Convective Bands Research Articles

Meso-scale transport characteristics and budget diagnoses of water vapor in binary typhoons

In this paper, we simulate a process of binary typhoon and the budget diagnoses of water vapor in this process to analyze the transportation characteristics of water vapor and their influences on the variation of typhoon intensity. The results show that the interactions between typhoon Fitow and surrounding systems, including subtropical high, mid-latitude trough, west of the continent high and Southeast trailing typhoon Danas, change the background wind fields of Fitow, and then adjust the transport channels of moisture. Those surrounding systems, especially the trailing typhoon Danas which can be called the collection-transfer station of water vapor, have important effects on the intensity maintenance and the northern strong precipitation in the offshore and landing period of Fitow. The distribution and evolution of water vapor flux convergence band are consistent with those of strong convection band, revealing that the water vapor transport has important influences on the structure and intensity of the inner-core convection band in typhoon. The budget results show that the time series of total water vapor flux and typhoon intensity change synchronously. And the eastern boundary is the main source of water vapor transport, and the southern and northern boundary are also important, while the western boundary makes a negative contribution. The inflow transport channel is mainly located at the bottom of the troposphere, while the outflow transport area of water vapor is located at middle- and low-level troposphere of western boundary. The vertical transportation of water vapor plays an important role in redistributing the internal moisture of typhoon. The duration of sever convection band in typhoon is accompanied by the strong vertical transport of water vapor, which indicates that the vertical transport of water vapor is important for developing the strong convection in U and V type typhoon.

Evolution of Mesoscale Convective System Organizational Structure and Convective Line Propagation

AbstractThis study examines organizational changes and periods of rapid forward propagation in an MCS on 6 July 2015 in South Dakota. The MCS case was the focus of a Plains Elevated Convection at Night (PECAN) IOP. Data from the Sioux Falls WSR-88D and a high-resolution WRF simulation are analyzed to examine two periods of rapid forward propagation (or surges) and organizational changes. During the first surge (surge A), the northern portion of the convective line propagates eastward faster than the southern portion, and the northern portion of the leading line transitions from a single convective core to a multicellular structure as it merges with convection initiation. Radar reflectivity factor Z and graupel concentrations decrease above the melting layer, while at lower altitudes Z increases. The MCS cold pool also intensifies and deepens beneath an expanded region of high rainwater content and subsaturated air. Throughout surge A, a mesoscale circulation with strong rear-to-front near-surface flow and front-to-rear midlevel flow is also evident. By the end of surge A, the leading edge of the MCS cold pool is beneath developing convection initiation ahead of the original convective line while the original convective updraft weakened and moved rearward. This MCS evolution is similar to discrete propagation events discussed in past studies, except with new convection developing along an intersecting convective band. During surge B, the MCS transitions from a multicellular structure to a single, intense updraft. Smaller microphysical and thermodynamic changes are observed within the MCS during surge B compared to surge A, and the mesoscale circulation continues to develop.

A numerical study of the convection triggering and propagation associated with sea breeze circulation over Hainan Island

AbstractBased on high‐resolution numerical simulations, the effects of sea breeze circulation (SBC) on associated convective initiation and propagation over Hainan Island are investigated. The results show that the blocking of synoptic wind on the windward side of the mountains inhibits the occurrence of windward‐side valley breeze and sea breeze. In the context of low mountains, weak synoptic wind not only favors the combination of the SBC with the valley breeze circulation on the leeward side of the mountains but also makes the convergence zone between the synoptic wind and the leeward‐side combined circulation stay near the tops of the mountains, leading to the accumulation of large quantities of water vapor taking place over the mountaintops. The combined effect of enhanced water vapor and high temperatures over the mountaintops can result in the generation of much larger convective available potential energy (CAPE) than the surrounding areas. The coupling of the large CAPE with the considerable converging ascending motion and abundant water vapor near the tops of the mountains promotes the occurrence of a strong convection band (CB) over the mountaintops. During the CB propagation under the steering of synoptic wind, the flow across the CB continuously and strongly opposes the SBC front and the fronts of the Kelvin‐Helmholtz billows behind, resulting in successive and strong lifting motion as well as large quantities of water vapor and high CAPE near these fronts. These lead to an apparent downstream wave‐like propagation of the CB.

