Mesoscale Exam 1

Dryline Triple Point

Synoptic scale cold fronts often "catch" and "interact" with dry lines • Location of enhanced convection • Front provides an additional source of lift • Front now has access to moist air • Severe thunderstorms often occur near the triple point on the warm moist side

Forcing of Mesoscale Weather

Synoptic-scale Geography/Topography Other organized mesoscale weather

The dryline

Can be defined as a near surface convergence zone between moist air flowing off the Gulf of Mexico and dry air flowing off the semi arid, high plateaus of Mexico and the southwest United States •Observed from southern Great Plains to the Dakotas → east of the Rockies •Occur between April and June when a surface high is located to the east with westerly flow aloft and a weak lee side surface low is located to the west

Jet Streak Vorticity increase

Convergence aloft Downward motion

Topographically Induced Fronts

Denver Convergence Zone: Generated by synoptic scale easterly flow converging with shallow cold air flowing down topography (ridges and mountains) • Cold air originates in the nocturnal boundary layer at high elevations • Air begins to flow down the slopes and valleys • Converges with synoptic scale easterly flow by mid morning and begins to push eastward onto the Great Plains • Usually dissipates by mid afternoon due to solar heating and surface fluxes warming the shallow cold air • Convergence line can help initiate deep convection → non supercell tornadoes often form during such events • The topography in the Denver area often leads to the development of a cyclonic circulation → enhances convergence

Stable Boundary Layer

Depth ~100 500 m Radiational cooling of the land creates a stable cold layer Layer deepens as night progress (no vertical mixing) Strong temperature inversion at top

Jet Streak Vorticity decrease

Divergence aloft Upward motion

Mesoscale Surface Phenomena

Dry front, Midget typhoon, Mesohigh, Gust front, Mesocyclone, Downslope wind, Macroburst, Microburst, Tornado

Dryline Bulges

Eastward "________" occasionally develop during the afternoon hours • 80-100 km in scale • Preferred location for convective initiation due enhanced low level convergence • Occur when mid tropospheric winds are strong • Result from the deep turbulent mixing west of the dryline transporting strong mid level westerly winds down to the surface

Origin of Mesoscale

First coined by Lidga in 1951 Meteorologist at MIT Used the term to classify weather phenomena observable by radar but too small to be observed by conventional (synoptic) observations

Gust Fronts

Generated within thunderstorms by either precipitation loading or evaporative cooling at mid tropospheric levels • Negative buoyancy brings cool air down to the surface, where it spreads out, creating outflow boundaries • Horizontal scale → 10 to 50 km • Vertical scale → 1 to 2 km • Time scale → 1 to 6 hours • Forward motion → 5 to 20 m/s • Often responsible for generating new convection due to the enhanced convergence and ascent along their leading edge • Under special conditions can help maintain intense long lived squall lines...more on this in the future

Convective Initiation on the Mesoscale

Given: • A synoptic scale environment conducive to deep convection (e.g. ample CAPE) • Some CIN which must be "overcome" to permit deep convection Required: • Boundaries " are needed to provide mesoscale regions of forced ascent • Not all boundaries produce deep convection • Deep convection is often not uniform along a given boundary Often Needed and/or Occurs: • Changes to the thermodynamic profile (i.e. lower CIN and increase CAPE)

Entrainment Zone

Located above the mixed layer during the day Depth ~100-200 m Transition layer between the well mixed convective boundary layer and the free atmosphere Strong vertical gradients in temperature and moisture Often contains a temperature inversion (source of CIN) Stability often absolutely stable (prevents cloud Strong inversions will prevent deep convection

Mixed Layer

Located above the surface layer during the day Depth ~1000 m Overturning thermals regularly transport (or "mix") heat and moisture from the surface layer to the entrainment zone Mixing often strongest ~1-2 hours after solar noon Heat and moisture are conserved (θ and w are constant) Stability often dry adiabatic

Middle and Upper-Level Phenomena

Low-level jet, Jet streak, Anvil cluster (MCC), Individual anvil, Supercell storm, Cumulonimbus, Cumulus, Overshooting dome, Tornado vortex signature

