drizzle than clouds associated with unimodal CCN spectra. Resulting brighter clouds and increased cloudiness could enhance both indirect aerosol effects (IAE). These stratus findings are opposite of analogous measurements in two cumulus cloud projects, which showed bimodal CCN associated with fewer larger droplets more broadly distributed and with more drizzle than clouds associated with unimodal CCN. Resulting reduced cumulus brightness and cloudiness could reduce both IAE. Physics of Stratocumulus Tops (POST) flights in air masses with higher CCN concentrations, N CCN , showed more extremes of the stratus characteristics. However, POST flights with lower N CCN showed opposite droplet characteristics similar to the cumulus clouds, yet still showed less drizzle in clouds associated with bimodal CCN, but not as much less as the flights with higher N CCN . Since all MASE clouds were in polluted air masses, while the two cumulus projects were in clean air masses we deduce from these four projects that both the dynamic stratus/cumulus differences (vertical wind) and N CCN are responsible for the microphysics and drizzle differences among these projects. This is because the clean POST characteristics are a hybrid between MASE/POST high N CCN and the two cumulus projects.
Abstract Rain in Cumulus over the Ocean (RICO) warm cloud microphysics and drizzle comparisons with cloud condensation nuclei (CCN) spectra displayed similarities with earlier analyses of low‐altitude Ice in Clouds Experiment‐Tropical (ICE‐T) warm maritime cumuli. These comparisons of high‐resolution CCN spectra measured at 100 m altitude with cloud and drizzle measurements were consistent within three altitude bands between 600 and 3,700 m. For both projects, clouds associated with bimodal CCN (accumulation mode dominant) displayed more drizzle than clouds associated with unimodal CCN (Aitken mode dominant). Higher concentrations in clouds associated with bimodal CCN than clouds associated with unimodal CCN extended throughout the ranges of 3 drizzle drop probes (260X, 2DC, and 2DP) between 60 and 5,200 μm diameter. Ratios of drizzle drop concentrations in clouds associated with bimodal CCN to those in clouds associated with unimodal CCN were as much as an order of magnitude. Confirmation of a relationship between CCN modality and cloud droplet spectra found in ICE‐T was often obscured in RICO by drizzle effects on droplet spectra. There was also an association between cloudiness and CCN bimodality that is consistent with clouds as a source of accumulation mode particles. These consistent results now found in two warm maritime small low‐altitude cumulus cloud experiments show that CCN bimodality could inhibit the indirect aerosol effect (IAE), especially second IAE (cloud lifetime) in warm maritime cumuli. These RICO and ICE‐T observations in maritime cumuli are opposite of the results in the Marine Stratus/Stratocumulus Experiment, which indicated that CCN bimodality could enhance both IAE.
Sea breeze winds are observed at various locations worldwide, but the spatially continuous mapping of sea breeze winds is rare. We have developed a scheme to map the areas of the southeastern United States (US) coast influenced by sea breeze winds using a range of surface re-analysis data to identify their occurrence. Changes in wind direction and dew point temperature are both used to detect a potential sea breeze signature, which is then confirmed by cumuliform clouds seen in satellite images or coastal fronts shown as cohesive lines in radar reflectivity images. Filters are employed to remove onshore winds not induced by the temperature difference between land and sea. From March to September 2019, this scheme identified 134 days with sea breeze occurrence somewhere in the southeastern US, a frequency of 63 percent. The number of sea breezes increased from March to July and then decreased to September. Deep inland propagation of sea breezes during this period left footprints in a band parallel to the coastline up to about 220 km inland, after which the sea breeze winds quickly diminished. Comparisons show that the findings using the scheme are consistent with site observations, theoretical estimates, and idealized and semi-idealized numerical model simulations.
Abstract Comparisons of high‐resolution extended range CCN spectra measured at 100 m altitude with cloud and drizzle microphysics in the Rain in Cumulus over the Ocean (RICO) aircraft field project are presented. CCN concentrations, N CCN , active at supersaturations, S , >0.1% showed positive relationships with cloud droplet concentrations, N c , measured at intermediate (606–976 m) and very high altitudes (1,763–3,699 m). These correlation coefficients, R , progressively increased with S while the two‐tailed probabilities, P2, progressively decreased with S to < 10 −6 at 1.6%S. More important were the positive relationships between N CCN active at S < 0.1% and drizzle drop concentrations, N d , at high (977–1,662 m), very high and high‐very high altitudes combined (977–3,699 m). All of these relationships were consistent for eight different cloud liquid water content, L c , thresholds (for N c ) and L c bins (for N d ) ranging from 0.0002 to 0.3 g/m 3 . Negative relationships between CCN modality and low altitude (76–475 m) cloudiness coupled with no relationship of N CCN active at any S with N c of these low clouds indicated a cloud effect on ambient aerosol. This is a demonstration of clouds causing bimodal aerosol.
Abstract Cloud microphysics and cloud condensation nuclei (CCN) measurements from two marine stratus cloud projects are presented and analyzed. Results show that the increase of cloud droplet concentrations Nc with CCN concentrations NCCN rolls off for NCCN at 1% supersaturation (S)N1% above 400 cm−3. Moreover, at such high concentrations Nc was not so well correlated with NCCN but tended to be more closely related to vertical velocity W or variations of W (σw). This changeover from predominate Nc dependence on NCCN to Nc dependence on W or σw is due to the higher slope k of CCN spectra at lower S, which is made more relevant by the lower cloud S that is forced by higher NCCN. Higher k makes greater influence of W or σw variations than NCCN variations on Nc. This changeover at high NCCN thus seems to limit the indirect aerosol effect (IAE). On the other hand, in clean-air stratus cloud S often exceeded 1% and decreased to slightly less than 0.1% in polluted conditions. This means that smaller CCN [those with higher critical S (Sc)], which are generally more numerous than larger CCN (lower Sc), are capable of producing stratus cloud droplets, especially when they are advected into clean marine air masses where they can induce IAE. Positive correlations between turbulence σw and NCCN are attributed to greater differential latent heat exchange of smaller more numerous cloud droplets that evaporate more readily. Such apparent CCN influences on cloud dynamics tend to support trends that oppose conventional IAE, that is, less rather than greater cloudiness in polluted environments.
Abstract The shapes of high‐resolution cloud condensation nucleus (CCN) spectra are compared with cloud and precipitation characteristics observed in the Marine Stratus/Stratocumulus Experiment. These high‐resolution spectra often revealed bimodality that is usually caused by in‐cloud processing. Bimodal CCN spectra were associated with clouds that had as much as double the concentrations of droplets with narrower distributions and as much as an order of magnitude less drizzle than clouds associated with the most extreme unimodal CCN spectra. We introduce objective CCN spectral metrics: (1) CCN concentration differences between the two modes, unprocessed (Aitken, N u ) and processed (accumulation, N p ), and (2) slopes of cumulative CCN spectra. Both of these CCN spectral shape classifications show similar cloud droplet concentration ( N c ) trends to that of an earlier subjective modal rating system. When there are two modes, they are separated either by minimal concentrations or concentration inflections. Enhancement of N c is traced to greater CCN bimodality that causes narrower droplet spectra that suppress drizzle. These effects could augment indirect aerosol effects and can only be revealed by high‐resolution CCN spectral shape comparisons with cloud and drizzle microphysics.