Physical and dynamic factors that drove the heavy rainfall event over the middle Korean Peninsula on 26-27 July 2011

This study used an observational analysis and a numerical model to investigate the heavy rainfall event that occurred on 26-27 July 2011 over the middle Korean Peninsula. Radar observations revealed a significant transition in the echo pattern, with strong convective systems along the coasts of southern Hwanghae-do (HHD) and Gyeonggi-do (0600-1200 UTC 26 July, stage 1), a medium-sized SW-NE precipitation band over the middle Korean Peninsula and major convective systems in the west (1500-2200 UTC, stage 2), and a narrow convective band from Gyeonggi Bay to the Seoul metropolitan area (2200 UTC 26-0100 UTC 27 July, stage 3). This study focused on the development of heavy precipitation systems during stages 1 and 2. A mesoscale ridge and trough developed over the middle peninsula at approximately 0600 UTC on 26 July and persisted throughout the first 2 stages. Both the observation and the numerical simulation suggest that a cold pool, which induced the mesoscale ridge, developed over the inland HHD in response to evaporative cooling of rain water. The outflow associated with the cold pool was found to trigger the strong convective systems along the southern coast of HHD in stage 1. The lifting of air in the precipitation band of stage 2 was mainly caused by convergence ahead of the strong low-level southwesterly flow. The numerical simulations indicated that the terrain over the Korean Peninsula contributed to the enhancement of the heavy rainfall primarily through its blocking effects on oncoming southwesterly airflow at low levels.

A Zona de Convergência do Atlântico Sul: uma visão e revisão críticas

A Zona de Convergência do Atlântico Sul (ZCAS) é uma alongada e persistente banda de intensa convecção que na época de verão e com certa frequência se estende da Amazônia ao oceano ao largo das costas brasileiras, exercendo papel significativo no regime pluviométrico do País. Juntamente com suas congêneres do Pacífico e do Índico, constitui uma nova categoria de organismo meteorológico de larga escala, identificado a partir da década de 1970 através da análise de observações de satélite. A explicação do fenômeno é ainda um desafio para a meteorologia e climatologia; essencialmente, resulta de uma intensa interação entre sistemas tropicais e de latitude média. Neste trabalho, recapitulamos uma parte da evolução histórica dos conhecimentos sobre o assunto, incluindo, além das contribuições de pesquisadores brasileiros, os providenciais estudos japoneses comparando a ZCAS com a Baiu, e apresentamos descrições resumidas de eventos de ZCAS no Brasil. Como complemento, reportamo-nos a dois eventos de precipitações destrutivas no Estado de São Paulo, de natureza frontal, a comparar com a atuação da ZCAS.

The Missing Link between Terrain-Induced Potential Vorticity Banners and Banded Convection

AbstractOn 1 February 2014, the southern side of the Alps was affected by a severe snowstorm that forced authorities to issue the highest level of avalanche danger in southern parts of Austria. The northern side of the Alps was mostly dry. Nevertheless, radar imagery captured the evolution of quasi-steady convective cloud bands over the northern Alpine foreland with a remarkable length of up to 300 km. This study illuminates the processes that generated these cloud bands based on numerical simulations. The storm was associated with a deep large-scale trough that caused strong southwesterly cross-Alpine flow, orographic precipitation on the southern side, and foehnlike subsidence on the northern side of the Alps. Orographic potential vorticity (PV) banners developed at small-scale topographic features embedded in the Alps and extended downstream over the northern Alpine foreland. Convective cloud bands were aligned parallel to these PV banners. They formed in an environment of inertial instability (negative absolute vorticity) and conditional instability. Sensitivity experiments reveal that the structure and size of these cloud bands are strongly sensitive to the small-scale terrain roughness. Removing small-scale topographic features suppresses the formation of orographic vorticity banners, which in turn suppresses the development of cloud bands. These results suggest that the release of inertial instability at negative orographic vorticity banners was crucial for establishing circulations and associated uplift that triggered conditional instability. To summarize, inertial instability was most likely responsible for the banded structure and conditional instability for the convective nature of these cloud bands.