Surface Layer

Lowest ~100 m AGL Layer where heat and moisture are exchanged between land and air Strong vertical gradients in winds, temperature, and moisture Stability often super adiabatic (daytime)

Capping Inversion

Remnant entrainment zone

Residual Layer

Remnant mixed layer

Sea-Breeze Fronts

Result from differential surface heating/cooling along coasts on "light wind" days Day → Heating over land (positively buoyant air → Onshore flow near surface - offshore flow aloft Night → Cooling over land (negatively buoyant air sinks) → Offshore flow near surface - onshore flow aloft • Front develops where onshore flow collides with "background" synoptic flow

Boundary Layer Convection Results

Shallow clouds often occur from noon to late afternoon •"Popcorn" or "Fair Weather" cumulus • Clouds can appear random, but are often organized into distinct structures •"Cloud Streets" •"Open/Closed Cells"

Coastal Fronts

Stationary boundary separating relatively warm moist air flowing off the ocean from relatively cold dry air flowing off the continent • Occur in the late fall and early winter from New England to Texas • Often form during cold air outbreaks and cold air damming events • Boundary between rain and freezing rain/snow • Temperature gradients of 5 1010°C over 5 10 km • Convergence zone enhanced by land sea friction contrasts

Dryline position

The 55°F isodrosotherm or the 9.0 g/kg isohume are used to indicate • Dewpoint gradient often 15°F per 100 km or larger • Wind shift and moisture gradient are not always collocated Also occur in India, China, and west Africa

Boundary Layer

The part of the atmosphere directly influenced by the Earth's surface that responds to surface forcing (i.e. friction and energy fluxes) within a time scale of ~1 hour or less

Mesoscale (Class)

Weather phenomena occurring on spatial scales of 2-2000 km and time scales of 3 minutes to 2 days

Non-homogeneous Surface Conditions

• Acts to modulate (or slightly alter) the convection generated by both solar heating and/or cold air advection • Strong gradient in surface properties

Mature Gust Front Passage

• Change in wind speed and direction • Direction may rotate 180° • Speed initially decreases prior to frontal passage and then rapidly increases soon after frontal passage • Decrease in temperature on the order of 2° to 5°C • Increase in pressure (~1 mb) • Initial rise is non hydrostatic, a dynamic effect created by the collisions of two fluids • Second rise is hydrostatic, the thermodynamic effect from the cold air • Onset of light precipitation

Pronounced sloping transition zones in the temperature, moisture, and wind fields

• Contain large vorticity gradients and vertical wind shears • Cross front scale (10 100 km) is often an order of magnitude smaller than along front scale (100 1000 km) • Shallow (1 5 km in depth) • Most often observed near the surface, but also occur aloft near the tropopause

Open Cell Convection

• Due to advection of cold air over a warm surface (either land or water) • Common late fall thru early summer (over land) • Cell has hexagonal structure (aspect ratio 10:1) • Descending motion at the core • Updrafts on edge are ~1 m/ s • Well observed by satellites (visible and low IR) • Difficult to detect on radar (looks like random "noise Form in weak shear environments Can trigger deep convection

Horizontal Convective Rolls (HCRs):

• Due to daytime solar heating of land • Mixed layer thermals organized into bands • Horizontal helices oriented nearly parallel to the ambient flow • Produce cloud streets • Commonly seen in satellite and radar imagery prior to the onset of deep convection (useful to forecasters)

Horizontal Convective Rolls Trigger

• If updrafts contain higher T, θ , and w then there should be less negative area ( to overcome and more positive area (CAPE) available for deep convection • On radar, higher reflectivity cells often correspond to the "deeper" convection along the bands that are more likely to reach their LFC and "trigger" the first deep convection (helpful for short term forecasts)

Dryline Agriculture

• Occur during the peak of growing season • Hot / Dry to the west (need to irrigate more) • Warm / Humid east

Dryline Convection

• Often develops into severe thunderstorms, producing strong winds, hail, and tornadoes • Over 90% of such convection develops within 100 km of the line on the moist side

