Cold Pool

1. Before After Onshore Offshore Using Ocean Observatory Data to Motivate Hurricane Ocean and Atmospheric Model Sensitivity Studies in the Mid-Atlantic Hurricane Irene Glider Sampling Sept 26 Real Time Global Sept 26 SST Difference Sept 26 Coldest Dark Pixel Hurricane Irene 0 26 depth poster 1904 Temperature Scott Glenn(glenn@marine.rutgers.edu), O. Schofield, J. Kohut, H. Roarty, Y. Xu, L. Palamara, T. Miles & G. Seroka + MARACOOS Rutgers University, Coastal Ocean Observation Lab’s Hurricane Tuesday Research Group, New Brunswick, New Jersey 08901 USA 14 55 0 33 SST Before Irene depth Sept 31 SST Difference Sept 31 Real Time Global Sept 31 Coldest Dark Pixel Salinity 29 55 Mid-Atlantic’s IOOS Regional Association Irene Hydrographic Respone Ocean Model Results & Interpretation Atmospheric Model Results & Interpretation 0 105 Before the eye passage near 0900 indicated by the Tuckerton air pressure, wind is from the east and CODAR surface currents are strongly onshore, peaking above 50 cm/sec in the onshore direction. Yet the observed depth-averaged glider velocity remains near zero, implying a compensating offshore flow in the bottom layer that peaks as the eye passes. After eye passage, the wind is from the west and surface currents rotate and accelerate offshore. Since the depth average glider velocity remains steady near zero, the bottom layer again must compensate and accelerate onshore. Sensitivities with the advanced Weather Research Forecast (WRF) model indicate the forecast intensity of Irene is strongly dependent on SST in the Mid-Atlantic. Specifically, the much colder temperatures observed in the regional product after the storm reduced intensities by over 5 m/s, bringing them in line with the best fit post-analysis. The Mid-Atlantic Regional Association Coastal Ocean Observing System (MARACOOS) extends the 1000 km distance between Cape Hatteras and Cape Cod. Its densely populated coast includes 10 states, ¼ of the U.S. population, 2 of the largest U.S. ports, the world’s largest Naval base, large commercial fisheries and a developing offshore wind energy industry. The wide continental shelf experiences hydrographic conditions with a strong seasonality. The extreme summer stratification between the warm surface layer and the bottom Cold Pool is not resolved by global models. Transitional steps to well-mixed winter conditions are forced by atmospheric cooling and accelerated by fall mixing storms. A new regional scale satellite Sea Surface Temperature (SST) product was compared to the operational global SST product before (top row) and after (bottom row) Irene. Before Irene the Mid-Atlantic shelf SST is 22-24C in both products, with more small-scale variability like coastal upwelling observed in the regional scale. After Irene, the regional product indicates the surface cooled to 14-18C, while shelf waters in the global product remain much warmer (20-24C). depth % Saturation (DO) 55 60 Aug 12 Sep 07 SST After Irene Fig. 10: WRF model winds when the eye is over the glider generated using the regional satellite SST product for fixed surface boundary conditions assuming the warm pre-storm conditions (left) and the cold post-storm conditions (right). Fig. 7: Left column: Tuckerton wind direction & speed (top), air pressure minimum (red dotted line), glider temperature profiles (middle), and cross-shore velocity (bottom) from the surface layer (CODAR), depth average (glider) and bottom layer (inferred). Right column: CODAR surface current maps before & after the eye. Warm SST Cold SST 10 States - 76 Million People 1000 km Cape to Cape RI U.. CT Cape Cod Middle Atlantic Regional Association Coastal Ocean Observing System MA NY PA NJ A WRF model sensitivity matrix is being explored with over 100 runs to date. The two largest sensitivities are the range of SST boundary conditions followed by the range of surface layer flux coefficients. DE MD Sandy Irene >40 PIs >20 Institutions >50 Members >2000 Contacts Fig. 11: Top: Time series of wind speed illustrating the Top Sensitivities to 2 SSTs (thick red line versus thick blue line) compared to best fit (thick black line) and actual forecast (thick green line). Bottom: Time series of wind speed difference from the best fit. The vertical thin black lines indicate when the eye was over the Mid-Atlantic. VA Fig. 4: Satellite SST maps derived from the new MARACOOS regional coldest dark pixel composite (1st column), the standard U.S. global product (2nd column), and the difference (3rd column). Row 1 is the last clear image before Irene and Row 2 is the first clear image after the storm. NC Eye over Mid-Atlantic The Regional Ocean Modeling System (ROMS) accurately reproduces the observed deepening and cooling of the surface layer during Irene. Before the eye passage, the onshore wind stress is balanced by an offshore pressure gradient, resulting in a strong onshore surface layer flow that is compensated by an offshore bottom layer flow. Mixing coefficients grow in each layer and interact when the vertical shear across the interface is large. The mixing terms dominate the heat equation, with cooling in the surface layer and warming of the interface as it moves down in the water column. Vertical advection plays a much smaller role in the heat equation, with other terms even smaller. Cape Hatteras Fig. 1: The wide continental shelf of the Mid-Atlantic. Inset illustrates a typical summer cross-shelf glider temperature transect (top) with an intense thermocline above the Cold Pool, and the corresponding RTOFS