INFLUENCE OF BIMODEL VERTICAL WIND SHEAR ON TYPHOON STRUCTURE AND INTENSITY

AbstractSimulations of Typhoon (Tropical Cyclone) Fitow (2013) are used in this study to analyze the influence of the vertical wind shear (VWS) on its structure and intensity. A vertical wave‐like distribution of VWS is revealed. This wave‐like distribution varies at different stage of typhoon Fitow (2013), and exhibits bimodal structure in the mature stage. It is found that the VWS between the top and bottom of middle troposphere is the major part of the total VWS. The typhoon intensity obviously changes about 6 hours after the distribution mutation of VWS. The different configuration of the secondary circulation induced by bimodal VWS and the typhoon vertical circulation causes the asymmetric structure of the deep convection bands, and they tend to be symmetric gradually with the enhancement of cyclonic circulation. The diagnostic analysis also indicates that the wave‐like distribution of VWS results in the inhomogeneous feature of vorticity forces in vertical direction. And the vorticity forces in the middle and low troposphere favor the development of convective instability. In accordance with the theoretical models, the maximum vertical velocity appears at the same altitude with the inflection of the vertical wind profile. Therefore, the bimodal VWS is crucial to the structure change of deep convection bands and the intensity maintenance in typhoon Fitow(2013). Furthermore, the wave‐like distribution of VWS might be the trigger of instability accounting for the convective‐rolls in typhoon.

A study of the formation mechanism of a long convection band over the Yellow Sea

ABSTRACT Observational and numerical investigations have been conducted to explain the formation of a long, quasi-stationary convection band (CB) that occurred over the Yellow Sea between August 1 and 2, 2008, in association with a synoptic-scale trough extending across the sea along the northwestern edge of the western Pacific subtropical high. The observations show that a line of weak echoes begins to appear over the Yellow Sea after 2030 UTC on August 1 and develops into a well-defined CB at approximately 0000 UTC on August 2, 2008. The life of the CB is divided into three stages: 1) initiation, 2) development, and 3) maturity. The initiation of the CB proceeds slowly and simultaneously along a long line over a period of approximately 2 h. The mature CB remains at nearly the same location until approximately 0300 UTC and is 10–20 km wide and greater than 300 km long. Both the observations and a numerical simulation show that the CB is initiated in an area of prevailing southwesterly surface winds to the south of an existing convection line, which is found to occur along or near a surface wind shift line. A cold pool or associated outflow is not observed at or near the existing convection line or the CB. The numerical simulation indicates that the convergence line along which the CB forms develops at the southern edge of a low-level convergence layer within a confluent flow associated with the extended trough. The formation of this convergence line appears to be related mainly to the southeastward movement of the low-level trough and convective circulation associated with an existing linear convective system.

The Utility of Convection-Permitting Ensembles for the Prediction of Stationary Convective Bands

Abstract This study examines convection-permitting numerical simulations of four cases of terrain-locked quasi-stationary convective bands over the United Kingdom. For each case, a 2.2-km-grid-length, 12-member ensemble and a 1.5-km-grid-length deterministic forecast are analyzed, each with two different initialization times. Object-based verification is applied to determine whether the simulations capture the structure, location, timing, intensity, and duration of the observed precipitation. These verification diagnostics reveal that the forecast skill varies greatly between the four cases. Although the deterministic and ensemble simulations captured some aspects of the precipitation correctly in each case, they never simultaneously captured all of them satisfactorily. In general, the models predicted banded precipitation accumulations at approximately the correct time and location, but the precipitating structures were more cellular and less persistent than the coherent quasi-stationary bands that were observed. Ensemble simulations from the two different initialization times were not significantly different, which suggests a potential benefit of time-lagging subsequent ensembles to increase ensemble size. The predictive skill of the upstream larger-scale flow conditions and the simulated precipitation on the convection-permitting grids were strongly correlated, which suggests that more accurate forecasts from the parent ensemble should improve the performance of the convection-permitting ensemble nested within it.

Effect of tropical cyclones on the tropical tropopause parameters observed using COSMIC GPS RO data