Closed Cell Convection

• Often occurs over cold surfaces (e.g. stratocumulus off California coast) • Forced by strong radiational cooling at cloud top • Cell has hexagonal structure • Ascending motion at the core Form in weak shear environments with minimal surface fluxes Rarely triggers deep convection

Deep Convection Initiation

• Quantitative Precipitation Forecasts (QPF) • Severe Weather Forecasting • Hydrology (flash flooding, stream levels) • Aviation Forecasting (microbursts)

Boundary Sub-layers

• Surface Layer • Mixed Layer (ML) • Entrainment Zone (EZ) • Stable Boundary Layer (SBL) • Residual Layer (RL)

Boundary Layer Convection

• Transport heat and moisture from the surface to the free atmosphere • Two common scenarios for boundary layer convection: Daytime Solar Heating and Cold air advection

Horizontal Convective Rolls Results

• Typical aspect ratio (horizontal to vertical scale) is 3:1 but can vary from 2:1 to 10:1 • Typical updrafts are 1 3 m/s Updrafts often contain higher values of T, θ , and w compared to adjacent downdrafts • Result from a combination of buoyancy and vertical wind shear within the boundary layer Most often occur in strong shear moderate heat flux environments ( the same environment most severe weather occurs in...)

Synoptic-scale weather forcing

•Cyclones and anticyclones •Warm / cold fronts •Warm / cold air advection •Large scale CAPE •Large scale vertical shear

Night time Westward Motion

•During the day, a heat low develops west of the dryline, driving low level air toward the line •When the sun sets, radiational coolingweakens the westerly flow (dry, cloud free) much quicker than it weakens the easterly flow (moist, cloudy) •Dryline surges westward

Geography / Topography forcing

•Flow over elevation gradients (hills) •Land sea boundaries •Gradients of surface vegetation

Dryline Daytime Eastward Motion

•Moves rapidly via sudden "leaps" (after sunrise) •Motion is much faster than would occur from advection alone...How? •Turbulent mixing induced by solar heating begins to erode the shallow west side of the dry line •Process continues throughout the day •In the late afternoon to early evening the dryline begins to move back westward...Why?

Therefore the greatest challenge for a weather forecaster is on the mesoscale

•Must be able to anticipate mesoscale weather based on the synoptic observations •Must be able to adapt quickly to any evolving mesoscale observations •Must be able to effectively disseminate information to the public in a timely manner •A considerable number of lives and property are often at stake

Important for mesoscale weather:

•Rapid local changes in weather •Associated with clouds and precipitation •Often provide the necessary "trigger" for initiating deep convection

Other organized mesoscale weather

•Sea breeze convergence •Thunderstorm outflow boundaries •Low level jet streaks •Mesoscale convective vortices (MCVs)

The vast majority of severe weather occurs in relation to organized mesoscale phenomena

•Supercells / Squall Lines → Tornadoes •Squall lines / Bow echoes → Derechos •Mesoscale Convective Complexes → Flash floods •Quasi stationary convective events → Flash floods •Mountain waves → Downslope wind storms

Current observation networks do not adequately resolve the mesoscale with regular observations of both winds and thermodynamics in space and time

•Surface observations (ASOS, AWOS, mesonets •NOAA rawinsonde network •NEXRAD WSR 88D Doppler radar network •NOAA Geostationary and polar orbiting satellites

Dryline Evening

→ Deeper (up to 750 mb) → Furthest eastward extension → Dry mixed layer on west side often extends up to 500 mb

The orientation of a surface front and an upper level jet streak can lead to suppressed (shallow) convection along the front

→ Left entrance or right exit region is above the front → Prevents destabilization of the potentially unstable air → Decreases the likelihood of deep convection

The orientation of a surface front and an upper level jet streak can lead to enhanced (deep) convection

→ Left exit or right entrance region is above the front → Helps destabilize the potentially unstable low level air → Increases the likelihood of deep convection

Dryline Morning

→ Shallow (below ~850 mb) → Furthest westward extension → Moist layer capped by strong nocturnal temperature inversion