forecast (bottom). Cold Pool A Slocum Glider deployed on the New Jersey Coast for an EPA Dissolved Oxygen (DO) monitoring mission before Irene was purposely not recovered but was instead flown offshore to the 40 m isobath as Irene approached. Strong summer stratification is observed before Irene. During the storm, the interface between the surface layer and the Cold Pool deepens and the surface layer is mixed with the water below. Based on user-prioritized needs, MARACOOS operates a sustained real-time regional-scale observatory that enhances the national backbone with the spatial data required for assimilation in an ensemble of regional-scale ocean forecast models. • Top Sensitivities: • SST • Flux Coefficients Warm • Flux Coefficients Cold Satellites HF-Radar Gliders Drifters Fig. 2: MARACOOS spatial observations include satellite surface temperature and color, CODAR surface current maps, Slocum Glider subsurface transects, and drifter tracks. An ensemble of weather forecasts validated with an industry-enhanced mesonet are available forcing for an ensemble of data assimilative ocean models. While Irene’s eye was over the Mid-Atlantic, error statistics for each model run compared to the best fit intensity are summarized in a box and whisker plot (below). The official forecast (green) averaged 10 m/s too high, even though the GFS model (pink) with low resolution physics and the overly warm SST did well. The warm SST runs (red) consistently overpredicted the intensity, with the smallest overprediction resulting from the least advanced flux formulation (warm, 0). Turning on WRF’s 1-D ocean mixed layer model (aqua) overpredicts the intensity when realistic mixed layer depths are used (OML,1) and requires the mixed layer to be reduced to the minimum 10 m even in the Gulf Stream to match the best fit (OML,2). The cold SST runs (dark blue) all cluster around the best fit, with the most advanced flux relation (Cold, 2) providing the best result. Fig. 5: Track of the EPA Glider (yellow line) and the complete time series of vertical temperature, salinity and dissolved oxygen (DO) profiles. The black bottom depth mask shows the glider zigzagging between deeper and shallower water. Fig. 8: ROMS model time series results at the location of the glider during Irene. Row 1 is the cross-shelf force balance. Row 2 is temp & cross-shelf velocity. Row 3 is vertical velocity & vertical mixing coefficient. Row 4 includes the two dominant terms in the heat equation, vertical advection & vertical diffusion. To seek, discover & apply new knowledge & understanding of our coastal ocean Population Density Weather Stations Ocean Forecast Ensemble • RU16 • Deployed for EPA • Map bottom DO • Provided data during mixing storm Hurricanes Irene and Sandy Irene propagated rapidly along the coast over a highly stratified shelf. Sandy was larger and approached slowly from offshore over a cooler, nearly mixed shelf after the fall transition was nearly complete. Fig. 12: WRF model sensitivities compared with the best fit intensity over the Mid-Atlantic. Error is the maximum wind speed difference. Zooming in on the 2+ days associated with Irene’s passage, much of the change occurs during the 6-12 hours preceding the eye under the high velocity winds of the leading edge. The surface layer deepens from 10 m to 25 m, cools from 24C to 18C, and DO levels drop from supersaturated to 81%. Bottom temperatures and DO levels remain relatively constant near 10 C and 60% during the event. Hurricane Irene August 26, 2011 NOAA/NHC Damage: #8 with >$15 Billion. Track Accurate; Intensity and Storm Surge Over-predicted. Since the Mid-Atlantic Cold Pool covers the full extent of the region, downwelling-induced shear across the interface is expected to produce mixing and cooling across the entire region. The SST difference map below indicates that significant cooling of 6-8 C can occur over much of the shelf. NOAA weather buoy observations indicate that much of the cooling occurs during the strong winds of the leading edge of the hurricane before the eye arrives. Conclusions Fig. 6: Glider time series of temperature (top row) and dissolved oxygen (bottom row) with profiles (left column) and surface/bottom values (right column) during Irene. Black dashed line indicates the time of eye passage over the glider. • Ocean response to Irene includes a 2 layer downwelling that reduces storm surge & generates shear across the intense summer pycnocline. • Mixing across the pycnocline causes the surface layer to deepen/cool. • A cooler surface results in a more accurate intensity forecast for Irene. Fig. 9: Left Column: Satellite regional SST product difference before and after Irene. The inset time series shows the value of the warm & cold SST under the eye as it propagates north. Right Column: NOAA weather buoy surface temperature (blue), wind speed (green) and timing of the pressure minimum (red) with the northern buoy at the top. Hurricane Sandy October 29, 2012 NOAA/NHC Damage: #2 with >$60 Billion. Track Accurate; Acceleration & Storm Surge Under-predicted. Acknowledgements Financial support provided by U.S. IOOS, NOAA, EPA, DHS, NJBPU, NJDEP, Teledyne Webb Research, Nortek & WeatherFlow. The Hurricane Tuesday working group gratefully acknowledges the support & scientific expertise of the MARACOOS community. Fig. 3: Hurricanes Irene and Sandy approaching the MARACOOS HF Radar network. Wind Stress = Pressure Grad. -------------------- Pressure Grad. = Coriolis Force Off On Vertical Diffusion > Vertical Advection > Other Terms