Abstract. Tropical cyclones (TCs) are deep convective synoptic-scale systems that play an important role in modifying the thermal structure, tropical tropopause parameters and hence also modify stratosphere–troposphere exchange (STE) processes. In the present study, high vertical resolution and high accuracy measurements from COSMIC Global Positioning System (GPS) radio occultation (RO) measurements are used to investigate and quantify the effect of tropical cyclones that occurred over Bay of Bengal and Arabian Sea in the last decade on the tropical tropopause parameters. The tropopause parameters include cold-point tropopause altitude (CPH) and temperature (CPT), lapse-rate tropopause altitude (LRH) and temperature (LRT) and the thickness of the tropical tropopause layer (TTL), that is defined as the layer between convective outflow level (COH) and CPH, obtained from GPS RO data. From all the TC events, we generate the mean cyclone-centred composite structure for the tropopause parameters and removed it from the climatological mean obtained from averaging the GPS RO data from 2002 to 2013. Since the TCs include eye, eye walls and deep convective bands, we obtained the tropopause parameters based on radial distance from the cyclone eye. In general, decrease in the CPH in the eye is noticed as expected. However, as the distance from the cyclone eye increases by 300, 400, and 500 km, an enhancement in CPH (CPT) and LRH (LRT) is observed. Lowering of CPH (0.6 km) and LRH (0.4 km) values with coldest CPT and LRT (2–3 K) within a 500 km radius of the TC centre is noticed. Higher (2 km) COH leading to the lowering of TTL thickness (2–3 km) is clearly observed. There are multiple tropopause structures in the profiles of temperature obtained within 100 km from the centre of the TC. These changes in the tropopause parameters are expected to influence the water vapour transport from the troposphere to the lower stratosphere, and ozone from the lower stratosphere to the upper troposphere, hence influencing STE processes.

The mechanism of growth of the low-frequency East Asia–Pacific teleconnection and the triggering role of tropical intraseasonal oscillation

The East Asia–Pacific (EAP) pattern is a well-known meridional teleconnection over East Asia during boreal summer. In this study, the mechanism for growth of the EAP on intraseasonal timescale is investigated through a vorticity budget. It is found that the beta-effect and high-frequency transient eddies have primary contributions to the growth of the low-frequency EAP. The former leads to a westward shift of disturbances associated with the low-frequency EAP and the latter favors an amplification of disturbances, respectively. The interaction between low-frequency disturbances and zonal flow has a damping effect by dragging disturbances eastward. The impact of boreal summer intraseasonal oscillation (BSISO) on the triggering of the low-frequency EAP is also examined in this study based on observational analysis and a linear model experiment. It is shown that an elongated anomalous convection band located in the vicinity of Philippines associated with the dominant mode of BSISO has a significant impact on the initiation of low-frequency EAP via Rossby wave propagation, whereas anomalous convection located over the North Indian Ocean has a limited impact. Based on the results of present study, the low-frequency EAP could be a self-sustained mode, and the BSISO plays a substantial role in triggering the low-frequency EAP.

Organized Convection Parameterization for the ITCZ*

Abstract Mesoscale convective systems (MCSs) are of fundamental importance in the dynamics of the atmospheric circulation and the climate system. They are often observed to develop over significant terrain in ambient shear flows in midlatitudes and embedded within the Madden–Julian oscillation (MJO) and convectively coupled equatorial wave (CCEW) envelopes, as well as in the intertropical convergence zone (ITCZ). Yet general circulation models (GCMs) fail to resolve these systems, and their underlying convective parameterizations are not directed to represent organized circulations. Shear-parallel MCSs, which are common in the ITCZ, have a three-dimensional structure and, as such, present a serious modeling challenge. Here, a previously developed multicloud model (MCM) is modified to parameterize MCSs. One of the main modifications is the parameterization of stratiform condensation to capture extended stratiform outflows, which characterize MCSs, resulting from strong upper-level jets. Linear analysis shows that, under the influence of a typical double African and equatorial jet shear flow, this modification results in an additional new scale-selective instability peaking at the mesoalpha scale of roughly 400 km. Nonlinear simulations conducted with the modified MCM on a 400 km × 400 km doubly periodic domain, without rotation, resulted in the spontaneous transition from a quasi-two-dimensional shear-perpendicular convective system, consistent with linear theory, to a fully three-dimensional flow structure. The simulation is characterized by shear-parallel bands of convection, moving slowly eastward, embedded in stratiform systems that expand perpendicularly and propagate westward with the upper-level jet. The mean circulation and the implications for the domain-averaged vertical transport of momentum and potential temperature are discussed.

A Study of Convective Band with Heavy Rainfall Occurred in Honam Region

On the study of the characteristics and life cycle of mesoscale convective band in type of airmass that occurred in the Honam area from June to September for only 4 years in the period of 2009~2012, 10 examples based on the amount of rainfall with AWS 24 hours/60 minutes rainfalls, Mt. Osung radar 1.5 km CAPPI/X-SECT images and KLAPS data for convective band with heavy rainfall event were selected. There were analyzed and classified by using the convective band with heavy rainfall occurred along the convergence line of sea wind in the form of individual multi-cellular cell and moving direction of convective band appeared in a variety of patterns; toward southwestern (2 cases), northeastern (4 cases), congesting (2 cases), and changing its moving direction (2 cases). The case study dated of the 17th Aug. 2012 was chosen and implemented by sequentially different evolution of its shape along the convergence line of sea wind cell and moving direction of convective band as equivalent potential temperatures at the lower layer have increased to the upper layer 500 hPa, that the individual cells were developed vertically and horizontally through their merger, but owing to divergence caused by weakened rainfall and descending air current, the growth of new cell was inhibited resulting in dissipation of convective cells.

