Springtime Cloud Properties in the Taiwan Strait: Synoptic Controls - PowerPoint PPT Presentation

Springtime Cloud Properties in the Taiwan Strait: Synoptic Controls and Local Processes Mien-Tze Kueh and Pay-Liam Lin Department of Atmospheric Sciences, National Central University, Chungli HKMS25: 2013 Nov 2

Outline • Background • Synoptic perspective • Composite cloud features • Local processes • Summary Primary source: Kueh and Lin: Springtime cloud properties in the Taiwan Strait: synoptic controls and local processes. Theoretical and Applied Climatology (2013) Accepted.

Background – Fog-related low visibility [1/2] The springtime is a high-occurrence season for low-clouds and fog events in the area. The fogs and very low clouds can impact the local marine and aviation transportation due to their low visibility.

Background: SST gradient & wind channeling [2/2] Remarkable northwestward decreasing SST and wind channeling effect are commonly observed over the strait. General features in the spring season: 1. northeast-orientated marine cloud belt is found above the large SST gradients over the shelf seas. 2. near-surface prevailing northeasterly flow. 3. a local wind speed maximum is found in northern TS, along with larger cloudiness located on the upwind side.

Synoptic perspective cold air surge versus frontal system

Composite cloud property: Space-borne MODIS cloud optical parameters: PDF of COT versus CER Stratocumulus dominate for both conditions Cold air surge: Frontal system: Separation of Vertical extension low cloud layers of low cloud layers cold air surge versus frontal system ISCCP

Composite cloud property: Ground-based Ground-based: Airport observation Cold air surge: Frontal system: 1. Stratocumulus 1. Stratocumulus and alto clouds dominate. associated with 2. Higher occurrence prevailing for stratus. northeasterly. 3. Fogs observed 2. Less stratus, with southwesterly no fog observed. cold air surge versus frontal system

Local processes: Composites [1/2] Significant divergence and Backing (veering) convergence within the wind observed within boundary layer in TS are the TS is indicative remarkable for cold surge of synoptic cold and frontal conditions, (warm) air advection respectively. emerges from the cold surge (frontal) Northward increasing condition. stabilities are found for both conditions, and a more stable environment emerges from the cold surge condition.

Local processes: Composites [2/2] Cold surge: Frontal condition: cold and dry air surge warmer and moister along with the local thermal structure is convergence subjected to the warm air advection and aloft, serves to compensate for the dynamical lifting that near-surface strong arose from the deep divergence and thus layer of the produce a subsidence southwesterly flow. � Vertical extension warming (result in a drying layer) of low cloud layers locally within the TS. � Separation of low cloud layers

Optical cloud physics: COT versus CER Frontal system: More rainfall Cold surge: Thicker clouds

Summary

Summary 1. The predominance of synoptic-scale controls in regulating the cloud properties. The springtime clouds over the TS are commonly a mixture of stratocumulus and alto clouds, with a preponderance of stratocumulus over the strait. A separation of low cloud layers is recognized during cold air surge, whereas vertical development of low cloud layers is commonly observed with frontal passage. 2. The sensitivity of cloud regimes to the local thermodynamic mechanisms (e.g., wind channeling effects and sea temperature conditions) within the TS is fundamentally dependent on synoptic-scale flow patterns. The most distinct difference between the local cloud formations associated with the two synoptic conditions is the suppression of very low cloud and fog along with cold air surge. Stratus cloud and fog are present within the northward prefrontal airflow from warmer to colder water sites, along with an increase in stability relating to lower altitudes of boundary layer clouds.

Comparison: Kuroshio region

Optical cloud types: COT versus CTP back

Methodology: the composite To describe the dependence of cloud properties on the two synoptic conditions, a set of composite fields are constructed based upon the daily SLP of the two airports using the upper quartile ( � � � � cold air surge) and the lower quartile ( � � frontal � � system) as thresholds.

Air-sea interaction ?

Data issues: Space-borne versus Ground-based