Why the South Pacific Convergence Zone is diagonal

During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest–southeast) oriented SPCZ are determined through a series of experiments with an atmospheric general circulation model. Continental configuration and orography do not have a significant influence on SPCZ orientation and strength. The key necessary boundary condition is the zonally asymmetric component of the sea surface temperature (SST) distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that, on its western flank, transports warm moist air from the equator into the SPCZ region. This moisture then intensifies (diagonal) bands of convection that are initiated by regions of ascent and reduced static stability ahead of the cyclonic vorticity in Rossby waves that are refracted toward the westerly duct over the equatorial Pacific. The climatological SPCZ is comprised of the superposition of these diagonal bands of convection. When the zonally asymmetric SST component is reduced or removed, the subtropical anticyclone and its associated moisture source is weakened. Despite the presence of Rossby waves, significant moist convection is no longer triggered; the SPCZ disappears. The diagonal SPCZ is robust to large changes (up to ±6 °C) in absolute SST (i.e. where the SST asymmetry is preserved). Extreme cooling (change <−6 °C) results in a weaker and more zonal SPCZ, due to decreasing atmospheric temperature, moisture content and convective available potential energy.

Genesis of Tropical Storm Debby (2006) within an African Easterly Wave: Roles of the Bottom-Up and Midlevel Pouch Processes

Abstract The “bottom up” generation of low-level vortices (LVs) and midlevel vortices (MVs) during the genesis of Tropical Storm Debby (2006) and the roles of a midlevel “marsupial pouch” associated with an African easterly wave (AEW) are examined using an 84-h simulation with the finest grid size of 1.33 km. Results show that several MVs are generated in leading convective bands and then advected rearward into stratiform regions by front-to-rear ascending flows. Because of different Lagrangian storm-scale circulations, MVs and LVs are displaced along different paths during the early genesis stages. MVs propagate cyclonically inward within the AEW pouch while experiencing slow intensification and merging under the influence of converging flows. The MVs’ merging into a mesovortex is accelerated as they come closer to each other in the core region. In contrast, the low-level Lagrangian circulation is opened as a wave trough prior to tropical depression (TD) stage, so the LVs tend to “escape” from the pouch region. Only after the low-level flows become closed do some LVs congregate and contribute directly to Debby’s genesis. The TD stage is reached when the midlevel mesovortex and an LV are collocated with a convective zone having intense low-level convergence. Results also show the roles of upper-level warming in hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls. It is found that the vertically tilted AEW with a cold dome below is transformed to a deep warm-core TD vortex by subsiding motion. A conceptual model describing the key elements in the genesis of Debby is also provided.

Hierarchical tubular structures grown from the gel/liquid interface.

Three dimensional hierarchical materials are widespread in nature but are difficult to synthesize by using self-assembly/organization. Here, we employ a gel-liquid interface to obtain centimeter-long ∼100 μm diameter tubes with complex mineral wall structures that grow from the interface into solution. The gel, made from gelatin, is loaded with metal chloride salt, whereas the solution is a high pH anion source. Tubes were obtained with a range of cations (Ca(2+) , Sr(2+) , Ba(2+) , Cu(2+) , and Zn(2+) ) and anions (CO3 (2-) and PO4 (3-) ). The crystalline phases found in the tube walls corresponded to expectations from solution chemistries and phase solubilities. The growth mechanism is found to be akin to that of chemical gardens. The divalent cations modify the strength of the gelatin gel in a manner that involves not only simple electrostatic screening, but also ion-specific effects. Thus, tubes were not obtained for those ions and/or concentrations that significantly changed the gel's mechanical structure. At high Cu(2+) loading, for example, vertical convection bands, not Liesegang bands, were observed in the gels.

Synoptic versus orographic control on stationary convective banding

Quasi‐stationary convective bands can cause large localised rainfall accumulations and are often anchored by topographic features. Here, the predictability of and mechanisms causing one such band are determined using ensembles of the Met Office Unified Model at convection‐permitting resolution (1.5 km grid length). The band was stationary over the UK for 3 h and produced rainfall accumulations of up to 34 mm. The amount and location of the predicted rainfall was highly variable despite only small differences between the large‐scale conditions of the ensemble members. Only three of 21 members of the control ensemble produced a stationary rain band; these three had the weakest upstream winds and hence lowest Froude number. Band formation was due to the superposition of two processes: lee‐side convergence resulting from flow around an upstream obstacle and thermally forced convergence resulting from elevated heating over the upstream terrain. Both mechanisms were enhanced when the Froude number was lower. By increasing the terrain height (thus reducing the Froude number), the band became more predictable.An ensemble approach is required to successfully predict the possible occurrence of such quasi‐stationary convective events because the rainfall variability is largely modulated by small variations of the large‐scale flow. However, high‐resolution models are required to accurately resolve the small‐scale interactions of the flow with the topography upon which the band formation depends. Thus, although topography provides some predictability, the quasi‐stationary convective bands anchored by it are likely to remain a forecasting challenge for many years to come.

Utility of Dense Pressure Observations for Improving Mesoscale Analyses and Forecasts

Abstract The use of dense pressure observations is investigated for creating mesoscale ensemble analyses and improving short-term mesoscale forecasts. By exploiting additional observation platforms, the number of pressure observations over the Pacific Northwest region is increased by an order of magnitude over standard airport observations. Quality control and bias correction methods for these observations are discussed, including the use of pressure tendency as an alternative observation type with fewer bias concerns. The enhanced station density provided by these observations contributes to localized adjustments for a variety of mesoscale phenomena. These adjusted analyses yield improved forecasts, including more accurate forecasts of frontal passages and convective bands. Assimilating dense 3-h pressure tendency observations also reduces the error in some forecast surface fields similarly to raw pressure observations, suggesting further investigation into pressure tendency as a mesoscale observation type.

A case study of mesoscale convective band (MCB) development and evolution along a quasi-stationary front

This paper presents a case study of mesoscale convective band (MCB) development along a quasi-stationary front over the Seoul metropolitan area. The MCB, which initiated on 1500 UTC 20 September 2010 and ended on 1400 UTC 21 September 2010, produced a total precipitation amount of 259.5 mm. The MCB development occurred during a period of tropopause folding in the upper level and moisture advection with a low-level jet. The analyses show that the evolution of the MCB can be classified into five periods: (1) the cell-forming period, when convection initiated; (2) the frontogenetic period, when the stationary front formed over the Korean peninsula; (3) the quasi-stationary period, when the convective band remained over Seoul for 3 h; (4) the mature period, when the cloud cover was largest and the precipitation rate was greater than 90 mm h−1; and (5) the dissipating period, when the MCB diminished and disappeared. The synoptic, thermodynamic, and dynamic analyses show that the MCB maintained its longevity by a tilted updraft, which headed towards a positive PV anomaly. Precipitation was concentrated under this area, where a tilted ascending southwesterly converged with a tilted ascending northeasterly, at the axis of cyclonic rotation. The formation of the convective cell was attributed in part by tropopause folding, which enhanced the cyclonic vorticity at the surface, and by the low-level convergence of warm moist air and upperlevel divergence. The southwesterly flow ascended in a region with high moisture content and strong relative vorticity that maintained the development of an MCB along the quasi-stationary front.

Satellite Regional Cloud Climatology over the Great Lakes

Thirty-one years of imager data from polar orbiting satellites are composited to produce a satellite climate data set of cloud amount for the Great Lakes region. A trend analysis indicates a slight decreasing trend in cloud cover over the region during this time period. The trend is significant and largest (~2% per decade) over the water bodies. A strong seasonal cycle of cloud cover is observed over both land and water surfaces. Winter cloud amounts are greater over the water bodies than land due to heat and moisture flux into the atmosphere. Late spring through early autumn cloud amounts are lower over the water bodies than land due to stabilization of the boundary layer by relatively cooler lake waters. The influence of the lakes on cloud cover also extends beyond their shores, affecting cloud cover and properties far down wind. Cloud amount composited by wind direction demonstrate that the increasing cloud amounts downwind of the lakes is greatest during autumn and winter. Cold air flows over relatively warm lakes in autumn and winter generate wind parallel convective cloud bands. The cloud properties of these wind parallel cloud bands over the lakes during winter are presented.